How do I calculate motor torque from current?

Step 1: P = √3 × V × I × cos φ × η (shaft power in W). Step 2: T = P × 9549 / n (in N·m), where n is speed in RPM. The constant 9549 converts kW and RPM to N·m.

What is the 9549 constant?

T = P × 9549 / n comes from T = P / ω, where ω = 2πn/60. So T = P × 60/(2π×n) = P × 9549/n when P is in kW and T in N·m. For P in watts: T = P × 9.549/n.

What is rated torque vs starting torque?

Rated torque is the continuous torque at full load and rated speed. Starting torque is the torque at zero speed (locked rotor). For standard induction motors, starting torque is typically 150-300% of rated torque.

How does speed affect torque?

For constant power: T = P/ω, so torque is inversely proportional to speed. Below base speed with VFD (constant V/f): torque stays roughly constant. Above base speed (field weakening): torque drops as 1/n.

What is the difference between shaft torque and air-gap torque?

Air-gap torque is the electromagnetic torque produced in the motor. Shaft torque is lower due to friction and windage losses. For practical purposes, T_shaft = P_output × 9549 / n.

Free Engineering Tool

Motor Torque from Current Calculator

Chain calculation: Current → Power → Torque. T = P × 9549 / n (N·m). Enter current or power, voltage, speed.

T = 9549 × P/nI → P → TN·m / lb·ft

Results

Shaft Torque

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Shaft Power (Mechanical)

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Electrical Input Power

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Angular Velocity ω

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Motor Torque from Current Calculator | T = P×9549/n | Free Online Tool

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026

Results

Current → Power → Torque Chain