How do you calculate bolt tightening torque?

A common engineering approximation is T = K × D × F, where T is torque, K is the nut factor (typically 0.15–0.2), D is the nominal diameter of the bolt, and F is the desired clamp load.

Does lubrication change the required torque?

Yes. Lubricated fasteners require less torque for the same clamp load because friction is reduced. Typical nut factors drop from about 0.2 (dry) to about 0.15 (lubricated).

Are these torque values valid for all bolt grades?

No. Recommended torque depends on bolt material, property class or grade, diameter and thread condition. Always verify against the manufacturer's data or applicable standards for critical joints.

Bolt Torque Calculator (Metric & Imperial) | Tightening Torque for Bolts

Calculator

System

Bolt size

Grade / Property class

Select the option that best matches your fastener. For a vendor-specific bolt, use vendor data.

Lubrication / condition

Target preload (% of proof load)

75% is common for structural joints.

Results

These are engineering estimates based on standard formula. Always compare with manufacturer or standard torque charts for critical applications.

Torque (Nm)

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Torque (ft-lb)

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Torque (in-lb)

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Clamp load (kN)

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Quick reference torque chart (typical)

Values below are typical, dry, for property class 8.8 / Grade 5 style bolts. Your application may differ.

Bolt	Thread	Torque (Nm)	Torque (ft-lb)
M6 (8.8)	1.0	~9–11	~7–8
M8 (8.8)	1.25	~23–28	~17–21
M10 (8.8)	1.5	~45–55	~33–41
3/8"-16	UNC	~30–35	~22–26
1/2"-13	UNC	~90–100	~66–74

How the bolt torque is calculated

The calculator uses the classic empirical relation:

T = K \times D \times F

T = tightening torque
K = nut factor / friction factor (typically 0.20 dry, 0.15 lubricated)
D = nominal bolt diameter (m or in)
F = desired clamp load (N or lbf)

Clamp load is estimated from the bolt's tensile stress area and proof strength for the selected class/grade, then scaled by your selected preload percentage. This mirrors typical engineering bolt torque tables you find from manufacturers.

When to use a chart instead

For safety-critical, pressure-containing or warranty-relevant joints, you should use the exact torque specified by the equipment manufacturer or by the fastener vendor. The values from this calculator are to be treated as engineering estimates.

Unit conversions

We output torque in:

Newton-meters (Nm) – SI unit, common in mechanical design
Foot-pounds (ft-lb) – common in automotive and construction in the US
Inch-pounds (in-lb) – used for smaller fasteners and electronics

Conversions used:

1 ft-lb = 1.3558179483314 Nm

1 in-lb = 0.11298482902762 Nm

FAQ

Is torque the only way to control preload?

No. Torque is easy to apply but quite sensitive to friction. For critical work you may use turn-of-nut, tensioning, or direct load-indicating fasteners.

Why do my values differ from the PDF charts?

Different charts assume different friction conditions, proof loads, and safety margins. If you know which chart your organization follows, follow that chart.

Can I use this for stainless steel bolts?

Only as a rough guide. Stainless often has different strength and galling behavior, so consult the supplier.