How do you calculate the failure rate from a reliability test?

Divide the number of samples that failed by the total tested, then multiply by 100. With 3 failures out of 180 units, that is 3 / 180 x 100 = 1.67%.

What is a good failure rate for an environmental test?

It depends on the acceptance criterion you set. Here the allowable target is 2%, and the observed 1.67% sits 0.33 points under it, so the lot passes - but only just. For safety-critical or automotive parts, targets are often a fraction of a percent.

Is a 1.67% failure rate a pass against a 2% target?

Yes by the point estimate - the 0.33-point gap to target means you are inside the limit. But with only 180 units the upper confidence bound can exceed 2%, so for high-stakes lots verify with a binomial interval before releasing.

Failure rate vs failure rate target - what is the difference?

The observed failure rate is what you actually measured (1.67%). The target is the maximum you allow (2%). The gap to target (0.33 points) is your margin; a negative gap means you exceeded the limit and failed.

Why does sample size matter for a failure-rate estimate?

Small samples make the estimate noisy. One more failure here (4 of 180) pushes you to 2.22%, over the 2% target. Larger samples tighten the estimate and shrink the confidence interval around it.

How do I convert this failure rate into a reliability figure?

Reliability over the test condition is roughly 1 minus the failure rate, so 1.67% failures implies about 98.33% survival for that stress profile - but only at the test duration and stress level applied, not a lifetime MTBF.

Environmental Test Chambers & Reliability Labs calculator

Failure Rate Estimate Calculator

The observed failure rate tells a reliability lab what fraction of a sample population failed during an environmental or accelerated-life test such as thermal cycling, HALT, or 85/85 humidity exposure. Reliability engineers, DVT teams, and supplier-qualification leads use it to decide whether a lot or design meets an acceptance criterion before it moves to PVT or production. It matters because a single point above target can stop a product launch, trigger a containment, or force a corrective-action loop with a supplier. This calculator also reports how many percentage points of margin you have to your allowable target so you know whether the result is a pass, a marginal pass, or a fail.

What this calculator does

Calculate observed test failure rate from failed samples, total samples tested, and the allowable failure-rate target.
a reliability engineer needs a quick observed failure rate from completed testing
It computes the observed failure rate as samples failed divided by total samples tested, times 100, and the gap in points to your allowable target.

Formula used

Observed failure rate = samples failed during test ÷ total samples tested × 100
Gap to target = allowable failure-rate target - observed failure rate

Inputs explained

Samples failed during test:
Total samples tested:
Allowable failure-rate target:

How to use the result

Use it at the end of a reliability or environmental test run when you have a final failed count and a defined acceptance criterion (AQL, lot tolerance, or design target).
A raw failure rate from a small sample is a point estimate, not a confidence-bounded reliability figure - 3 of 180 looks like 1.67% but the true rate could be meaningfully higher; use a binomial confidence interval for go/no-go on critical lots.

Common questions

How do you calculate the failure rate from a reliability test? Divide the number of samples that failed by the total tested, then multiply by 100. With 3 failures out of 180 units, that is 3 / 180 x 100 = 1.67%.
What is a good failure rate for an environmental test? It depends on the acceptance criterion you set. Here the allowable target is 2%, and the observed 1.67% sits 0.33 points under it, so the lot passes - but only just. For safety-critical or automotive parts, targets are often a fraction of a percent.
Is a 1.67% failure rate a pass against a 2% target? Yes by the point estimate - the 0.33-point gap to target means you are inside the limit. But with only 180 units the upper confidence bound can exceed 2%, so for high-stakes lots verify with a binomial interval before releasing.
Failure rate vs failure rate target - what is the difference? The observed failure rate is what you actually measured (1.67%). The target is the maximum you allow (2%). The gap to target (0.33 points) is your margin; a negative gap means you exceeded the limit and failed.
Why does sample size matter for a failure-rate estimate? Small samples make the estimate noisy. One more failure here (4 of 180) pushes you to 2.22%, over the 2% target. Larger samples tighten the estimate and shrink the confidence interval around it.

Last reviewed 2026-05-12.