About this tool
The Factor of Safety Calculator tells you how much stronger a part is than the load it carries. Enter the material's yield and/or ultimate strength and the applied stress, and it returns the yield-based and ultimate-based factors of safety, the corresponding margins of safety, and a pass/fail check against a target factor. All arithmetic runs locally in your browser and nothing you type leaves your device.
Factor of safety is simply strength divided by applied stress: n_yield = σ_yield / σ_applied and n_ultimate = σ_ultimate / σ_applied. The margin of safety is the factor of safety minus one, expressed as a percentage — a margin of 0% means the part is exactly at its limit. When a part sees both direct (normal) stress and shear at once, the tool combines them into the von Mises equivalent stress, σ_vm = √(σ² + 3τ²), and divides the strength by that equivalent value, which is the standard ductile-material yield criterion. It also reports the allowable stress at your target factor of safety, strength ÷ target, so you can see how much headroom you have.
As a worked example, a steel with a 250 MPa yield strength under 100 MPa of applied normal stress has a factor of safety of 2.5 and a margin of safety of 150%; if it also carried 40 MPa of shear, the von Mises stress would rise to about 122 MPa and the yield factor of safety would fall to about 2.05. Typical design factors run from about 1.5 for well-known static loads up to 4 or more for uncertain loads, brittle materials or safety-critical parts.