Clamp load calculator (torque to bolt preload)
Enter the torque, the nominal bolt diameter and the thread condition to estimate the clamp load. 75 ft-lb on a 1/2 in bolt with plain dry threads (K 0.20) gives about 9,000 lbf, or 40.0 kN, with roughly ±25% scatter from friction.
Contents6 sections
Key takeaways
- Clamp load F = T ÷ (K × D): the torque equation run backwards 1.
- The nut factor dominates. The same 75 ft-lb gives 9,000 lbf dry (K 0.20) and 12,000 lbf oiled (K 0.15) on a 1/2 in bolt.
- Treat the answer as a band, not a number: about ±25% with a torque wrench 2, 35% for dry or as-received parts in NASA's design default 3.
- Check the result against the bolt's proof load. If it is close, the bolt has no margin for the scatter.
Quick facts
| Formula | F = T ÷ (K × D) |
|---|---|
| D | Nominal diameter, not minor or pitch diameter |
| Default | 75 ft-lb, 1/2 in, K 0.20 → 9,000 lbf |
| Scatter | About ±25% 2 |
How it works
T = K × D × F rearranges to F = T ÷ (K × D) 1. The calculator converts the torque to N·m and the diameter to meters, divides, and shows the result in lbf (rounded to the nearest 10) and kN.
- T is the torque actually applied, the final reading on the wrench.
- D is the nominal diameter: 0.5 in for a 1/2-13 bolt, 12 mm for M12. Fastenal’s equation uses nominal bolt diameter 1.
- K is the nut factor, picked from the same five conditions as the other calculators: 0.20 plain dry, 0.18 zinc, 0.15 oiled 1, 0.16 anti-seize 4, 0.11 moly 5.
Thread pitch and grade don’t appear. Grade sets how much clamp load the bolt can take, not how much a given torque produces.
Worked example
Fastenal’s guide uses a 1/2-13 zinc-plated Grade 5 cap screw at 905 in-lb (75 ft-lb) with K 0.20 1. Run it backwards with the rounded 75 ft-lb (900 in-lb):
F = 900 in-lb ÷ (0.20 × 0.5 in) = 9,000 lbf (40.0 kN)
| Thread condition | K | Clamp load | Share of Grade 5 proof load (12,070 lbf) |
|---|---|---|---|
| Plain dry | 0.20 | 9,000 lbf | 75% |
| Zinc-plated dry | 0.18 | 10,000 lbf | 83% |
| Copper anti-seize | 0.16 | 11,250 lbf | 93% |
| Lubricated | 0.15 | 12,000 lbf | 99% |
| Moly paste | 0.11 | 16,360 lbf | 136% |
Scroll the table sideways to see every column.
The proof load column uses 85,000 psi × 0.142 in² for 1/2-13 Grade 5 1. The torque that is right for a dry bolt brings an oiled one to its proof load, and a moly-coated one well past it. Fastenal makes the same point with an over-lubricated bolt at K 0.10, which reaches 18,100 lb at 905 in-lb, more than a 1/2-13 Grade 5 bolt’s tensile strength 1.
A metric case: 100 N·m on an M12 bolt, plain dry, gives 100 ÷ (0.20 × 0.012 m) = 41.7 kN (9,370 lbf). That is 85% of an M12 x 1.75 class 8.8 bolt’s 48.9 kN proof load, or 60% of a class 10.9’s.
Reading the result
The number is an estimate of the average preload, not a measurement. Most of the torque you apply goes into friction: about 90% according to Fastenal 1 and usually 85 to 95% according to Bolt Science 6. Small changes in friction therefore move the clamp load a lot. The IFI figures reproduced by NASA put torque-wrench control at ±25%, against ±15% for turn-of-nut and ±3 to 5% for measuring bolt elongation 2.
So read 9,000 lbf as roughly 6,750 to 11,250 lbf. If the top of that band is above the bolt’s proof load, lower the torque or control the friction. For a joint that needs a known preload, measure it: NASA-STD-5020B requires the torque-preload relationship to be established by test on the actual hardware for spaceflight joints 3, and RCSC requires pre-installation verification in a bolt tension measuring device for structural bolts 7.
Common mistakes
- Using the minor or pitch diameter for D. The short-form equation is built on nominal diameter. Using a smaller D inflates the clamp load.
- Including running torque. On a prevailing-torque (locking) nut, part of the reading turns the nut against its locking element. NASA notes that running torque does not generate preload 3. Subtract the measured running torque before you calculate.
- Treating K as the friction coefficient. K combines thread geometry, thread friction and underhead friction 1. A friction coefficient of 0.15 on threads and underhead corresponds to a K of about 0.19 2.
- Applying it to angle-tightened bolts. After a torque-plus-angle step, the final torque no longer predicts preload. Use the maker’s procedure.
- Forgetting relaxation. Clamp load drops after tightening as surfaces settle and as neighboring bolts are tightened 1. The calculator gives the value right after tightening.
The opposite direction, from a target clamp load to a torque, is the bolt torque calculator. Why the two never line up exactly is covered in torque vs tension.
Sources
- 1
- 2
- 3
- 4
- 5Henkel Corporation (LOCTITE), LOCTITE LB 8012 Moly Paste TDS, April 2015Checked 21 Oct 2026
- 6Bolt Science Ltd, Bolt Science, Methods of Tightening Threaded FastenersChecked 21 Oct 2026
- 7
Spotted a value that doesn’t match its source? Report an error
Related
- Torque vs tensionTorque only estimates bolt tension: about 90% goes into friction, so preload scatters ±25% or more. The figures from NASA-STD-5020B, NASA RP-1228, Fastenal and RCSC.
- Bolt torque calculatorBolt torque calculator for SAE and metric bolts by size, thread, grade and lubrication, using T = K × D × F with the proof loads from SAE J429 and ISO 898-1.
- Dry vs lubricated torqueWhy oiled bolts need less torque than dry ones for the same clamp load, with nut factor K values from Fastenal, NASA and Henkel for oil, zinc, anti-seize and moly.
- 1/2 in1/2-13 Grade 5 bolt torque is 75 ft-lb dry and Grade 8 is 107 ft-lb. Charts for 1/2-13 and 1/2-20 in Grade 2, 5 and 8, dry, zinc-plated and oiled.