Factor of Safety Calculator

How it is calculated

The tool divides the strength you enter by the working stress: FoS = strength ÷ stress. It then reports the margin of safety as (FoS − 1) × 100 in percent, and a verdict: Safe when FoS is 1 or more, Unsafe when it is below 1. A FoS of exactly 1.00 is therefore labelled safe even though it leaves no reserve at all.

There are no unit fields. Strength and stress are divided as typed, so both must be in the same unit (for example MPa and MPa); the ratio itself has no unit. The stress must be greater than zero and the strength cannot be negative, otherwise no result is shown. When FoS is below 1 the margin is negative: a FoS of 0.40 gives -60.00%. The tool does not decide which strength to use (yield, ultimate, buckling) or what FoS your code or standard requires; the verdict only compares with 1.

Common mistakes

• Mixing units, for example strength in MPa and stress in kPa or psi. The tool divides raw numbers, so the ratio comes out wrong by the conversion factor.
• Using the part's ultimate strength where the design basis is yield strength, or the other way round. The same part then shows a very different FoS.
• Treating a Safe verdict as approval. It only means FoS is at least 1; most codes require a larger factor, and 1.00 leaves zero reserve for load spikes or defects.
• Entering the load (force) instead of stress. FoS here compares strength with stress, so convert force to stress (force ÷ area) first.
• Confusing the factor of safety (2.50) with the margin of safety (150.00%). The margin is FoS − 1 as a percentage, so a FoS of 2.50 is not a 250% margin.