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Torque to Horsepower Converter

Convert between torque and horsepower with efficiency, gearbox ratios, and kW equivalents

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. Enter any two of the three values (torque, RPM, power) and the calculator solves for the third. The efficiency chain input lets you account for motor efficiency, gearbox efficiency, and coupling or belt drive losses, so you can calculate the required motor HP for a known load torque at the driven shaft, or determine the actual torque delivered to the load from a known motor HP. Gearbox ratio input converts between motor-side and load-side RPM and torque. Results display in both imperial (HP, ft-lbs) and metric (kW, Nm) units. This is the calculation every millwright does when replacing a motor, sizing a gearbox, or troubleshooting a drive that is not delivering enough torque to the load.

Pro Tip: When sizing a replacement motor, always calculate the required torque at the driven shaft first, then work backward through the gearbox ratio and efficiency chain to find the motor HP. Do not assume a one-to-one replacement is correct. A worn gearbox that was 92 percent efficient when new may be down to 82 percent after years of service, meaning the replacement motor needs to be larger to deliver the same load torque. Measure the actual load torque with a strain gauge or torque transducer if possible, rather than relying on the old motor's nameplate.

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Torque to Horsepower Converter
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How It Works

  1. Enter Known Values

    Enter any two of the three quantities: torque (ft-lbs or Nm), speed (RPM), and power (HP or kW). The calculator solves for the missing value using HP = (T x RPM) / 5252. Toggle between imperial and metric units as needed.

  2. Add Efficiency and Gearbox (Optional)

    Enter motor efficiency (typically 90 to 96 percent for NEMA premium motors), gearbox efficiency (typically 92 to 98 percent depending on type and age), and gearbox ratio if applicable. The calculator adjusts the output to show both input (motor) and output (load) values for torque, RPM, and power.

  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. Use these values to specify motor size, verify gearbox selection, or troubleshoot insufficient torque at the driven equipment.

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

  • Marks' Standard Handbook for Mechanical Engineers (McGraw-Hill)
  • Machinery's Handbook (Industrial Press)
  • NEMA MG 1 - Motors and Generators

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.
Motor efficiency is listed on the nameplate as a percentage at full load. NEMA premium efficiency motors range from about 90 percent (1 HP) to 96 percent (200 HP and above). For gearboxes, the efficiency depends on the type: helical gears are 95 to 98 percent per stage, worm gears are 50 to 90 percent (highly dependent on ratio), and planetary gears are 95 to 97 percent per stage. Belt drives lose 3 to 5 percent (V-belt) or 1 to 2 percent (synchronous belt). Chain drives lose about 2 to 3 percent. Multiply all efficiencies in the chain together to get the overall efficiency from motor to load.
Yes. The HP = (T x RPM) / 5252 relationship is universal for any rotating shaft. It applies equally to electric motors, diesel engines, gas turbines, and hydraulic motors. For engines, the torque and HP values are typically measured on a dynamometer at various RPM points. The only difference is that engine torque varies with RPM (following a torque curve), while electric motors maintain relatively constant torque up to their rated speed. Enter the specific RPM and torque values for the operating point you need to analyze.
A standard AC induction motor produces approximately constant torque from zero to rated speed (ignoring the starting transient). Since HP = T x RPM / 5252, if torque is constant and RPM increases, HP increases linearly. At rated speed, the motor reaches rated HP. Above rated speed (in field-weakening or extended speed range), the motor operates at constant HP and torque decreases inversely with speed. This is why VFD-driven motors are rated for constant torque below base speed and constant HP above base speed. The motor's mechanical capability does not change. The HP-torque-RPM relationship is just math.
Disclaimer: This calculator applies the standard HP = (T x RPM) / 5252 relationship for steady-state conditions. It does not account for transient loads, acceleration torque requirements, or dynamic loading. Motor and gearbox selection should consider service factor, duty cycle, and environmental conditions in addition to steady-state torque and power.

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