Torque Calculator: Force, Lever Arm or Engine Power
How do you calculate torque? Enter a force and lever arm length for a bolt or wheel, or switch to engine power and rpm for crankshaft torque. Both answers are shown in N·m and lb-ft. This page finds torque; to turn torque and rpm into horsepower, use the horsepower calculator.
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Formula
- From power:
T (N·m) = P (W) / (2 x pi x rpm / 60); in lb-ft this ishp x 5252 / rpm. - From force:
T = F x r, withrmeasured perpendicular to the force. 1 lb-ft = 1.3558179 N·m,1 hp = 745.69987 W,1 PS = 735.49875 W,1 lbf = 4.4482216 N.
Worked example
300 hp at 5000 rpm: power = 223,710 W, angular speed = 523.6 rad/s, so torque = 223,710 / 523.6 = 427.25 N·m = 315.13 lb-ft.
Force example: 100 N on a 0.3 m lever gives 30 N·m = 22.13 lb-ft. 50 lbf on a 2 ft bar gives 100 lb-ft = 135.58 N·m.
Torque vs horsepower
Torque is a twisting force and is measured at one instant; horsepower is the rate of doing work, torque multiplied by rotational speed. Two engines with the same peak torque can have very different power if one makes it at higher rpm. Use this page for "how much twist": wrench and lug nut work, or the torque behind a power figure. Use the horsepower calculator when you have torque and rpm and want power.
Use this when
You are setting a torque wrench, working out how long a breaker bar must be, or turning a quoted hp and rpm into N·m or lb-ft.
Typical values
Typical car wheel nuts are tightened to roughly 80 to 140 N·m, but always follow the vehicle maker's figure. Road-car engines typically make roughly 150 to 500 N·m at their peak.
Common mistakes
- Measuring the lever arm along the bar at an angle instead of perpendicular to the force.
- Mixing lb-ft with lbf and ft-lb conventions, or N·m with N·mm.
- Using peak power with the wrong rpm: torque depends on the rpm you pair it with.
- Treating the result as a spec for safety-critical fasteners instead of reading the manual.
Frequently asked questions
How do I calculate torque from horsepower and rpm?
Torque in lb-ft = hp x 5252 / rpm. For 300 hp at 5000 rpm that is 300 x 5252 / 5000 = 315.1 lb-ft, which equals 427.25 N·m.
How do I calculate torque from force and distance?
Multiply the force by the length of the lever arm, measured at right angles to the force. 100 N on a 0.3 m wrench gives 100 x 0.3 = 30 N·m, or 22.13 lb-ft.
How do I convert lb-ft to N·m?
Multiply lb-ft by 1.3558179. For example 150 lb-ft is 150 x 1.3558179 = 203.37 N·m. To go the other way, divide N·m by 1.3558179.
What is the difference between torque and power?
Torque is the twisting force on a shaft; power is the rate of doing work, which is torque times rotational speed. The same torque at twice the rpm means twice the power. The pull on a bolt wrench is a static torque, not power.
How do I calculate torque from kW and rpm?
Torque in N·m = power in watts / (2π × rpm / 60). An engine making 150 kW at 5500 rpm produces 260.44 N·m, which is 192.09 lb-ft. A quick check is 9549 × kW / rpm.
How much force do I need for 100 lb-ft with a breaker bar?
Divide the torque by the arm length. On a 2 ft bar you need 50 lbf to make 100 lb-ft, which is also 135.58 N·m. A longer bar needs less force for the same torque.
How much torque does a 120 hp engine make at 3000 rpm?
Using lb-ft = hp × 5252 / rpm, 120 hp at 3000 rpm is 210.08 lb-ft, or 284.84 N·m. The same hp at a lower rpm means more torque, which is why diesels feel strong at low revs.