If you are selecting a rolling-element bearing, the single most important number you will calculate is its rated fatigue life. ISO 281 defines this as L10 life — the number of revolutions that 90% of a population of identical bearings will reach or exceed before the first signs of subsurface fatigue. This guide walks through the rating equation, the difference between dynamic load rating and applied load, and a complete worked example you can reproduce in the L10 bearing life calculator.
The basic rating life equation
ISO 281 expresses basic rating life in millions of revolutions:
Where:
- C — basic dynamic load rating (kN), taken from the manufacturer’s catalog. It is the constant radial load a bearing can theoretically carry for one million revolutions.
- P — equivalent dynamic bearing load (kN), the constant radial load that produces the same fatigue life as the actual combined loading.
- p — the life exponent: 3 for ball bearings and 10/3 for roller bearings.
Converting revolutions to operating hours
Catalog life in millions of revolutions is rarely the unit you actually care about. For a constant shaft speed n in rev/min, convert to hours:
Finding the equivalent dynamic load P
Bearings rarely see pure radial load. When both a radial load Fr and an axial load Fa act on the bearing, ISO 281 combines them with two factors:
X and Y depend on the ratio Fa/Fr relative to a reference value e. For a single-row deep-groove ball bearing, a common case is X = 0.56 and Y ≈ 1.5 once the axial component is large enough to matter. The equivalent dynamic load calculator handles the X/Y interpolation for you.
Worked example: a 6205 deep-groove ball bearing
Step 1 — equivalent load. With Fa/Fr = 0.25, take X = 0.56 and Y = 1.5:
Step 2 — basic rating life. Ball bearing, so p = 3:
Step 3 — convert to hours.
So this bearing has a 90% probability of surviving roughly 4,000 operating hours under the stated load and speed. If that falls short of your design target, you increase C (a larger bearing), reduce P (better load distribution), or accept the trade-off.
Adjusted life: when L10 is not enough
Basic L10 assumes good lubrication, normal cleanliness, and reasonable temperature. ISO 281 also defines a modified rating life, Lnm = a1 · aISO · L10, where a1 adjusts for a reliability other than 90% and aISO bundles lubrication, contamination, and the fatigue load limit. Clean, well-lubricated bearings can far exceed their basic L10; contaminated or starved bearings fall well short. Speed and lubrication regime feed directly into this — check the DN value and speed-limit calculator before finalizing a high-speed selection.
A quick reality check before you buy
Measure your actual bore and shaft with a calibrated digital caliper before ordering — a 0.02 mm error on a fit class is the difference between a press fit and a spinning inner ring. Then confirm the fit class with the bearing fit and tolerance calculator.
Frequently asked questions
Why is it called L10 and not average life?
Because bearing fatigue is statistical. L10 is the 10% failure point (90% survival). Median life (L50) is roughly five times L10 — quoting average life would dangerously overstate reliability.
Do I use exponent 3 or 10/3?
Use p = 3 for ball bearings (point contact) and p = 10/3 for roller bearings (line contact). Roller bearings reward higher load capacity more steeply, which is why they dominate heavy-load applications.
What if my load varies over time?
Compute an equivalent constant load using a cube-weighted (or 10/3-weighted) average of the load spectrum, then apply the same equation. The bearing life calculator accepts a single equivalent load — derive it first.
Three L10 mistakes that wreck the estimate
Even with the right equation, the inputs trip people up. First, using static load rating C0 in place of dynamic rating C — C0 governs permanent deformation under no rotation, not fatigue life, and the two numbers differ substantially. Second, ignoring the axial component: dropping Fa from the equivalent load underestimates P and badly overstates life on any bearing that takes thrust. Third, treating a duty cycle as a single steady load — a machine that idles, runs, and peaks needs a load-weighted equivalent (cube-weighted for ball bearings) before the rating equation means anything. Get those three right and the L10 number actually predicts field behavior instead of flattering the spec sheet.
Run your numbers in the L10 Bearing Life Calculator →
Browse the full set of rotation tools on the Bearings & Rotation hub, including equivalent load, fit, speed, and the selection wizard.
Stop looking it up twice — run the numbers and keep the record.
This reference is free forever. When you need to actually calculate, document, and defend the result, do it in the app — then export a clean report you can hand to a reviewer.
- ✓46 engineering calculators, unlimited
- ✓Save & reload every calculation
- ✓Organize work into project folders
- ✓Branded, PE-stamped PDF reports
Free plan, no credit card. Upgrade only when it's paying for itself.