Factor of Safety Calculator

About this calculator

Factor of safety (FoS) compares a material's known strength to the stress it actually experiences in service: FoS = material strength ÷ applied working stress. A value above 1 means the design has margin before failure, and this calculator also reports the margin of safety (FoS − 1, expressed as a percentage) along with a plain safe/unsafe verdict based on whether FoS clears 1.

Mechanical and structural engineers use factor of safety throughout design to account for uncertainties that pure stress calculations can't capture — variability in material quality, unexpected load spikes, fatigue over time, and manufacturing defects. Different applications target very different FoS values by code or convention: structural steel buildings often design to roughly 1.5-2, while pressure vessels, cranes, and lifting equipment — where failure risk to life is higher — commonly require 3 to 10 or more.