FIT Calculator

A FIT number only means something when tied to mission life, fleet size, confidence, acceleration, and test evidence. Convert between reliability metrics, then check whether a test plan actually supports the claim.

I know my…

FIT rate (failures / 1e9 h)

Mission context

Mission hours (t)Fleet size (n)

Reliability over 10,000 h

99.9990%

5.0 nines

R(t) mission profile

Converted valuesread-only

Failure rate λ

1.000e-9/h

Failure probability

10ppm

MTTF or MTBF

1.000e+9h

Expected fleet failures

≈ 1

At 1 FIT over 10,000 h, a fleet of 100,000 should see about 1 failure(s) — 5.0 nines of reliability. High reliability is not zero risk.

Formula & assumptions

R(t)=e^{-\lambda t},\quad \text{FIT}=\lambda\times 10^{9},\quad \text{MTTF}=\tfrac{1}{\lambda}

λ = constant failure rate (per hour)
t = mission hours, n = sample size or fleet size
FIT = failures per 1e9 device-hours

Have a more complex case than this calculator covers? Get in touch.

Assumptions & limitations

The converter assumes a constant failure rate (exponential model). It does not represent infant-mortality or wear-out phases — that is what the Weibull module covers.
The evidence check uses the chi-square upper confidence bound for time-terminated testing with r observed failures, applied to equivalent device-hours (real hours × acceleration factor).
Acceleration factor is treated as a single scalar; it should come from a validated physics-of-failure model (e.g. Arrhenius, Coffin-Manson).
This tool is educational and for planning. It does not replace a formal FMEDA, safety case, or ISO 26262 assessment.