Free tool · no sign-up · runs in your browser
Bolt Torque to Tension Calculator (T = K × F × d)
Enter the bolt diameter and your nut factor K. The tool estimates the tension a torque gives, the torque for a target tension, or the K from a torque and tension you measured.
Bolt and nut factor
Torque to tension
| Estimated tension F = T / (K × d) | – |
Tension to torque
| Estimated torque T = K × F × d | – |
Nut factor from a measured pair
If you measured the tension at a known torque (for example with a load cell under the nut), the tool backs out the K for that bolt and condition. Uses the torque and force units above and the diameter at the top.
| K = T / (F × d) | – |
Nothing you type is uploaded or saved.
The formula
T = K × F × d, with T the tightening torque, F the bolt tension (preload), d the nominal diameter and K the nut factor, a single number that lumps together thread friction, friction under the nut and thread geometry. The tool works in N, m and N·m internally; unit factors are exact (1 lbf = 4.4482216152605 N, 1 in = 0.0254 m, 1 ft·lbf = 1.3558179483314 N·m). Check: d = 12 mm, K = 0.2, F = 20 kN gives T = 0.2 × 20,000 N × 0.012 m = 48 N·m, and 48 N·m back to tension gives 20 kN. Results show tension to 0.001 kN and 1 lbf, torque to 0.1 N·m, 0.1 ft·lbf and 1 in·lbf, K to 0.001.
The tool does not choose K. It changes with lubrication, plating, thread condition and washer, so take it from the fastener or anchor manufacturer, your spec, or your own torque-tension test, and treat the result as an estimate: the same torque can give a quite different tension when friction changes. Where the tension itself matters, measure it rather than infer it from torque. This is not a design tool and it does not set an installation torque; that comes from the anchor manufacturer's instructions or your spec. To convert a torque from a data sheet, use the installation torque converter; to turn the tension into stress, the bolt tensile stress area calculator; to read a load cell in mV/V, the load cell mV/V to load calculator.
Record the results in the full report workbook
The Fastener & Anchor Pull-Test Report Templates pack has a 40-row proof-test report with automatic PASS/FAIL, ultimate-load statistics, a calibration log, a unit converter, a printable procedure checklist and a standards index. $49 one-time, from Quality Tester Solutions LLC, the company behind Force-Test pull testers.