B10 life calculator
B10 life is the age at which 10% of units have failed. Reliability engineers use it for warranty, bearing catalogs, and early-life risk — it is much more conservative than MTTF or characteristic life η. Weibull.Genie evaluates Bx life from the current workbook fit; this page shows the identity so you can check the arithmetic.
Open the Weibull.Genie workbook Open Weibull Calculator Compute BX% from a fit
Definition
For any life distribution, Bx is the quantile at unreliability x/100. B10 ⇒ F = 0.10. On a two-parameter Weibull,
tx = η [−ln(1 − x/100)]1/β
Because −ln(0.90) ≈ 0.10536 is far below 1, B10 sits well below η whenever β is not tiny. Do not substitute MTTF or η for B10 on a drawing: they answer different questions.
Worked example (β = 2, η = 1 000 h)
Take a wear-out Weibull with shape 2 and characteristic life 1 000 hours — round numbers you can redo on paper.
B10 = 1 000 × [−ln(0.90)]1/2 = 1 000 × √0.105361 ≈ 325 h
| Life | Unreliability | Time |
|---|---|---|
| B1 | 1% | 100 h |
| B10 | 10% | 325 h |
| B50 (median) | 50% | 833 h |
| Characteristic life η | 63.2% | 1 000 h |
| MTTF | mean, not a quantile | 886 h |
Enter those β and η on Weibull Calculator (or fit your own grid on the workbook) and set BX% = 10. The bearing-sample fit on the Weibull analysis page gives B10 ≈ 254 h for β ≈ 2.17, η ≈ 716 h — same formula, different LifeFit.
FAQ
What is B10 life?
B10 is the time at which 10% of the population is expected to have failed (F = 0.10, R = 0.90). It is a quantile of the life distribution, not a test duration and not MTTF.
How is B10 calculated for a 2P Weibull?
t_0.10 = η [−ln(1 − 0.10)]^(1/β) = η [−ln(0.90)]^(1/β). The same formula with 0.01 or 0.50 gives B1 and B50. Characteristic life η is the 63.2% point, not B63 as a named Bx in most handbooks.
Is B10 the same as 90% reliable life?
Yes. Reliable life t_R at R = 0.90 is the same quantile as B10. Weibull.Genie reports BX% life and reliable life from the same fitted LifeFit so the two cannot drift apart.
Where do I compute B10 in Weibull.Genie?
Fit life data on the Weibull Workbook, then open Weibull Calculator → Life → BX% life with 10. Test vs use also lists B1, B10, and B50 on the use-case curve after an ALT projection.