How to Calculate Bearing Life (L10) — with a Worked Example
Bearing life is normally expressed as the basic rating life, L10 — the life that 90% of a group of identical bearings will reach or exceed under a given load and speed (so 10% may fail before it). It is calculated from the bearing's load rating and the load you apply, using one simple formula.
The formula
where:
- C = basic dynamic load rating (from the catalogue), in newtons.
- P = equivalent dynamic load actually applied, in the same unit.
- p = the life exponent: 3 for ball bearings, 10/3 for roller bearings.
The result is in millions of revolutions. To get hours, bring in the speed:
where n is the speed in rpm.
A worked example
Take a deep-groove ball bearing (so p = 3) with a catalogue dynamic rating C = 42,000 N, carrying a steady radial load P = 4,000 N, at n = 1,500 rpm.
- Load ratio: C / P = 42,000 / 4,000 = 10.5
- L10 = 10.53 = ≈ 1,158 million revolutions
- In hours: L10h = 1,000,000 / (60 × 1,500) × 1,158 = 11.11 × 1,158 = ≈ 12,860 hours
That is roughly 1.5 years of continuous running — or much longer in intermittent service.
What about combined loads?
If the bearing carries both radial and axial load, you first work out the equivalent load P:
where Fr and Fa are the radial and axial loads, and X and Y are factors from the manufacturer's catalogue (they depend on the bearing and the ratio of axial to radial load). For a purely radial load, P is simply Fr.
Adjusting for reliability
L10 is the life at 90% reliability. If you need a higher reliability, multiply by a factor a1:
| Reliability | Life | a1 |
|---|---|---|
| 90% | L10 | 1.00 |
| 95% | L5 | 0.64 |
| 96% | L4 | 0.55 |
| 99% | L1 | 0.25 |
So L5 (95% reliability) in our example = 0.64 × 1,158 ≈ 741 million revolutions. (These follow ISO 281:2007; some older references differ slightly.)
A note on real-world life
L10 covers fatigue only — it assumes good lubrication, clean running and correct mounting. The fuller ISO 281 "modified" life adds factors for lubrication and contamination, because in practice those are what usually end a bearing's life early. In other words, the formula tells you the best case; how close you get depends on how well the bearing is looked after.
How to get more life from a bearing
- Reduce the load, or choose a larger bearing (higher C) — life rises with the cube of C/P for ball bearings.
- Lubricate correctly and keep it clean — the biggest real-world levers.
- Get the fit, alignment and clearance right.
Frequently asked questions
What is L10 bearing life?
The basic rating life — the number of revolutions (or hours) that 90% of identical bearings will reach or exceed under a given load and speed.
What is the difference between the ball and roller exponent?
The life exponent p is 3 for ball bearings and 10/3 for roller bearings, reflecting the different way load is carried (point vs line contact).
Does doubling the load halve the life?
No — it has a much bigger effect. For ball bearings life varies with the cube of C/P, so doubling the load cuts life to about one-eighth.
Run your own numbers in the bearing calculators, and see the Standards Hub for load ratings and clearance.