RPN Calculator (FMEA Risk Priority Number)
Calculate the FMEA risk priority number RPN = severity × occurrence × detection, criticality S × O, an action-threshold check and how much each improvement would lower the RPN.
How it works
RPN = Severity × Occurrence × Detection (each rated 1–10, so RPN runs from 1 to 1,000) · Criticality = Severity × Occurrence
The risk priority number ranks failure modes in a failure mode and effects analysis (FMEA), as described in the AIAG FMEA manual and IEC 60812. Each failure mode gets three 1–10 ratings: how severe the effect is, how often the cause occurs and how unlikely current controls are to detect it. Because very different risks can share one RPN, teams also look at severity first — a 9 or 10 needs action whatever the product — and the 2019 AIAG-VDA handbook replaced pure RPN ranking with action priority tables for that reason.
Worked example
A failure mode rated severity 8, occurrence 4 and detection 6 has RPN = 8 × 4 × 6 = 192, which is 19.2% of the 1,000 maximum and above an action threshold of 100. Its criticality is 8 × 4 = 32. Lowering occurrence to 3 with a prevention control would bring the RPN to 144, a 25.00% reduction.
Sources: Failure mode and effects analysis — RPN and rating scales · ASQ — Failure mode and effects analysis (FMEA)
Frequently asked questions
How do you calculate RPN in FMEA?
Multiply the three ratings: severity × occurrence × detection. With S = 8, O = 4 and D = 6 the RPN is 192. Each rating uses a 1–10 scale defined in your FMEA rating tables.
What RPN value requires action?
There is no universal cut-off. Many teams act on RPN ≥ 100 or 125, or on the top 20% of failure modes, but any severity of 9 or 10 should be addressed regardless of the RPN.
Why is RPN criticized?
The scales are ordinal, so multiplying them is not strictly meaningful, and different combinations give the same number (S 10 × O 2 × D 5 = S 2 × O 10 × D 5 = 100). That is why the AIAG-VDA FMEA handbook introduced action priority (high, medium, low), which weights severity most.
Which rating should I try to lower first?
Severity usually only drops with a design change. Prevention controls lower occurrence, and better inspection or testing lowers detection. Preventing the cause (occurrence) is generally preferred to catching it (detection).
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This calculator is for educational purposes. Double-check important results before acting on them.