Electric Motor Torque Calculator (from voltage, current, efficiency and rpm)

Understanding the electric motor torque calculator — concept and principle

This calculator estimates the shaft torque of an electric motor from quantities you can measure at its terminals: voltage, current, power factor, efficiency and speed. It works for DC, single-phase and three-phase AC motors and is useful when checking whether a motor can drive a pump, fan, conveyor or gearbox.

8.83 kW in, 7.95 kW on the shaft at 1,460 rpm gives 52 N·m.

How it is calculated

Electrical input: P_in = V I (DC), V I cos φ (single-phase), √3 V_L I cos φ (three-phase)

Shaft power: P_out = η × P_in

Torque: T = P_out ÷ ω, with ω = 2π n ÷ 60

V in volts (line-to-line for three-phase), I in amperes, cos φ the power factor, η the efficiency, n the speed in rpm.
1 lbf·ft = 1.35582 N·m; 1 hp = 745.70 W.

The principle

Power is the rate of doing work. In rotation, power equals torque times angular speed, just as in a straight line power equals force times velocity. So once you know how much mechanical power reaches the shaft and how fast it turns, the torque follows. The electrical side gives the input power: in AC circuits only the in-phase part of the current does work, which is why the power factor appears, and a balanced three-phase supply delivers √3 times the line voltage times the current. The efficiency removes copper, iron, friction and fan losses.

Worked example

Default inputs: three-phase, 400 V, 15 A, power factor 0.85, efficiency 90%, 1,460 rpm.

P_in = √3 × 400 × 15 × 0.85 = 8,833 W = 8.833 kW.
P_out = 0.90 × 8,833 = 7,950 W = 7.95 kW, or 10.66 hp. Losses = 0.883 kW.
ω = 2π × 1,460 ÷ 60 = 152.9 rad/s.
T = 7,950 ÷ 152.9 = 52 N·m, which is 38.35 lbf·ft.

Each point is the calculator re-run with only “Shaft speed (rpm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Shaft speed (rpm)	Shaft torque (N·m)
365	207.99
730	104
1,095	69.33
1,460	52
1,825	41.6
2,190	34.67
2,920	26

How to read the result

Compare the torque with what the load needs at that speed. For the same power, a slower motor gives more torque, which is why gearboxes trade speed for torque. A four-pole motor on 50 Hz runs a little under 1,500 rpm, so 1,460 rpm with 7.95 kW is typical of a motor rated around 7.5 kW. Remember that starting torque and breakdown torque differ from the running value.

Common mistakes

Using phase voltage instead of line-to-line voltage for three-phase.
Leaving out the power factor for AC motors.
Using full-load nameplate efficiency and power factor at light load, where both are lower.
Mixing up rpm and rad/s.

Keep in mind: This is an estimate from electrical measurements. Real motor selection should follow manufacturer data, applicable electrical codes and advice from a qualified engineer.

Frequently asked questions

How do I get torque from motor kW and rpm?

Torque in N·m ≈ 9,549 × kW ÷ rpm.

Why does the same power give more torque at low speed?

Power is torque times speed, so at lower speed more torque is needed to carry the same power.

Does the calculator give starting torque?

No. Starting torque depends on motor design and is listed on the data sheet.

Sources: Standard formulas P = VI cos φ, P = √3 V_L I cos φ and P = Tω

Checked 9 Nov 2026. Electrical power P = VI, VI cos φ and √3 V_LI cos φ; mechanical power P = Tω — standard formulas. 1 lbf·ft = 1.35582 N·m and 1 hp (mechanical) = 745.70 W (standard values).

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