Calculations with context
FMEA and RPN calculator
Build FMEA rows; explore severity, occurrence, detection and risk-priority numbers with their scoring limitations.
Educational model · assumptions remain visible
A worksheet before a score
Define the function and failure chain, then explore the ordinal arithmetic. The first row is an invented marine cooling case; the optional second row lets you compare another profile.
Whole numbers 1–10. Larger S means a more serious effect; larger O means a higher occurrence category; larger D means poorer opportunity for timely detection. These are ranks, not probabilities.
Worked example · equal products, different consequences
The existing RPN-limitations guide uses profile A (S 10, O 2, D 5) and profile B (S 4, O 5, D 5). Both give 100, yet their severity and occurrence categories differ. These are invented arithmetic profiles, separate from the cooling worksheet.
4 × 5 × 5 = 100
What an RPN can carry
RPN = S × O × D. The input categories and their rationale are part of the result. Dividing the product by 1,000 does not turn it into a probability, and 200 does not mean twice the physical risk of 100.
Separate cause, failure mode and effect. A failed main pump does not necessarily mean loss of the cooling function if standby flow is genuinely available. Shared electrical supply, a common suction blockage and a late response can defeat that assumption.
A proposed action does not justify lowering a score. Record what was implemented, what was tested, the operating configuration, acceptance criterion and remaining uncertainty. Better detection alone does not change the physical severity of the defined effect.
This worksheet does not implement a certified action-priority procedure, calculate failure probability or validate vessel safety. It is a small review aid, not a complete FMEA or a model of interacting failures.
Illustrative ordinal rubric
This authored teaching rubric is not an IEC, AIAG/VDA or organizational scoring table. Adjacent ranks have no promised equal spacing. Agree a project-specific procedure and evidence before rating real equipment.
| Rank | Severity · S | Occurrence · O | Detection difficulty · D |
|---|---|---|---|
| 1 | No material effect in the stated boundary | Lowest occurrence category | Strongest timely detection opportunity |
| 2 | Small local disturbance | Very low occurrence category | Very strong detection opportunity |
| 3 | Limited local degradation | Low occurrence category | Strong detection opportunity |
| 4 | Recoverable functional degradation | Lower-middle occurrence category | Generally effective detection opportunity |
| 5 | Noticeable loss of function | Middle occurrence category | Mixed detection opportunity |
| 6 | Substantial interruption | Upper-middle occurrence category | Limited detection opportunity |
| 7 | Major loss of required function | Elevated occurrence category | Weak detection opportunity |
| 8 | Serious system-level consequence | High occurrence category | Very weak detection opportunity |
| 9 | Very serious system-level consequence | Very high occurrence category | Detection unlikely before the relevant stage |
| 10 | Most severe consequence in this study | Highest occurrence category | No effective timely detection identified |
Method sources and scope
- IEC 60812:2018 · public method scope
- ASQ · Failure Mode and Effects Analysis
- Shebl, Franklin & Barber · FMEA outputs: are they valid? (2012)
Source checks: 8 October 2026. Numerical examples and rubric are authored teaching assumptions, not values prescribed by these sources.
The calculation interface could not load. The example and explanation below remain readable; reload the page to recalculate.
Read the method and its limits
- FMEA practical guide
- FMEA risk-priority numbers: equal scores and unequal consequences
- Fault tree analysis practical guide
Related project: AURA
All calculators