How to use the Factor of Safety Calculator
- 1. Choose the taskFind FOS, find allowable stress, or find the rod diameter.
- 2. Enter the strengthYield stress for ductile metals, ultimate stress for brittle ones.
- 3. See the resultFOS, margin of safety, allowable stress or required area and diameter.
What factor of safety means
Factor of safety = failure stress / working stress. For ductile materials like mild steel, failure is taken at yield. For brittle materials like cast iron, use the ultimate strength. The allowable or permissible stress is the failure stress divided by the FOS, and the margin of safety = FOS − 1.
The FOS covers uncertainty in loads, material properties, workmanship and analysis. Shock, fatigue and brittle materials need higher values.
Worked example and exam tips
A steel rod with yield stress 250 MPa must carry 60 kN with FOS 1.5. Allowable stress = 250 / 1.5 = 166.7 MPa. Area = 60,000 / 166.7 = 360 mm². Diameter = √(4 × 360 / π) = 21.4 mm, so choose the next standard size, 22 or 25 mm.
- Always state which strength (yield or ultimate) you used.
- Round sizes up, never down.
- Long compression members can buckle well below this stress, so check them separately.
Frequently asked questions
What is a typical factor of safety?
It depends on the code, material and loading. Take the value from your design code or the question; the tool does not assume one.
Is FOS the same as load factor?
Not exactly. FOS divides the material strength (working stress method). Limit state design multiplies loads by partial safety factors and also divides the material strength.
Can FOS be less than 1?
Mathematically yes, and it means the stress is above the failure stress, so the part fails.
Is this calculator free?
Yes. You enter your name and WhatsApp number once to see your result, and all other Uncodemy tools then open without the form. Everything is calculated in your browser.