Estimates, not a procedure. Torque sets bolt load only as accurately as K is known, and real bolts commonly scatter ±25% or more. Where a joint has an engineered torque or bolt stress, that governs. Gasketed flanges need a pattern and multiple passes. See the tightening sequence guide.
What the calculator does
It applies the short-form torque equation, T = K × D × F, the same way the bolt torque chart does:
- Tensile stress area comes from diameter and thread pitch: As = 0.7854 × (D − 0.9743 ÷ n)² for inch threads, π/4 × (d − 0.9382 P)² for metric.
- Preload is your chosen percentage of the material's reference strength times the stress area. For B7 studs and custom materials the reference is minimum yield. For SAE grades and metric property classes it is proof stress, the way those standards are written.
- Torque is K × D × preload, converted to ft-lb and N·m.
The reverse calculation rearranges the same equation to F = T ÷ (K × D). It's useful for checking what an existing procedure is actually asking of a bolt.
Picking the inputs
Target preload
Defaults are 50% of yield for B7 and 75% of proof for SAE grades and metric classes. Gasketed flanges are usually assembled somewhere between 40% and 70% of yield, chosen for the gasket and flange. ASME PCC-1 covers how to select it. Going higher buys clamp load and costs margin against yield, gasket crush and flange rotation.
Nut factor
K is the input that matters most and the one known least. The presets are typical published ranges. Your lubricant's datasheet value for steel-on-steel is better, and a value measured on your own bolts with a bolt tension calibrator is better still. See nut factor explained.
Stud size above 1 inch
Inch sizes above 1 in. default to 8 threads per inch, the standard series for flange studs. If your bolts are UNC (7 or 6 TPI at 1-1/8 to 1-1/2 in.), change the thread count and the stress area updates.
Frequently asked questions
How do you calculate bolt torque?
Use T = K × D × F: torque equals the nut factor times the nominal diameter times the target preload. For inch units, divide by 12 to get ft-lb. Preload is usually set as a percentage of the bolt's yield or proof strength times its tensile stress area.
What nut factor should I use?
Use the value published by the lubricant manufacturer for steel-on-steel threads. Without one, 0.20 is the customary assumption for clean, as-received steel threads with light oil, 0.15–0.18 for many anti-seize compounds, and 0.10–0.14 for molybdenum disulfide pastes. Measuring K on your actual bolts with a bolt tension calibrator is better than any table.
What percentage of yield should a bolt be tightened to?
Reusable structural and machinery bolts are commonly tightened to about 75% of proof load. Gasketed flange joints are usually assembled to a target bolt stress between roughly 40% and 70% of yield, chosen for the specific gasket and flange, per ASME PCC-1. Do not exceed the joint's engineered value.
Can I calculate bolt load from torque?
Yes, with the same equation rearranged: F = T ÷ (K × D). The answer is only as good as your estimate of K, so treat it as an estimate. Expect real bolts to scatter ±25% or more around it.