Calculate tightening torque and clamp load for metric bolts to ISO 898-1. Pick the size, property class (8.8, 10.9, 12.9 and others) and lubrication condition, and the calculator returns the torque, preload, proof load and a torque table across all sizes at the same settings.
What it gives you
- Tightening torque from the K-factor relationship T = K F d
- Preload as a percentage of proof load, with a warning if it exceeds proof
- Proof load, yield load and tensile stress area for the thread
- Bolt stress at the chosen preload
- Torque table for M3 to M64 at the current class and condition
Method and standards
The method is the standard lumped-friction relationship used in most bolting guidance: torque equals the friction factor K times preload times nominal diameter. K is an approximate value for the surface condition and lubricant, so for critical joints verify on the actual hardware, ideally by measuring elongation or using a calibrated method.
How to use it
- Select the bolt size and property class.
- Choose the lubrication condition to set the friction factor, or override K.
- Set the target preload as a percentage of proof load and read the torque.
Questions engineers ask
- Which K-factor should I use?
- Around 0.20 for dry plain steel, 0.15 to 0.18 for zinc plated, and 0.10 to 0.14 for lubricated or waxed threads are typical. Manufacturer data for the specific lubricant is better than any generic figure.
- Why does the calculator warn about proof load?
- If the target preload exceeds the proof load the bolt may yield during tightening. Most guidance targets 65 to 90 percent of proof load.
- Does it cover UNC and UNF bolts?
- Not yet. It covers metric coarse threads to ISO 898-1.
Need this calculation done properly, with a signed-off calculation pack or FEA behind it? Forgepoint provides mechanical design, analysis and pressure equipment engineering from Manchester, UK-wide.
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