Bolt Torque Calculator

Formula

• Tightening torque T = K × F × d
• T in N·m, F in N, d in metres (d_mm ÷ 1000), K is dimensionless

About this calculator

Part of our Mechanical Calculators collection. Beams, bolts, springs, gears, bearings, and core ME calculations.

How it works

Tightening torque follows T equals K times F times d. K comes from the lubrication preset or your custom value. Typical K: 0.20 dry, 0.15 oiled, 0.10 moly grease. Confirm final torque with the fastener supplier chart for critical joints.

Enter your values in the inputs above and click CALCULATE. Results appear on the right without a page reload. No login and no server upload.

Worked example

M12 bolt, lightly oiled (K = 0.15), target clamp load 25 kN.

1. F = 25 kN = 25,000 N
2. d = 12 mm = 0.012 m
3. T = 0.15 × 25,000 × 0.012

Result: Tightening torque = 45 N·m (33.2 ft·lb).

When to use

• Assembly planning and torque wrench setup
• Comparing lubrication conditions before final spec
• Quick screening when supplier charts are unavailable

Limitations

• K-factor method has ±25% scatter in real joints
• Not a substitute for VDI 2230 on critical joints
• Does not check thread stripping or gasket relaxation

FAQ

What is the K factor?

K is an empirical nut factor that lumps thread friction and bearing friction into one number. Dry steel is about 0.20; lightly oiled is about 0.15.

Why do I get different torque in ft·lb?

The same torque in N·m converts to ft·lb using 1 N·m ≈ 0.738 ft·lb. Both values describe the same tightening force.

When should I not use this calculator?

Safety-critical joints, dynamic loading, and code-controlled flanges need detailed analysis or the fastener manufacturer's chart.