Factor of Safety Calculator
The factor of safety of a component — its strength divided by the stress it will actually carry — from a material strength and a working load on a cross-section, with the load at which it would fail, the allowable stress for a required factor, and the section that would achieve it.
Factor of safety is strength over stress: how many times the working load the part could carry before it yields or breaks.
How the factor of safety calculator works
Factor of safety is strength over stress: how many times the working load the part could carry before it yields or breaks. Design codes require a minimum — 1.5 to 2 for well-known static loads, 3 to 4 for uncertain or dynamic ones, higher where failure kills — and the required factor turns a material's strength into an allowable stress. Working the other way, it tells you the load that would fail the part and the section area that would meet the required factor.
Formula: FoS = strength / working stress; working stress = load / area; allowable = strength / required FoS
Worked examples
| Inputs | Factor of safety | Note |
|---|---|---|
| 40 kN on 600 mm² of mild steel | 3.75 | FoS 3.75 |
| A notch | 1.5 | the factor drops to 1.5 |
| A higher requirement | 3.75 | a larger section needed |
FAQFrequently asked questions
What is a factor of safety?
The ratio of what a part can withstand to what it is asked to carry. A factor of 2 means the working load could double before failure — the margin for the things the calculation did not know.
What factor is required?
Depends on the consequences and the uncertainty: 1.5 to 2 for well-characterized static loads on ductile materials, 3 to 4 for brittle materials, dynamic loads or uncertain data, 5 to 10 for lifting gear and pressure vessels. Codes specify it.
Yield or ultimate strength?
Yield for ductile metals where permanent deformation is the failure; ultimate for brittle materials, or where the code says so. Use the same basis the required factor was written for.
What is a stress concentration factor?
The multiplier a hole, notch or sharp corner puts on the local stress — 2 to 3 is common. It cuts the real factor of safety at that spot, which is where fatigue cracks start.
Is a higher factor always better?
It is safer but heavier and dearer. The reading row flags heavily over-designed parts; aerospace lives near 1.5 with very good data, civil structures above 2 with less.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Institute of Standards and Technology — the US measurement authority
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.