How to Calculate Bearing Life: Formula, L10 Life and Easy Step-by-Step Guide

Knowing how to calculate bearing life is essential when selecting a bearing for motors, pumps, gearboxes, conveyors, fans, machine tools and other rotating equipment. A bearing may have the correct inner diameter and outer diameter, but that does not automatically mean it will provide the required service life.

Bearing life depends primarily on factors like dynamic load rating, actual operating load, rotational speed, lubrication, temperature, alignment, and contamination. The standard metric for measuring durability is L₁₀ life, which defines the operating hours at which 90% of a given population of identical bearings will survive (and only 10% fail) under specified conditions. The term B₁₀ life is often used interchangeably in industrial settings to denote the point where 10% of the bearings have failed, though manufacturer documentation should always be verified for specific definitions.

This guide explains the complete method to calculate bearing life, including the formula, five different bearing examples, bearing life in hours, load carrying capacity, ID and OD identification, clearance formula, C3 rating, service life, common problems and a simple calculation tool.

What Is Bearing Life?

Bearing life is generally expressed as the number of revolutions a bearing can complete before the first signs of fatigue, such as rolling-element or raceway fatigue, occur under defined operating conditions.

Bearing life can be expressed in two common ways:

• Million revolutions
• Operating hours

For practical machine design, bearing life in hours is often easier to understand.

For example, if a bearing has a calculated L10 life of 50 million revolutions and operates at 1,000 RPM, its life in hours can be calculated by converting those revolutions into hours.

L10 Life : L10 life is the basic rating life corresponding to 90% reliability.

The basic bearing life equation is:

• For ball bearings: L10 = (C/P)³ million revolutions
• For roller bearings: L10 = (C/P)^(10/3) million revolutions

Where:

• L10 = basic rating life in million revolutions
• C = basic dynamic load rating
• P = equivalent dynamic bearing load

The exponent is approximately:

• 3 for ball bearings
• 10/3 for roller bearings

Bearing Life Formula

The bearing life formula is one of the most important calculations used in bearing selection.

Ball Bearing Formula :

(1) For a deep-groove ball bearing or other applicable ball bearing: L10 = (C/P)³

Example:

Suppose:

• C = 15,000 N
• P = 5,000 N

Then:

L10 = (15,000 / 5,000)³

L10 = 3³

L10 = 27 million revolutions

Therefore, the basic rating life is 27 million revolutions.

(2) Roller Bearing Formula : L10 = (C/P)^(10/3)

The higher exponent means that increasing the dynamic load rating relative to the applied load can produce a substantial increase in calculated rating life.

Important: Always use the exact bearing manufacturer‘s load ratings and life method for the selected bearing. The examples below are educational calculations.

How to Calculate Bearing Life in Hours ?

The conversion formula is:

L10h = (L10 × 10⁶) / (60 × n)

Where:

• L10h = bearing life in hours
• L10 = life in million revolutions
• n = rotational speed in RPM

Example :

If:

• L10 = 40 million revolutions
• Speed = 1,500 RPM

Then:

L10h = (40 × 10⁶) / (60 × 1,500)

L10h = 444.44 hours

So the calculated bearing life is approximately 444 hours.

Step-by-Step Method to Calculate Bearing Life

Calculating bearing life is easier when you follow a fixed sequence. The key point is that bearing ID and OD are used to select the correct bearing, while the basic L10 life calculation mainly uses the dynamic load rating (C), equivalent dynamic load (P), and speed (RPM).

Step 1: Identify the Bearing Type

First identify the bearing type and bearing number.

The bearing number helps identify the bearing series, bore size and construction, but the complete manufacturer catalogue should be used for exact dimensions and ratings.

Step 2: Find the Bearing’s Inner Diameter (ID) and Outer Diameter (OD)

The ID is the diameter of the hole through the bearing.

The OD is the outside diameter of the bearing.

The width is normally represented by B for many radial bearings.

Typical bearing diagram

For many metric deep-groove ball bearings, bore codes follow a standard system. For example, a 6205 commonly has a 25 mm bore. However, do not assume ID and OD from the bearing number alone for every bearing type. Always confirm dimensions from the manufacturer’s catalogue or technical drawing.

Example:

For a typical 6205 bearing:

• ID = 25 mm
• OD = 52 mm
• Width = 15 mm

These dimensions confirm whether the bearing physically fits the shaft and housing.

Common examples

Bearing	Typical ID	Typical OD	Width
6205	25 mm	52 mm	15 mm
6306	30 mm	72 mm	19 mm
6208	40 mm	80 mm	18 mm
6310	50 mm	110 mm	27 mm
22215 E	75 mm	130 mm	31 mm

Dimensions can vary by bearing design or manufacturer, so catalogue verification is recommended.

Step 3: Find the Dynamic Load Rating C

The basic dynamic load rating C is normally provided in the manufacturer’s bearing catalogue.

It represents the bearing’s standardized load-rating capability for life calculations.

Do not confuse:

• C = basic dynamic load rating
• C₀ = basic static load rating

A higher C generally provides greater calculated rating life when all other factors remain constant.

Step 4: Determine the Equivalent Dynamic Load P

The actual bearing may experience radial load, axial load or a combination. For a purely radial load, the calculation may be relatively simple. For combined loading, the equivalent dynamic load may be represented by:

P = XFr + YFa

Where:

• Fr = radial load
• Fa = axial load
• X = radial load factor
• Y = axial load factor

Example:

Suppose the radial load is:

Fr = 4,000 N

For this simplified example:

P = 4,000 N

The correct X and Y values depend on the bearing type, internal design, contact angle and operating conditions. They must be obtained from the appropriate manufacturer’s catalogue.

Step 5: Calculate L10 Life

After finding C and P, use the appropriate formula.

Ball bearing

L10 = (C/P)³

Example:

Suppose the radial load is:

Fr = 4,000 N

For this simplified example:

P = 4,000 N

Roller bearing

L10 = (C/P)^10/3

The result is in million revolutions.

Step 6: Convert L10 to Bearing Life in Hours

Use:

L10h = (L10 × 10⁶)/(60n)

where:

• L10h = Bearing life in hours
• L10 = Life in million revolutions
• n = Rotational speed in RPM

This gives the calculated rating life in operating hours.

Example :

Assume:

• L10 = 42.875 million revolutions
• Speed = 1,500 RPM

Then:

L10h = (42.875 × 10⁶) / (60 × 1,500)

L10h ≈ 476.4 hours

Therefore:

Bearing L10 life ≈ 476 hours

Step 7: Compare the Calculated Life With Required Service Life

Now compare the calculated L10 life with the machine’s required service life.

Example :

Suppose the machine requires:

Required service life = 400 hours

Calculated:

L10 life = 476 hours

Since:

476 > 400

the calculated L10 rating life exceeds the basic required life in this simplified example.

However, actual bearing service life can still be affected by lubrication, contamination, temperature, misalignment, installation, clearance and other operating conditions.

Step 8: Check Bearing Clearance and C3 Rating

After the basic life calculation, check whether the bearing’s internal clearance is suitable for the application.

C3 means the bearing has greater-than-normal radial internal clearance. C3 is not a higher dynamic load rating.

For example, a bearing may be available in:

• Normal clearance
• C3 clearance
• C4 clearance

The appropriate clearance depends on factors such as shaft and housing fits, operating temperature and thermal expansion.

Complete Worked Example

Suppose you need to calculate the basic rating life of a 6205 ball bearing.

Given:

• Bearing = 6205
• ID = 25 mm
• OD = 52 mm
• Width = 15 mm
• Dynamic load rating, C = 14,000 N
• Radial load, Fr = 4,000 N
• Equivalent dynamic load, P = 4,000 N
• Speed = 1,500 RPM

Formula: L10 = (C/P)³

Calculation:

L10 = (14,000/4,000)³

L10 = 42.875 million revolutions

Convert to hours:

L10h = (42.875 × 10⁶)/(60 × 1,500)

L10h ≈ 476 hours

Final Answer:

Basic L10 life = 42.88 million revolutions

Bearing life ≈ 476 hours at 1,500 RPM

Quick Formula Summary

Calculation	Formula
Ball bearing L10	L10 = (C/P)³
Roller bearing L10	L10 = (C/P)^(10/3)
Bearing life in hours	L10h = (L10 × 10⁶)/(60 × RPM)
Simple radial load	P ≈ Fr
Combined load	P = XFr + YFa

Easy Method to Remember

1. Identify bearing → 2. Check ID/OD → 3. Find C → 4. Calculate P → 5. Calculate L10 → 6. Convert to hours → 7. Compare with required service life → 8. Check clearance and operating conditions.

Important: For real engineering selection, use the exact C rating, C₀ rating, equivalent-load factors, clearance, speed limits and life method specified by the bearing manufacturer.

5 Different Bearing Examples With Complete Calculations

The following examples use representative values for demonstrating the calculation method. They are not substitutes for the manufacturer’s catalogue ratings.

Example 1: 6205 Deep Groove Ball Bearing

Assume:

• Bearing = 6205
• C = 14,000 N
• P = 4,000 N
• Speed = 1,500 RPM
• Bearing type = Ball bearing

Step 1: Apply the formula

L10 = (C/P)³

L10 = (14,000/4,000)³

L10 = (3.5)³

L10 = 42.875 million revolutions

Calculate bearing life in hours :

L10h = (42.875 × 10⁶)/(60 × 1,500)

L10h = 476.39 hours

Result :

L10 = 42.875 million revolutions

Bearing life ≈ 476 hours

Example 2: 7306 Angular Contact Ball Bearing

Assume:

• Bearing = 7306
• C = 19,500 N
• P = 5,000 N
• Speed = 2,000 RPM

For this educational example, use the ball-bearing exponent of 3.

Calculation :

L10 = (19,500/5,000)³

L10 = (3.9)³

L10 = 59.319 million revolutions

Now convert to hours:

L10h = (59.319 × 10⁶)/(60 × 2,000)

L10h = 494.33 hours

Result :

L10 = 59.319 million revolutions

Bearing life ≈ 494 hours

For an actual angular-contact bearing calculation, axial load and the manufacturer’s equivalent-load method must also be considered when applicable.

Example 3: NU308 Cylindrical Roller Bearing

Assume:

• Bearing = NU308
• C = 36,000 N
• P = 9,000 N
• Speed = 1,200 RPM
• Bearing type = Roller bearing

Use: L10 = (C/P)^(10/3)

Calculation :

L10 = (36,000/9,000)^(10/3)

L10 = 4^(10/3)

L10 ≈ 101.59 million revolutions

Now:

L10h = (101.59 × 10⁶)/(60 × 1,200)

L10h ≈ 1,411 hours

Result :

L10 ≈ 101.59 million revolutions

Bearing life ≈ 1,411 hours

Example 4: 30208 Tapered Roller Bearing

Assume:

• Bearing = 30208
• C = 33,000 N
• P = 8,000 N
• Speed = 1,000 RPM

For the simplified educational calculation:

L10 = (33,000/8,000)^(10/3)

L10 ≈ 112.57 million revolutions

Convert to hours:

L10h = (112.57 × 10⁶)/(60 × 1,000)

L10h ≈ 1,876 hours

Result :

L10 ≈ 112.57 million revolutions

Bearing life ≈ 1,876 hours

For an actual tapered roller bearing application, use the manufacturer’s complete equivalent-load and rating-life procedure, particularly when radial and axial loads are both present.

Example 5: 22215 E Spherical Roller Bearing

Assume:

• Bearing = 22215 E
• C = 62,000 N
• P = 15,000 N
• Speed = 900 RPM

Use the roller bearing equation:

L10 = (62,000/15,000)^(10/3)

L10 ≈ 113.33 million revolutions

Convert to hours:

L10h = (113.33 × 10⁶)/(60 × 900)

L10h ≈ 2,099 hours

Result :

L10 ≈ 113.33 million revolutions

Bearing life ≈ 2,099 hours

Summary of the 5 Bearing Calculations

Example	Bearing	Type	C (N)	P (N)	RPM	L10 (million rev.)	Life (hours)
1	6205	Ball	14,000	4,000	1,500	42.88	476
2	7306	Ball	19,500	5,000	2,000	59.32	494
3	NU308	Roller	36,000	9,000	1,200	101.59	1,411
4	30208	Roller	33,000	8,000	1,000	112.57	1,876
5	22215 E	Roller	62,000	15,000	900	113.33	2,099

These figures demonstrate an important point: bearing life is highly sensitive to the ratio between C and P.

Bearing Load Carrying Capacity and Life

Bearing selection should never be based only on bore size.

Two bearings may have the same ID but significantly different load carrying capacity.

For example, a larger bearing series may provide a higher dynamic load rating C, which can significantly improve calculated L10 life under the same applied load.

The basic relationship is:

Higher C / Lower P = Longer calculated rating life

Conversely:

Lower C / Higher P = Shorter calculated rating life

However, load capacity is not the only consideration. Speed, lubrication, temperature, contamination, alignment, clearance and installation also affect actual service performance.

Clearance Formula

Bearing clearance is the amount of internal movement available between the bearing’s rolling elements and raceways when the bearing is not mounted.

A simplified representation of radial internal clearance is:

Cr = Cmax − Cmin

Where:

• Cr = radial internal clearance
• Cmax = maximum measured internal clearance
• Cmin = minimum measured internal clearance

The actual manufacturer’s clearance specification should be used because the measurement procedure and tolerance depend on bearing design and standard.

Incorrect clearance can cause:

• Excessive vibration
• Increased temperature
• Noise
• Premature wear
• Reduced bearing life

What Is C3 Rating?

C3 refers to a bearing with radial internal clearance greater than normal.

A simplified clearance sequence is:

C2 < Normal < C3 < C4 < C5

C3 does not mean that the bearing has a higher load rating.

It means the bearing has greater internal clearance than the normal-clearance version.

C3 clearance may be selected for applications where operating temperature, interference fits or other conditions reduce the operating clearance.

Examples can include certain:

• Electric motors
• Pumps
• Fans
• High-temperature applications
• Applications using interference fits

But C3 should not be selected simply because it sounds “stronger.” The correct clearance depends on the application.

How Does C3 Clearance Affect Service Life?

A bearing with inappropriate clearance can experience increased stress, heat and vibration.

If the internal clearance becomes too small during operation, rolling elements may experience excessive preload or reduced running clearance.

If clearance is excessive, load distribution may become less favorable and vibration can increase.

Therefore:

Correct clearance → Better operating conditions → Better reliability

C3 is an internal-clearance specification, not a direct measure of bearing life.

Bearing Service Life vs L10 Life

It is important to understand that calculated L10 life is not necessarily the same as actual service life.

A bearing can fail before the calculated rating life because of:

• Poor lubrication
• Contamination
• Incorrect installation
• Excessive misalignment
• Excessive temperature
• Electrical damage
• Corrosion
• Excessive vibration
• Improper clearance
• Shock loading

Conversely, a bearing may operate much longer than the basic L10 calculation if operating conditions are favorable.

Therefore, L10 is a rating-life calculation, not a guaranteed failure date.

Simple Bearing Life Calculation Tool

You can create a basic bearing life calculator using the following inputs:

Inputs:

1. Bearing type
2. Dynamic load rating C
3. Equivalent dynamic load P
4. Speed in RPM

Calculation logic

For a ball bearing:

L10 = (C/P)³

For a roller bearing:

L10 = (C/P)^(10/3)

Then:

Life in hours = L10 × 1,000,000 / (60 × RPM)

Common Bearing Life Problems and Solutions

PROBLEM	POSSIBLE CAUSE	SOLUTIONS
Bearing Fails Earlier Than Calculated	The calculated L10 life assumed a clean and correctly lubricated bearing, but the actual application may have contamination, excessive load or incorrect installation.	Check: Actual radial load Axial load Lubrication Contamination Alignment Shaft and housing fits Operating temperature Then recalculate the bearing life using realistic operating conditions.
Excessive Bearing Temperature	Too much grease Incorrect lubricant Excessive preload Incorrect clearance Excessive speed	Check the manufacturer’s recommended lubrication quantity, operating speed and clearance specification.
Bearing Has High Noise and Vibration	Noise can result from: Contamination Raceway damage Incorrect installation Misalignment Excessive clearance Lubrication problems	Inspect the bearing, shaft, housing and lubricant. Verify that the selected clearance is appropriate.
Calculated Life Is Too Low	The applied load P may be too high compared with the bearing’s dynamic load rating C.	Consider a bearing with: Higher dynamic load rating Appropriate bearing geometry Suitable speed capability Correct internal clearance Reducing unnecessary load can also improve calculated life.
Wrong C3 Bearing Selected	C3 was selected only because it is commonly used in motors or because the user assumed C3 means higher strength.	Select clearance based on: Shaft fit Housing fit Operating temperature Thermal expansion Bearing arrangement Manufacturer recommendations Remember that C3 means greater-than-normal internal clearance, not greater load capacity.

Important Factors That Affect Actual Bearing Life

The basic L10 calculation is useful, but real bearing service life depends on many factors.

1. Load	Higher operating load generally reduces calculated fatigue life.
2. Speed	Higher RPM increases the number of revolutions accumulated per hour and can also increase temperature and lubrication demands.
3. Lubrication	Correct grease or oil is essential for separating rolling surfaces and controlling friction and heat.
4. Contamination	Dust, water and abrasive particles can significantly reduce bearing service life.
5. Alignment	Misalignment can create additional stresses and uneven load distribution.
6. Temperature	Excessive temperature can degrade lubricant and affect bearing material and clearance.
7. Installation	Incorrect mounting force, shaft damage, improper fits or contamination during installation can cause early failure.
8. Clearance	Correct internal clearance is important for maintaining suitable operating conditions.

Quick Bearing Life Calculation Checklist

Before calculating bearing life, collect:

• Bearing number
• Bearing type
• Inner diameter (ID)
• Outer diameter (OD)
• Width
• Dynamic load rating C
• Static load rating C0
• Radial load Fr
• Axial load Fa
• Equivalent dynamic load P
• Operating speed RPM
• Internal clearance
• Lubrication condition
• Operating temperature
• Required service life

Then follow this method:

Why Engineers Use L10 Instead of “Average Life”

When engineers calculate bearing service life, they usually use L10 life rather than an “average life” value because L10 provides a standardized and conservative basis for comparing bearings under a defined load and operating condition.

The important point is that rolling bearings do not all fail at exactly the same number of revolutions. Even bearings made to the same specification and operated under apparently identical conditions can have different fatigue lives because of variations in material, manufacturing, lubrication, surface condition, contamination and operating environment.

Therefore, simply saying that a bearing has an “average life” can be misleading.

What Does L10 Mean?

L10 life is the basic rating life at which 10% of an identical group of bearings are statistically expected to have reached fatigue failure, while approximately 90% are expected to survive.

For a ball bearing:

L10 = (C/P)³ million revolutions

For a roller bearing:

L10 = (C/P)^(10/3) million revolutions

Where:

• L10 = basic rating life
• C = basic dynamic load rating
• P = equivalent dynamic bearing load

L10 is therefore not a guaranteed failure point. It is a statistical rating used for bearing selection and comparison.

L10 vs L50 vs L90

The terms L10, L50 and L90 describe different statistical reliability levels.

Life Value	Approximate Survival	Approximate Failure	Meaning
L10	90%	10%	Standard basic rating life
L50	50%	50%	Median life
L90	10%	90%	Very high failure probability by this point

The terminology is easiest to understand by thinking about a group of 100 identical bearings.

L10

At L10 life:

• Approximately 90 bearings are expected to survive.
• Approximately 10 bearings may have failed from the relevant fatigue mechanism.

This is why L10 is the standard rating-life reference used in bearing calculations.

L50

At L50 life:

• Approximately half the population has survived.
• Approximately half has failed.

L50 is therefore the median life, not necessarily the arithmetic average life.

L90

At L90 life:

• Approximately 10% of the original population remains.
• Approximately 90% have failed.

Therefore, L90 should not be interpreted as “90% reliability.” In this notation, the subscript refers to the percentage of the population expected to fail.

Why Not Simply Use Average Bearing Life?

An average can hide the spread of actual bearing failures.

Imagine 10 bearings operating under the same nominal conditions:

• Some may fail relatively early.
• Some may operate close to the calculated rating life.
• Others may continue operating much longer.

If an engineer only uses the average, the value may not adequately represent the risk of an early failure.

L10 provides a more useful engineering reference because it establishes a recognized 90% survival basis.

This is particularly important in machinery where unexpected bearing failure can cause:

• Production downtime
• Equipment damage
• Maintenance costs
• Safety problems
• Loss of productivity

For this reason, engineers generally begin bearing selection with the standardized L10 rating-life calculation and then consider application-specific reliability requirements.

L10 vs L50 vs L90: Simple Example

Suppose a bearing population has a calculated statistical life distribution in which the approximate life points are:

• L10 = 10 million revolutions
• L50 = 50 million revolutions
• L90 = 200 million revolutions

These numbers would mean:

At 10 million revolutions: approximately 90% of the bearings are expected to survive.

At 50 million revolutions: approximately 50% are expected to survive.

At 200 million revolutions: approximately 10% are expected to survive.

This demonstrates why the three values represent very different reliability levels.

Important: The numerical relationship between L10, L50 and L90 is not universal. It depends on the statistical life distribution and assumptions used. Do not convert L10 to L50 or L90 using an arbitrary multiplier unless the applicable reliability model or bearing manufacturer’s data supports it.

When to Use L10 Bearing Life

L10 bearing life is normally the appropriate starting point when:

1. Comparing different bearings.
2. Checking whether a bearing meets a required design life.
3. Selecting a bearing based on dynamic load rating.
4. Estimating bearing life under a known load and speed.
5. Comparing bearing sizes or series.
6. Performing preliminary machine-design calculations.
7. Checking bearing life in hours for maintenance planning.

For example, suppose a machine requires a bearing to operate for 20,000 hours.

The engineer can calculate the required L10 life:

Required L10 = (20,000 × 60 × n) / 10⁶

If the shaft operates at 1,000 RPM:

Required L10 = (20,000 × 60 × 1,000) / 10⁶

Required L10 = 1,200 million revolutions

The engineer can then compare this requirement with the calculated L10 rating life of candidate bearings.

When L10 May Not Be Enough

Although L10 is extremely useful, it should not be the only consideration for every application.

For critical equipment, engineers may need a higher reliability target and additional life calculations.

Examples include:

• Aircraft systems
• Critical industrial machinery
• Large turbines
• Power-generation equipment
• High-cost production machinery
• Safety-critical rotating equipment

In such cases, the engineer may use a manufacturer’s modified rating-life calculation, reliability adjustment factors, application-specific life models or other reliability methods.

The calculation may account for factors such as:

• Material quality
• Lubrication
• Contamination
• Load spectrum
• Temperature
• Reliability
• Speed
• Bearing operating conditions

L10, B10 and Reliability: Are They the Same?

The terms L10 and B10 are closely related in many engineering contexts because both can describe the point at which approximately 10% of a population has failed, corresponding to approximately 90% survival.

However, terminology can vary between industries, standards and manufacturers.

For rolling-bearing rating calculations, L10 is the conventional term used for basic rating life in million revolutions.

When a manufacturer specifies B10 life, always check its exact definition and test method before treating it as directly interchangeable with an L10 rating.

The Key Engineering Lesson

The main reason engineers use L10 instead of “average life” is simple:

L10 provides a standardized statistical reference for bearing selection, while an average-life number alone does not adequately describe the spread of possible bearing failures.

A good bearing-life analysis should therefore follow this sequence:

Bearing selection → C rating → Equivalent load P → L10 calculation → Bearing life in hours → Reliability/application check

L10 is an excellent starting point, but actual service life can be affected by lubrication, contamination, mounting, alignment, temperature, clearance, vibration and other operating conditions.

Quick Takeaway

• L10: approximately 90% survival — the standard basic rating-life reference.
• L50: approximately 50% survival — median life.
• L90: approximately 10% survival — approximately 90% of the population has failed.
• Average life: a statistical average and not necessarily a suitable reliability-design target.
• B10: often describes the life at which 10% of a population has failed, but the exact definition should be verified for the relevant industry or manufacturer.
• For most preliminary bearing-selection calculations: start with L10, then evaluate application-specific reliability and operating conditions.

Conclusion

Learning how to calculate bearing life is essential for proper bearing selection. The basic process involves identifying the bearing, checking C (dynamic load rating) and calculating the equivalent dynamic load P.

Bearing life is also affected by load, lubrication, speed, temperature, contamination, alignment and clearance. Remember, C3 means greater-than-normal internal clearance, not higher load capacity.

Always use the latest manufacturer catalogue data for accurate bearing selection.

FAQs :