Bolt Torque Calculator

Find the tightening torque and clamp load for a bolt from its size, grade and finish.

–

Ft-Lb.

Metric torque
Inch-pounds
Clamp load
Stress area
Proof strength
Thread

Torque Chart

Items you add are saved in this browser.

Joint	Bolt	Grade	K	Ft-Lb.	N·m	Remove

How to Calculate Bolt Torque

Tightening a bolt stretches it, and that stretch is what clamps the joint. The usual target clamp load is 75% of the bolt's proof load. The torque to reach it depends on the bolt diameter and on friction, which the torque coefficient K captures.

Torque (in-lb) = K × diameter (in) × clamp load (lb) ÷ 12 for ft-lb

Clamp load = 75% × proof strength × tensile stress area

Tensile stress area (inch) = 0.7854 × (D − 0.9743 ÷ threads per inch)² (metric) = 0.7854 × (d − 0.9382 × pitch)²

Worked Example

Proof Strength by Grade

Grade	Sizes	Proof Strength
SAE Grade 2	¼″ to ¾″	55,000 psi
SAE Grade 2	Over ¾″ to 1½″	33,000 psi
SAE Grade 5	¼″ to 1″	85,000 psi
SAE Grade 5	Over 1″ to 1½″	74,000 psi
SAE Grade 8	¼″ to 1½″	120,000 psi
ISO 8.8	M16 and smaller	580 MPa
ISO 8.8	Over M16	600 MPa
ISO 10.9	All	830 MPa
ISO 12.9	All	970 MPa

SAE J429 values as published by Portland Bolt; metric values from ISO 898-1. K values of 0.20 (plain), 0.25 (galvanized) and 0.10 (waxed) follow Portland Bolt's torque chart.

Tips

Friction is the biggest unknown: the same torque on a lubricated bolt gives far more clamp load than on a dry one, and can stretch it past yield. Always use the equipment maker's torque spec when there is one. Structural bolts on steel buildings are tightened by turn-of-nut or tension-indicating methods under the RCSC specification, not by a torque chart.