Bolt Torque Calculator
Torque from force and wrench length, in ft-lb and N-m with exact factors.
What it does
Torque is the twisting effort that tightens a bolt. Pull on a wrench and two things set the result: how hard you pull and how far from the bolt center you pull. Fifty pounds on a one foot wrench gives 50 ft-lb, and the same pull on a two foot wrench gives 100.
The tool above covers both unit systems. Imperial mode takes pounds and inches and prints ft-lb, in-lb, and N-m. Metric mode takes newtons and centimeters and prints N-m, ft-lb, and N-cm. Both modes assume the pull lands at a right angle to the handle.
The conversion factor is exact, not approximate. NIST lists the pound-force foot as 1.355818 N-m, which follows from the exact definitions of the pound-force and the foot. Georgia State HyperPhysics states the base rule the same way: torque equals force applied times lever arm.
Garage work runs on these numbers. Lug nuts, head bolts, and spark plugs each carry a spec, and a click wrench set ten units off can strip threads or leave a joint loose. Always confirm whether the spec reads ft-lb or in-lb before pulling.
How to use
- Pick imperial (lbf, in) or metric (N, cm).
- Enter the pull force at the handle.
- Enter the wrench length from the bolt center to the pull point.
- Pull at a right angle. For angled pulls, multiply by sin of the angle.
- Read torque first, then the converted unit and the fine unit (in-lb or N-cm).
- Compare against the fastener spec in the same unit before tightening.
Formula
- F
- Pull force: lbf in imperial mode, N in metric mode.
- r
- Wrench length from bolt center: inches or cm.
- 1.355818
- Exact NIST factor between ft-lb and N-m.
The 1.355818 factor follows from exact definitions: 1 lbf is 4.4482216152605 N and 1 ft is 0.3048 m. Metric defaults mirror the imperial ones (222.4 N on 30.5 cm gives 67.83 N-m).
Worked example
Example: 50 lbf on a 12 in wrench
Imperial defaults: 50 lbf of force on a 12 inch wrench, pulled at a right angle.
- Inputs: 50 lbf on a 12 in wrench, perpendicular pull.
- Torque in imperial: 50 x 12 = 600 in-lb = 50 ft-lb.
- Convert force: 50 x 4.4482216152605 = 222.41 N.
- Convert arm: 12 x 0.0254 = 0.3048 m.
- SI check: 222.41 x 0.3048 = 67.79 N-m. Tool shows 50 ft-lb, 67.79 N-m, 600 in-lb.
Reference table
| N-m | ft-lb | Everyday check |
|---|---|---|
| 10 | 7.38 | Small fasteners |
| 30 | 22.13 | Lug nut range starts here |
| 67.79 | 50 | The worked example |
| 135.58 | 100 | Head bolt territory |
| 200 | 147.51 | Heavy truck specs |
Limits
- Right angle pulls only. Angled pulls need the sin correction.
- Static tightening only. Impact guns and moving loads differ.
- Ignores friction in the threads, which eats part of the torque.
- Ideal lever: flex in long extensions is outside the math.
Common mistakes
- Mixing ft-lb and in-lb specs, a factor of 12 error.
- Pulling at an angle while using the plain formula.
- Reading the wrench scale in the wrong unit.
- Adding extensions without correcting the length.
- Jerking the wrench instead of pulling smoothly to the click.
Try it yourself
Quick checks. Answers sit tucked under each question.
200 N on a 15 cm wrench?
200 x 0.15 = 30 N-m. 30 / 1.355818 = 22.13 ft-lb.
25 lbf on an 8 in wrench?
25 x 8 = 200 in-lb = 16.67 ft-lb, which is 22.6 N-m.
Questions
What is torque?
Twisting effort: force times the perpendicular distance from the bolt center. A 50 lbf pull on a 1 ft wrench gives 50 ft-lb.
How do ft-lb and in-lb relate?
Multiply by 12 for in-lb, divide by 12 for ft-lb. The tool prints both, plus N-m.
How do I convert ft-lb to N-m?
Multiply ft-lb by 1.355818 to get N-m. So 50 ft-lb is 67.79 N-m.
Does the formula need a right angle?
Only at 90 degrees. At any other angle multiply force times arm times sin of the angle, which shrinks the result.
Why do long wrenches help?
Double the arm doubles the torque for the same pull. That is why long wrenches break loose tight bolts with less effort.
How do I use a click torque wrench?
Set the wrench to the spec in the correct unit and pull smoothly to the click. Confirm whether the spec reads ft-lb or in-lb first.