Free engineering tool
Bolt Size Estimator (from load)
Arrive with a load, not a size: the smallest metric bolt and property class whose proof load carries your working load.
How this is calculated
Required force = working load × safety factor. Proof load of each size = tensile stress area × proof strength (ISO 898-1) for the chosen class. We recommend the smallest size whose proof load meets the required force. Reference estimate; verify critical joints against the drawing.
Required force
20 kN
Recommended size
M8
Proof load (this size)
21.2 kN
Headroom over required
+6%
Reference estimate — not a joint design
- •Proof load is the safe working limit; keep the working load below it with the safety factor.
- •A higher property class carries the same load in a smaller bolt — but needs the right material + heat treatment.
- •Shear, fatigue, prevailing torque and embedment are not covered here — confirm critical joints against the drawing.