Torque Calculator
One tool for the way torque actually comes up: from a force on a lever, from a motor’s power and RPM, when you add an extension to a torque wrench, around a pulley, as stress in a shaft, or just converting between units. Pick a mode and get the answer in lb-in, lb-ft, N·m, and kgf·m at once.
Uses T = 5252 × HP / RPM (or 9550 × kW / RPM). Great for motors, gearboxes, pumps, and fans.
For an in-line extension. The wrench is set lower than the target so the fastener still sees the desired torque: setting = target × L / (L + E).
Torque = net belt tension × pulley radius (diameter / 2).
Solid round shaft, shear stress = 16 T / (pi × d^3). Safety factor compares the shear yield (about 0.577 × yield) to the working shear stress.
Enter any torque and read it in all four common units below.
Torque Calculator: force, power, wrench, pulley, and shaft in one place
Torque is the twist a force makes around a point of rotation. The same word covers a wrench tightening a bolt, a motor spinning a pump, a belt turning a pulley, and the stress winding up inside a drive shaft. This calculator keeps all of those in one place and always reports the answer in lb-in, lb-ft, N·m, and kgf·m, because the unit you need is rarely the unit you start with.
What is torque?
Torque is rotational force, the twisting effort that turns something around an axis or pivot. Where a straight push is measured in pounds or newtons, a twist is measured in those force units multiplied by the distance from the center of rotation, which is why torque is quoted in pound-feet, pound-inches, newton-meters, and kilogram-force meters rather than in plain force. It is the quantity that loosens a stubborn bolt, spins a drill bit, drives a car’s wheels, and winds up a shaft.
The core idea is simple: torque equals the applied force times the length of the lever arm, as long as the force acts at right angles to the arm. A small force on a long lever makes the same torque as a large force on a short one, which is the principle behind breaker bars, long-handled wrenches, and gear reductions. Only the perpendicular part of a force produces torque, so pulling straight along a wrench handle does nothing useful, while pulling square to it delivers the full effect.
Torque is not the same as power. Torque is the turning effort available at any instant, while power also accounts for how fast that effort is delivered, which is where RPM enters. An engine can make high torque at low speed or modest torque at high speed and still produce the same power, so sizing a motor, choosing a gear ratio, or picking a drive pulley means thinking about both at once.
The common units convert directly: 1 lb-ft equals 12 lb-in, about 1.356 N·m, and about 0.138 kgf·m. Because shops, spec sheets, and standards mix all four freely, this calculator always shows the result in every unit at once so you never have to stop and convert by hand.
Torque from a force on a lever
The most basic case: a force F applied at a distance r from the pivot produces a torque equal to force times lever arm. Push with 50 lb at the end of a 12 in wrench and you generate 600 lb-in, which is the same as 50 lb-ft. Only the part of the force acting at right angles to the lever counts, so keep the push perpendicular for the full effect.
Torque from power and RPM
Rotating equipment is rated in power, but what actually turns the load is torque. For a given power, torque rises as speed drops, which is why gearboxes trade RPM for twist. Use the horsepower form for imperial work and the kilowatt form for metric.
The same relationship solved for power is the horsepower formula engineers reach for constantly:
This is the everyday formula for sizing motors, gearboxes, pumps, and fans.
Torque wrench with an extension
Add an in-line extension to a torque wrench and the fastener sees more torque than the dial shows, because the extension lengthens the lever. To land on the torque you actually want, set the wrench lower using the ratio of the wrench length to the combined length.
Here L is the wrench length from the drive to your hand and E is the extension length. This applies to a straight in-line extension; an extension held at an angle changes the math.
Pulley and belt torque
A belt pulling on a pulley creates torque equal to the net belt tension times the pulley radius. Doubling the pulley diameter doubles the torque for the same tension, which is the whole idea behind sizing drive pulleys.
Shaft torsion and safety factor
This is where torque becomes a stress problem. A solid round shaft carrying a torque develops a shear stress that peaks at its surface. Slimmer shafts build stress fast, because the diameter enters to the third power.
To judge whether the shaft is safe, the calculator compares the working shear stress to the material shear yield, taken as about 0.577 of the tensile yield strength. A safety factor of two or more is a comfortable starting point for steady loads; shock or reversing loads call for more.
Torque units at a glance
| 1 of | lb-ft | lb-in | N·m | kgf·m |
|---|---|---|---|---|
| lb-ft | 1 | 12 | 1.3558 | 0.13826 |
| lb-in | 0.08333 | 1 | 0.11298 | 0.011521 |
| N·m | 0.73756 | 8.8507 | 1 | 0.10197 |
| kgf·m | 7.2330 | 86.796 | 9.80665 | 1 |
Common torque values
Handy real-world references. Always follow the manufacturer specification for your exact part; these are ballpark ranges to sanity-check a result, not torque specs.
| Application | Typical torque |
|---|---|
| Automotive lug nuts | 80 to 150 lb-ft |
| Spark plugs | 10 to 25 lb-ft |
| Bicycle crank bolts | 25 to 55 N·m |
| Small electric motors | Varies widely with size and speed |
Frequently asked questions
What is the difference between torque and power? Torque is the turning effort at an instant; power is how fast that effort does work, which folds in speed. A small motor at high RPM and a large motor at low RPM can make the same power while producing very different torque.
Why do I have to change my torque wrench setting when I use an extension? An in-line extension lengthens the lever, so the bolt feels more than the dial reads. Setting the wrench lower with setting = target × L / (L + E) cancels the extra leverage.
How do I get more torque? Apply more force, use a longer lever or a larger pulley, or gear down to trade speed for twist. In a shaft, a larger diameter cuts stress sharply.
Is more torque always better? No. Over-torquing a fastener can strip threads or snap the bolt, and oversizing a shaft wastes weight and cost. The goal is the right torque, not the most.
Related engineering tools: the Beam Load Calculator, the Beam Deflection Calculator, and the full Engineering Calculators hub. More torque-related tools such as gear ratio, horsepower, and pulley ratio are on the way.
This calculator is for preliminary estimating and education only. Shaft, fastener, and drive designs should be verified by a qualified engineer against the actual materials, loads, and duty cycle for your application.