Reliability Demonstration Test Calculator
Plan exact mission samples and declared Weibull duration alternatives.
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- Free preview: the minimum fixed-mission sample count in full with the model’s notes and the first rows of the field table; every other figure is hidden.
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Mission samples and duration alternatives
Your inputs stay on this device, and every result states the model it assumes.
The reliability required at the stated mission.
Strictly between zero and one.
A whole failure count.
All alternative durations use these same units.
This is an assumption supplied from separate life evidence, never inferred from this plan.
One whole unit count per line. Each must exceed allowed failures. The calculation retains every alternative.
Your inputs
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Engineering estimate — not a professional design, safety approval or verification certificate. Review the stated independence, censoring and sampling assumptions with a qualified engineer.
In the CSV, text that starts like a spreadsheet formula is escaped so that it cannot run; the “Original value JSON” column keeps your exact text.
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Results are estimates from the formulas shown, not a professional design or certification. Have a qualified engineer verify anything safety-critical (structures, electrical installations, gas or pressure systems).
About the Reliability Demonstration Test Calculator
Choose a target mission reliability, confidence and prespecified allowed-failure count. The fixed-mission plan uses the exact binomial acceptance probability and verifies that one fewer sample does not meet the target. With an externally justified Weibull shape, compare complete sample-count alternatives at their required per-unit durations. Fixed-mission outcomes and a constant-rate time-on-test MTBF plan are distinct sampling models and are shown separately.
How to use it
- Enter target reliability and confidence strictly between zero and one, and the whole number of failures permitted by the plan.
- Enter the mission duration and a known Weibull shape only when the life model is justified independently of this demonstration.
- Add each sample-count alternative. Read its per-unit duration, aggregate exposure and exact allowed-failure acceptance probability.
- Keep the original plan and assumptions. Unlock the full alternatives table, copied calculation and downloadable report.
Examples
Target reliability 0.5 and confidence 0.75 at the fixed mission
Minimum sample counts: 2 with no failures allowed, 5 with one allowed, and 7 with two allowed. Each previous sample count fails the confidence inequality.
Exact mission-sample criterion
Accept at most c failures among n independent units. At target reliability R, the binomial acceptance probability is sum(j=0..c) choose(n,j)(1-R)^j R^(n-j), and must be at most alpha = 1-confidence. With zero allowed failures, n = ceil[ln(alpha)/ln(R)]. This is a planning criterion, not a statement that the tested product is already demonstrated.
Samples versus duration
For externally known Weibull shape beta, reliability at duration d is R(mission)^((d/mission)^beta). The same exact binomial criterion is used for each duration alternative, including allowed failures. With zero failures, n(d/mission)^beta = ln(alpha)/ln(R). A separate NIST HPP exposure plan uses MTBF objective × one-half a chi-square quantile and aggregates operating unit-hours under constant-rate assumptions.
Limitations
- Engineering estimate — not a professional design, safety approval or verification certificate. Confirm the model and sampling assumptions with a qualified engineer.
- Each unit on test is assumed to fail independently under the mission conditions. The plan does not model accelerated testing.
- Reset clears the page: download a backup of your inputs to keep them.
Privacy
On this device; its files download once
Calculations and the files you open stay on this device: nothing is uploaded. Downloading a backup of your own inputs never needs a pass.
Frequently asked questions
What do I get without a pass?
Without a pass, Reliability Demonstration Test Calculator shows the minimum fixed-mission sample count in full with the model’s notes and the first rows of the field table; every other figure is hidden. Until you unlock it, the result can’t be downloaded or copied. A Pro, Premium or Ultimate pass, a one-time payment that never renews, unlocks the full result. The pricing page lists the passes and their prices.
Why do allowed failures require more samples?
The plan must still make acceptance unlikely at the target reliability. The calculator uses the full binomial lower tail and checks minimality rather than replacing it with a normal approximation.
Can I estimate the Weibull shape from this test plan?
No. The duration trade-off requires a shape supported by separate life evidence. A guessed shape is not a demonstrated acceleration or ageing model.
Is total unit-hours the same as elapsed test time?
No. Several units can run together. Per-unit duration is the calendar exposure for each unit; aggregate unit-duration sums exposure over all units.
Does this approve a medical-device or vehicle verification plan?
No. Confirm failure definitions, censoring, mission conditions and the application’s required protocol with a qualified engineer.