Bolt Torque Calculator

How to use this bolt torque calculator

What this calculator does

This calculator estimates the tightening torque needed to create a chosen axial clamping force in a threaded fastener. It uses the practical short-form relationship T = K × F × d × (1 – l/100), where torque depends on the nut factor, target preload, nominal bolt diameter, and an estimated lubrication reduction. It is useful for preliminary planning, checking a torque worksheet, comparing dry and lubricated assumptions, or converting one calculated torque into common metric and imperial units. It does not determine whether the bolt, nut, threads, joint material, or gasket can safely carry the selected preload. Final assembly values should come from the equipment manufacturer, an approved drawing, a qualified engineer, or validated torque – tension testing.

When to use it

• Estimate a starting torque when a target clamp load and an experimentally supported nut factor are known.
• Compare the effect of changing lubricant or coating assumptions before writing an assembly procedure.
• Convert a torque result among lbf·ft, lbf·in, and N·m while retaining the same physical preload estimate.
• Audit a hand calculation or build a documented spreadsheet for design review.

How to calculate

1. The calculator opens with a complete demonstration: a 0.75 in zinc-plated steel bolt, K = 0.20, SAE 30 oil represented by a 40% lubrication factor, and a 25,000 lbf clamping force. The example workbook is already validated and Download Excel is available immediately.
2. Choose Bolt type / nut factor. Select a preset only when it matches your fastener condition, or choose Custom and enter a tested K value.
3. Enter Bolt diameter and select inches or millimetres. Use nominal shank diameter, not the bolt-head width or wrench size.
4. Choose Lubricant. For a custom coating or lubricant, enter its estimated percentage torque reduction.
5. Enter the required Clamping force and choose lbf, N, or kN. Then select the preferred Torque output unit. Results update live.
6. Review Required torque, Dry torque, Lubrication reduction, Effective nut factor, Torque per 1,000 lbf, and the Calculation detail table. Download Excel exports the current canonical values, not rounded screen text.
7. Reset clears the demonstration and all calculated content. Download Excel then becomes unavailable until a new complete valid state is entered.

Input guide

Bolt type / nut factor is required and supplies the dimensionless K coefficient. Presets show their assumed values; Custom accepts 0.05 to 0.40, for example 0.18. A larger K produces proportionally more required torque for the same preload. Do not treat a generic preset as a material strength rating. The Bolt diameter is a required positive decimal in inches or millimetres, such as 0.75 in or 19.05 mm. Larger diameter increases calculated torque linearly. A common mistake is entering head size instead of nominal thread diameter.

Lubricant is required. Dry means 0% reduction; SAE 30 oil is represented here by 40%; moly paste by 50%; Custom accepts 0% to 80%. Higher lubrication reduction lowers the estimated torque for the same target preload. Actual friction depends on the complete fastener and joint system, so use tested data whenever the application is critical. Clamping force is required and accepts a positive decimal in lbf, N, or kN, for example 25,000 lbf. Increasing it increases torque linearly. Do not enter proof load, yield strength, or tensile strength unless your design process has explicitly converted that value into a permissible preload. Torque output unit is required and only changes presentation; lbf·ft, lbf·in, and N·m all represent the same torque.

Output guide

Required torque is the estimated wrench torque in the selected output unit. Dry torque is the result before the lubrication reduction. Lubrication reduction is the arithmetic difference between dry and adjusted torque. Effective nut factor equals K multiplied by the remaining friction fraction, so the startup example gives 0.20 × 0.60 = 0.1200. Torque per 1,000 lbf normalizes the current geometry and friction assumption for quick scaling; it is not a universal specification. The three header pills repeat torque, preload, and effective K from the same canonical model. The Calculation detail table shows SI canonical values, selected-unit values, and each quantity's role in the equation.

Worked example

For the startup values, the diameter is 0.75 in = 0.0625 ft, K is 0.20, preload is 25,000 lbf, and the lubrication factor is 40%. First calculate dry torque: 0.20 × 25,000 × 0.0625 = 312.50 lbf·ft. The remaining fraction after lubrication is 1 – 0.40 = 0.60. Therefore adjusted torque is 312.50 × 0.60 = 187.50 lbf·ft, equivalent to 2,250.00 lbf·in or about 254.22 N·m. These exact values appear on first open and in the workbook checkpoints.

Learn more

For engineering context, review NASA's preloaded joint analysis methodology, which discusses preload uncertainty and nut-factor ranges, and Fastenal's torque – tension reference guide, which emphasizes that torque is only an indirect indication of tension.