Bolt Torque Calculator
Bolt torque calculator to estimate the tightening torque of metric and imperial bolts based on size, grade and lubrication. Includes formula, conversion (Nm, ft-lb, in-lb) and FAQ.
Torque Inputs
75% is common for structural joints.
How to Use This Calculator
Choose the system (metric or imperial), select the nominal diameter and bolt grade, pick the lubrication state, and adjust the preload target to match the joint. Click Calculate to refresh the estimated torque in Nm, ft-lb, and in-lb.
Methodology
The calculator uses the empirical T = K × D × F relation, where the nut factor (K) is tied to lubrication, D is the bolt diameter, and F is the desired clamp load derived from proof strength and preload percent. Clamp load is computed from the tensile stress area and proof strength of the selected grade.
- Because bolt grip depends on preload, you can adjust the percent-of-proof-load input to model tighter or looser joints.
- Lubricated fasteners lower the nut factor, so torque decreases for the same clamp load.
- Results are shown in Nm, ft-lb, and in-lb so you can compare with manufacturer tables in different units.
Full original guide (expanded)
The calculator and accompanying resources reproduce the bolt torque guidance previously published on CalcDomain. Use this material for quick reference and keep manufacturer data handy for mission-critical joints.
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 × D × 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.
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About this calculator
Author: CalcDomain Engineering Desk
Reviewed: 6 November 2025
Category: Math & Conversions → General