What Is a Risk Priority Number (RPN)?

A risk priority number (RPN) is the score an FMEA team attaches to a potential failure by multiplying three judgments — severity of the effect, likelihood of occurrence, and the chance that current controls fail to detect it — each rated on a 1-to-10 scale. The product runs from 1 to 1,000, and higher means more urgent. The RPN exists to solve a practical problem: an honest failure analysis produces dozens of worries, and a team can only work a few at a time.

How it’s used

Each rating comes from an agreed scale. Severity 1 means the customer would never notice; 10 means a safety or regulatory catastrophe. Occurrence runs from practically impossible to near-certain. Detection is scored in reverse — 1 means the failure is almost certain to be caught before it escapes, 10 means it would sail through untouched. The team scores every failure mode, sorts the table, and assigns corrective actions from the top down. After an action is implemented, the row is re-scored; a falling RPN is the receipt proving the fix changed something real.

Two cautions keep the tool honest. First, severity outranks arithmetic: a failure scored 9 or 10 for severity demands attention even when low occurrence gives it a modest product. Second, RPNs are rankings, not measurements — a 240 is not exactly twice as bad as a 120, and close scores are effectively ties. Newer automotive guidance has shifted toward action-priority tables for exactly this reason, but the three underlying ratings remain the working core of the method.

An illustration: the space heater

Picture an appliance maker assessing a space heater. Failure mode: the tip-over switch fails to cut power. Severity is 9. Occurrence is 2 — the component is proven. Detection is 6, because the end-of-line test exercises the switch only briefly. RPN: 108. The team adds an automated full-cycle switch test, and detection improves to 2. The new RPN is 36. Nothing about the design changed except the odds that this failure reaches a living room — which is precisely what the number is built to track.

• Never compare RPNs across different FMEAs; the scales are calibrated per team
• A severity 9 or 10 is an action item regardless of its product
• Only a process change should move a score — re-rating a number down is fiction
• Recalculate after every corrective action closes
• Treat close scores as ties; the tool ranks attention, it does not measure risk

An RPN doesn’t measure risk; it ranks where your attention goes first.

The RPN is taught alongside FMEA in our Green Belt curriculum, where you score a simulated process end to end. If the mechanics feel abstract, start with the free White Belt — about six hours and a 30-question exam — for the vocabulary, then meet the tool properly at Green.