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
| Field | What to enter |
|---|---|
| Material strength or failure load | A number |
| Design (applied) stress or load | A 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 load | Factor of safety |
|---|---|
| 125 | 1.25 |
| 187.5 | 1.875 |
| 250 | 2.5 |
| 312.5 | 3.125 |
| 375 | 3.75 |
| 500 | 5 |
Formula
FoS = strength / design stress, margin of safety = FoS − 1
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.
Frequently asked questions
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.