The factor of safety (FoS) is how many times stronger a part is than it strictly needs to be: FoS = strength ÷ working stress. A value of 4 means the strength is four times the applied stress. This calculator gives the FoS, the margin of safety (FoS − 1) and a quick design rating.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: the FoS & margin-of-safety definitions, recomputed in code.
The definition
Use yield strength if permanent deformation is the failure mode, or ultimate strength if fracture is. The factor must exceed 1 to be safe; how far above depends on the application. Ductile parts under static, well-known loads often use 1.5–4; brittle materials, fatigue, impact and uncertain loads call for more; weight-critical aerospace parts run lower because loads are tightly controlled.
Worked examples
400 MPa strength, 100 MPa stress:
250 MPa yield, 125 MPa stress:
500 MPa, 600 MPa stress:
To find the allowable working stress for a target factor, divide the strength by the desired FoS — for example 400 MPa with a target FoS of 2 gives a 200 MPa allowable. The 60,000 psi over 20,000 psi case gives exactly 3, showing the calculation works in any consistent units.
Frequently Asked Questions
FoS = strength ÷ stress. 400 MPa / 100 MPa = 4.
1.5–4 for ductile static loads; higher for brittle, fatigue or uncertainty.
Stress exceeds strength — overstressed and unsafe. Redesign.
FoS − 1, the spare capacity. FoS 2 = 100% margin.
Yield if no permanent set allowed; ultimate if fracture is the limit. Keep units consistent.