Unit system
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Common motor presets
Shaft torque
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How electric motor torque is calculated
Power, torque, and rotational speed are locked together by one physical identity: power = torque × angular speed. Rearranged for the units engineers actually use, you get two handy formulas that solve any one variable from the other two:
T (lb-ft) = (HP × 5252) / RPM
T (N·m) = (kW × 9550) / RPM
The constants 5252 (= 33,000 / 2π, from the definition of horsepower) and 9550 (= 60,000 / 2π) do the unit conversion for you. They are exact identities, not approximations — as long as HP/kW is the true mechanical shaft output power at that RPM, the answer is exact.
Worked examples
- Find torque: a 5 HP motor at 1750 RPM → T = (5 × 5252) / 1750 ≈ 15.0 lb-ft (20.3 N·m).
- Find power: 49.4 N·m at 1450 RPM → first convert: 49.4 N·m = 36.4 lb-ft, then HP = (36.4 × 1450) / 5252 ≈ 10.05 HP ≈ 7.5 kW.
- Find speed: a 2 HP motor delivering 3.04 lb-ft → RPM = (2 × 5252) / 3.04 ≈ 3455 RPM.
Important: this gives rated (full-load) torque — steady torque at rated power and speed. Starting (locked-rotor) torque and breakdown (pull-out) torque are higher, typically 150–300% of rated, and depend on the motor design, not this formula.
Frequently asked questions
How do you calculate electric motor torque from HP and RPM?
Use the formula T = (HP × 5252) / RPM, which gives torque in pound-feet (lb-ft). For example, a 5 HP motor at 1750 RPM produces (5 × 5252) / 1750 ≈ 15 lb-ft of torque.
What is the 5252 constant in the motor torque formula?
5252 comes from the definition of horsepower: 1 HP = 33,000 ft-lb per minute. Since power = torque × angular speed, rearranging with RPM gives torque = HP × 33,000 / (2π × RPM) = HP × 5252.11 / RPM. It is the speed at which a torque curve and a horsepower curve (in lb-ft and HP) cross on a graph.
How do you calculate motor torque from kW in metric units?
Use T = (kW × 9550) / RPM to get torque in newton-meters (N·m). The constant 9550 = 60,000 / 2π. For example, a 7.5 kW motor at 1450 RPM gives (7.5 × 9550) / 1450 ≈ 49.4 N·m.
How do you find HP from torque and RPM?
Rearrange the formula to HP = (T × RPM) / 5252, where T is torque in lb-ft. For instance, 15 lb-ft at 1750 RPM = (15 × 1750) / 5252 ≈ 5 HP.
How do you find RPM from motor horsepower and torque?
Use RPM = (HP × 5252) / T. For example, a 5 HP motor producing 15 lb-ft of torque runs at (5 × 5252) / 15 ≈ 1751 RPM.
How do you convert lb-ft of torque to N·m?
Multiply lb-ft by 1.35582 to get N·m. To go the other way, multiply N·m by 0.73756. For example, 15 lb-ft = 20.3 N·m.
How do you calculate electric motor torque from watts?
Convert watts to kilowatts first (divide by 1000), then use T = (kW × 9550) / RPM. For example, a 1500 W motor at 1450 RPM: 1.5 kW × 9550 / 1450 ≈ 9.88 N·m.
What is the difference between rated torque, starting torque, and breakdown torque?
Rated (full-load) torque is the torque at the motor's rated HP and rated RPM — what this calculator computes. Starting (locked-rotor) torque is the torque at standstill, typically 150–250% of rated for induction motors. Breakdown (pull-out) torque is the maximum torque before the motor stalls, typically 200–300% of rated.
Why does motor torque increase when RPM decreases at constant power?
Because power equals torque times speed (P = T × ω). If electrical power stays the same and speed drops — for example through a gearbox — torque must rise proportionally. A 10:1 gearbox roughly multiplies torque by 10 while dividing speed by 10.
What is a typical torque for a 1 HP electric motor?
A standard 4-pole 1 HP motor runs at about 1750 RPM, giving (1 × 5252) / 1750 ≈ 3 lb-ft (4.07 N·m). A 2-pole 1 HP motor at 3450 RPM gives about 1.52 lb-ft (2.07 N·m).
What is the difference between horsepower and torque in an electric motor?
Torque is the rotational force the motor shaft produces, measured in lb-ft or N·m. Horsepower is the rate of doing work: HP = T × RPM / 5252. Torque without speed does no work; horsepower combines both. Two motors can have the same torque but different HP if their speeds differ.
How does a VFD (variable frequency drive) affect motor torque?
A VFD keeps the volts-per-hertz ratio constant, so an induction motor delivers roughly constant rated torque from near zero up to its base speed (e.g. 50/60 Hz). Above base speed, the motor enters field weakening: torque falls while power stays roughly constant.
What is service factor and how does it affect torque calculations?
Service factor (SF), commonly 1.15, is the overload a motor can handle continuously. A 5 HP motor with SF 1.15 can safely deliver 5.75 HP, and its usable torque rises proportionally: T = (5 × 1.15 × 5252) / RPM. Apply SF only for short-term or occasional overload, not continuous sizing.
How do you measure electric motor torque in practice?
Use a dynamometer (dyno) coupled to the shaft, which measures reaction torque directly. Alternatives: a torque transducer between motor and load, or a calibrated brake arm with a scale. Electrical measurements of voltage, current, and power factor can estimate shaft torque but require efficiency data.
Is the electric motor torque formula accurate for all motors?
Yes — T = (HP × 5252) / RPM is an exact identity whenever HP is the true shaft (mechanical) output power at that RPM. The common error is using electrical input power or nameplate HP at a different operating speed; shaft torque depends on actual output power and actual shaft RPM.
Does this torque formula work for AC and DC motors?
Yes. The relationship between power, torque, and speed is physics, not motor technology — it applies equally to AC induction motors, DC motors, BLDC motors, servo motors, and single-phase motors, as long as you use mechanical shaft power.