# Factor of Safety Calculator

URL: https://speedycalculator.com/calculators/physics/factor-of-safety/
Last updated: 2026-10-30

> The factor of safety calculator finds the ratio between a part's strength (ultimate or yield) and the actual or design stress or load, and the margin of safety.

## Inputs

- Material strength or failure load
- Design (applied) stress or load

## Results

- Factor of safety
- Margin of safety
- Assessment

## Formula

```
FoS = strength / design stress,  margin of safety = FoS − 1
```

## Explanation

Engineers never design right to the limit. The factor of safety covers uncertain loads, material variability and wear: aircraft use about 1.5, buildings 2–3, lifting equipment 5 or more.

### How to use the Factor of Safety Calculator

- **Material strength or failure load**
- **Design (applied) stress or load**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Enter the failure (yield or ultimate) strength or load and the applied design stress or load in the same units.

### Inputs explained

FieldWhat to enterMaterial strength or failure loadA numberDesign (applied) stress or loadA number

### What each result means

- **Factor of safety** = [Material strength or failure load]÷[Design (applied) stress or load]
- **Margin of safety** = [Material strength or failure load]÷[Design (applied) stress or load] - 1

### Example values

How the main result changes when **Material strength or failure load** changes and everything else stays at the example values above:

Material strength or failure loadFactor of safety1251.25187.51.8752502.5312.53.1253753.755005

## Worked example

Inputs — Material strength or failure load: 250; Design (applied) stress or load: 100.

Results — **Factor of safety = 2.5**; Margin of safety = 1.5; Assessment = Adequate for most uses.

## Key terms

- **Factor of safety**: Ratio of capacity to demand.

## FAQ

**What is a typical factor of safety?**
1.5–2 for aerospace and well-known loads; 3–5+ for uncertain or life-critical loads.

**Yield or ultimate strength?**
Yield for ductile materials (no permanent deformation), ultimate for brittle ones.

**Is a higher factor always better?**
It adds weight and cost, so it is balanced against risk.

**What is the margin of safety?**
FoS − 1 – the extra capacity beyond the load.


## Sources

- [MIT OpenCourseWare – Mechanics of Materials](https://ocw.mit.edu/courses/2-001-mechanics-materials-i-fall-2006/)
- [The Engineering ToolBox](https://www.engineeringtoolbox.com/)