Bolt Torque Calculator

Bolt Torque Calculator for Clamping Force and Tightening Specs

The Bolt Torque Calculator converts between tightening torque and clamping force for threaded fasteners. Select bolt material, diameter, and lubrication, then solve in either direction. Proper torque prevents loose joints and avoids bolt stretch or breakage.

Bolt tightening torque formula

$$T = K \times F \times d \times \left(1 - \frac{l}{100}\right)$$

Where $T$ is torque, $K$ is the material constant, $F$ is clamping force, $d$ is bolt diameter, and $l$ is lubrication reduction percent.

Typical K values

Mild steel dry bolts often use $K \approx 0.20$. Zinc-plated steel is similar. Black finish and cadmium-plated finishes use different constants. Lubricants such as SAE 30 oil reduce required torque by lowering friction.

Related tools: Torque Calculator and Bolt Circle Calculator.

Frequently Asked Questions

What happens if I under-torque a bolt?

Under-torqued bolts may loosen from vibration and fail to maintain clamping force. Critical joints can separate over time.

What happens if I over-torque a bolt?

Over-torquing can stretch the bolt past its elastic limit, reduce clamping force, or snap the fastener during assembly or service.

Why does lubrication change torque?

Lubrication lowers thread friction, so less wrench torque is needed to reach the same clamping force. Dry and lubricated joints need different torque values.

Are these results exact for all bolt grades?

This tool uses standard approximation constants valid for common bolt sizes. Always follow manufacturer specs and torque charts for critical applications.

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