Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of factor of safety. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
The factor of safety absorbs everything the calculation does not model: load uncertainty, material variation, stress concentrations, wear, and the consequence of failure. Its magnitude is a judgement about those, not a number to look up.
Where loads are well known and consequences are minor, factors near 1.5 are common; where either is uncertain, 3 or more is normal. Applying a large factor to a stiffness-limited problem achieves nothing, because stiffness does not fail suddenly.
Common misuse
Using a large factor of safety as a substitute for knowing the load. It buys strength, not understanding, and it does nothing for a deflection limit.