Motor Power Calculator

Rotating machinery converts electrical energy into mechanical shaft power characterized by torque and angular speed. Electric vehicles, CNC spindles, conveyor drives, and cordless drills all specify power in horsepower or kilowatts alongside torque in newton-meters or pound-feet. Converting between these quantities lets you match motors to loads, size VFDs, and sanity-check manufacturer claims.

Shaft power from torque and speed:

P (watts) = τ × ω = τ × (2π × RPM / 60)

Torque from power and speed:

τ (N·m) = P / ω = P × 60 / (2π × RPM)

Where τ is torque in newton-meters, ω is angular velocity in radians per second, and RPM is revolutions per minute. One mechanical horsepower equals 745.7 watts. One N·m ≈ 0.7376 lb·ft.

Electric input power exceeds shaft power by motor efficiency — a 90% efficient 5 kW output motor draws roughly 5.56 kW from the line. Breakaway torque, peak vs continuous ratings, and gear reduction between motor and load all affect sizing. Gearboxes multiply torque and divide RPM: a 10:1 reducer delivers ten times output torque at one-tenth speed.

Worked example: A workshop drill delivers 40 N·m at 600 RPM. P = 40 × 2π × 600/60 = 40 × 62.83 = 2,513 W (2.5 kW or 3.4 HP). A bicycle hub motor rated 250 W at 300 RPM produces τ = 250 × 60/(2π×300) = 7.96 N·m — adequate for flat urban riding. An EV motor producing 320 N·m at 4,000 RPM delivers P = 320 × 2π × 4000/60 ≈ 134 kW (180 HP) — peak performance before thermal limits.

Use the power calculator for electrical input side and the transformer calculator when stepping drive voltage down for low-RPM high-torque applications.