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Torque to Horsepower Converter

Steady-state torque, horsepower, and kW identities with efficiency and gearbox prompts

Field guide: Ohm's Law, Watt's Law, and the AC Power Triangle

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Free torque to horsepower converter for millwrights, maintenance mechanics, and rotating equipment engineers who need to convert between torque (ft-lbs or Nm) and power (HP or kW) at a known RPM. The fundamental relationship is HP = (Torque x RPM) / 5252, where 5252 is derived from 33,000 ft-lbs/min per HP divided by 2 x pi. Pick a conversion mode (HP to torque, torque to HP, kW to torque, or torque to kW), enter the known value and shaft RPM, and the calculator returns the steady-state equivalent. The optional efficiency chain inputs let you screen motor efficiency, gearbox efficiency, and coupling or belt drive losses to estimate the input power a known load torque implies. The optional gearbox ratio input screens motor-side vs load-side RPM and torque. Results display in both imperial (HP, ft-lbs) and metric (kW, Nm) units. This is steady-state arithmetic only; verify against nameplate data, manufacturer curves, and duty cycle before motor, gearbox, or drive decisions.

Pro Tip: For replacement or troubleshooting work, keep the arithmetic separate from the selection decision. Measure or obtain the driven-shaft torque and speed where possible, then verify motor nameplate data, gearbox type, load inertia, duty cycle, service factor, VFD region, and manufacturer curves before changing equipment.

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How It Works

  1. Enter Known Values

    Pick the conversion mode (HP to torque, torque to HP, kW to torque, or torque to kW), then enter the known value and the shaft speed in RPM. The calculator solves the steady-state identity HP = (T x RPM) / 5252 and shows HP, kW, ft-lbs, and N-m equivalents together.

  2. Add Efficiency and Gearbox (Optional)

    Enter user-sourced motor, gearbox, and other efficiency prompts plus gearbox ratio if applicable. The calculator shows idealized input and load-side values, but real efficiency depends on load, type, ratio, temperature, wear, and manufacturer data.

  3. Review Full Results

    The output displays torque, RPM, and power at both the motor shaft and the load shaft (if a gearbox is included), along with kW equivalents and a standard-synchronous-speed reference table. Use these values as a screening step; final motor size, gearbox selection, and troubleshooting decisions need nameplate data, manufacturer curves, and qualified review.

Assumptions

  • The load is at steady-state (constant torque and RPM). Acceleration and deceleration torque are not included.
  • Efficiency values are assumed constant across the operating range (actual efficiency varies with load percentage).
  • Gearbox ratio is exact (no slip). For belt drives, slip of 1 to 3 percent may apply.
  • Torque is measured at the shaft centerline (no overhung load or radial force components).

Limitations

  • Does not calculate acceleration torque or time to speed for inertia loads.
  • Does not model variable-speed motor characteristics (torque derating at low speed, field weakening above base speed).
  • Does not include service factor or duty cycle analysis for motor selection.
  • Does not account for altitude derating or temperature derating of motor HP ratings.

References

  1. Machinery's Handbook, 31st Edition - source pointer for mechanical power identities
  2. NEMA MG 1 - Motors and Generators source pointer
  3. NIST SP 811 - unit conversion source pointer

Frequently Asked Questions

One horsepower is defined as 33,000 ft-lbs of work per minute. Torque in ft-lbs times RPM gives ft-lbs per minute if you account for the circular motion: Work per minute = Torque x 2 x pi x RPM. Dividing by 33,000 gives HP = (T x 2 x pi x RPM) / 33,000. The constant 2 x pi / 33,000 simplifies to 1/5252.11, so HP = (T x RPM) / 5252. The number 5252 is also the RPM at which 1 HP and 1 ft-lb of torque are exactly equal, which is why the torque and HP curves on a motor or engine dyno chart always cross at 5252 RPM.
Use selected-equipment data: motor nameplate or manufacturer efficiency, gearbox manufacturer data at the operating ratio and load, and measured or specified losses for belts, chains, couplings, VFDs, or other components. Generic efficiency ranges are not reliable enough for selection decisions.
The steady-state HP = (T x RPM) / 5252 identity applies to rotating shafts, but the input torque and speed must represent the actual operating point. Engines, turbines, hydraulic motors, and electric motors each have different curves, transient behavior, ratings, and duty limits that this calculator does not model.
Because shaft power is torque multiplied by angular speed. That identity does not mean a real motor can supply constant torque at every speed. Use the selected motor or drive curve, VFD region, cooling limits, overload rating, and duty cycle for equipment decisions.
Disclaimer: This calculator applies steady-state shaft-power identities only. It does not account for transient loads, acceleration torque, breakaway torque, load inertia, service factor, duty cycle, real efficiency curves, derating, VFD regions, guarding, lockout, or safe-work requirements. Verify equipment choices with nameplate data, manufacturer curves, and qualified review.

Guides for this job

Electrical

Ohm's Law, Watt's Law, and the AC Power Triangle

The 12 Ohm's Law and Watt's Law relationships explained for DC and AC circuits. Power factor, real vs apparent vs reactive power, and worked motor load examples.

Read Guide →
Electrical

Torque, Horsepower, and Power Transmission Fundamentals

The HP formula and where 5252 comes from. Torque vs speed, drivetrain efficiency, gearbox selection, motor speeds, and VFD vs gearbox decisions.

Read Guide →

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