What Is the Motor Horsepower Calculator?
This tool turns a motor's twisting force and spinning speed into a horsepower rating. Enter torque in ft-lbs, shaft speed in RPM, and the motor's efficiency, and it returns three numbers: the mechanical (shaft) horsepower delivered to your load, the electrical horsepower the motor draws to produce it, and the equivalent power in watts. It is built around the two relationships every motor engineer uses daily — HP = (Torque × RPM) ÷ 5,252 for shaft power, and 746 watts per horsepower for the electrical side.
A Worked Example
Take the default values: 50 ft-lbs of torque at 1,750 RPM (a common four-pole induction speed) with 90% efficiency. Shaft horsepower is (50 × 1,750) ÷ 5,252 ≈ 16.7 HP. Because the motor is only 90% efficient, the electrical horsepower it pulls from the line is 16.7 ÷ 0.90 ≈ 18.5 HP. Converting the shaft output to SI units, 16.7 × 746 ≈ 12,430 watts, or about 12.4 kW. Scale it up and the formula holds: 300 ft-lbs at 3,500 RPM comes to (300 × 3,500) ÷ 5,252 ≈ 200 HP.
Reading the Torque-Speed Relationship
Horsepower is not a fixed property of a motor — it depends on where on the torque-speed curve the motor is running. A motor making its rated torque low in the RPM band produces less horsepower than the same torque higher up, because HP scales directly with speed. That is why 5,252 matters: torque and horsepower are numerically equal at 5,252 RPM, below it torque leads, and above it horsepower leads. If your torque or RPM figure is an estimate, run a few values to see how sensitive the horsepower result is before you commit to a frame size.
Motor Horsepower Calculator
How to Use This Calculator
- Enter Your Torque (ft-lbs): Type the shaft torque in pound-feet. Use the rated or measured value from the load; if the spec is in Nm, divide by 1.356 to get ft-lbs first.
- Enter Your Motor Speed (RPM): Type the shaft speed in revolutions per minute. Common induction speeds are 3,450 (two-pole), 1,750 (four-pole), and 1,150 (six-pole) at rated load.
- Enter Your Motor Efficiency (%): Type the motor's efficiency. Use the nameplate value if you have it, or roughly 90% for a standard motor near full load, 92-96% for a premium-efficiency motor.
- Click Calculate: Hit Calculate to apply (Torque × RPM) ÷ 5,252 for shaft HP, then divide by efficiency for electrical HP. Results appear instantly below.
- Review Your Results: Compare mechanical HP (what the shaft delivers) against electrical HP (what the motor draws) and the watts figure. Use the electrical HP when sizing supply and the mechanical HP when matching a load.
How It Works
Shaft horsepower is the product of how hard the motor twists (torque) and how fast the shaft spins (RPM), scaled by the constant 5,252. This calculator takes torque in ft-lbs and speed in RPM to give you mechanical horsepower at the shaft, then divides by efficiency to reveal the electrical horsepower the motor actually pulls from the line.
The basic rule:
- HP = (Torque x RPM) / 5252
- Electrical HP = Mechanical HP / Efficiency
Mechanical HP is the useful work delivered at the coupling; electrical HP is what the motor draws to produce it. The gap between the two is the loss you pay for as heat. Nameplate values from the motor label always override these estimates for warranty and sizing decisions.
Tips & Considerations
- Torque in Nm won't work here — convert to ft-lbs first by dividing Nm by 1.356.
- The torque and horsepower curves always intersect at 5,252 RPM; use that as a quick gut-check when reading a dyno chart.
- Size overload protection and wiring from the electrical HP (and the nameplate full-load amps), not the shaft HP.
- Efficiency falls off at light load, so a lightly loaded motor draws proportionally more electrical HP than its rating suggests.
- For a three-phase electric motor with volts and amps instead of torque, use HP = (Volts × Amps × Efficiency × 1.732) ÷ 746 with the power factor included.
Frequently Asked Questions
How do I calculate horsepower from torque and RPM?
Multiply torque in ft-lbs by shaft speed in RPM, then divide by 5,252. A motor producing 300 ft-lbs at 3,500 RPM makes (300 × 3,500) ÷ 5,252 ≈ 200 HP. At the calculator's default 50 ft-lbs and 1,750 RPM you get (50 × 1,750) ÷ 5,252 ≈ 16.7 HP at the shaft.
Why is 5,252 the magic number in the horsepower formula?
One horsepower is defined as 33,000 ft-lbs of work per minute. A shaft turning one full revolution moves torque through 2π radians, so to convert ft-lbs × RPM into ft-lbs per minute of work you divide by 2π and multiply by the HP definition: 33,000 ÷ (2 × π) ≈ 5,252. A useful side effect is that torque and horsepower curves always cross at exactly 5,252 RPM on any dyno chart, because that is the speed where the two quantities are numerically equal.
How do I find horsepower for an electric motor from volts and amps?
When you have electrical inputs instead of shaft torque, use HP = (Volts × Amps × Efficiency) ÷ 746, since 746 watts equals one horsepower. A 240 V motor drawing 25 A at 90% efficiency makes (240 × 25 × 0.90) ÷ 746 ≈ 7.2 HP of usable output. For three-phase motors multiply the volts × amps term by 1.732 (√3) and include the power factor.
What is the difference between mechanical HP and electrical HP here?
Mechanical HP is the shaft power the motor delivers to your load. Electrical HP is the larger figure the motor pulls from the supply to make that shaft power, because no motor is 100% efficient. This calculator divides mechanical HP by your efficiency percentage, so at 90% efficiency a 16.7 HP shaft output needs about 18.5 HP of electrical input.
What efficiency should I enter for my motor?
Standard-efficiency induction motors run about 85-90%. NEMA Premium and IE3/IE4 motors reach 92-96%. Efficiency sags at light loads, so a motor loafing at 25-50% of rating may drop below 80%. If you have the nameplate, use its rated efficiency; otherwise 90% is a safe default for a mid-size motor near full load.
How do I convert the result into watts or kilowatts?
Multiply horsepower by 746 to get watts. The 16.7 HP shaft output from the default inputs is about 16.7 × 746 ≈ 12,430 watts, or 12.4 kW. Because 746 W = 1 HP exactly, this is the standard bridge between the US horsepower rating on a nameplate and the kilowatt rating used on IEC motors sold outside North America.