Motor Horsepower Calculator | Torque & RPM | Electric Motor Sizing

Motor Horsepower Calculator

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What problem does this solve? Engineers and mechanics need to determine the horsepower of motors from torque and RPM, or estimate electrical input power from voltage and current. This calculator provides both mechanical and electrical horsepower calculations for proper motor sizing and performance analysis.

Mechanical HP (Torque & RPM)

Electrical HP (V, I, η, PF)

Find Torque

Find RPM

Torque

lb-ft

Rotational Speed (RPM)

HP = (Torque × RPM) / 5252 (for lb-ft) or HP = (Torque × RPM) / 7121 (for Nm)

Voltage (V)

Current (I)

Efficiency (η, %)

Power Factor (PF)

HP = (V × I × η × PF) / 746 (for single-phase AC or DC)

For three-phase motors, multiply by √3 (≈1.732). Use the three-phase toggle below.

Three-phase motor (×√3)

Horsepower (HP)

Rotational Speed (RPM)

Torque Unit

Torque (lb-ft) = (HP × 5252) / RPM | Torque (Nm) = (HP × 7121) / RPM

Horsepower (HP)

Torque

lb-ft

RPM = (HP × 5252) / Torque (lb-ft) or RPM = (HP × 7121) / Torque (Nm)

Common Motor Examples

Presets fill typical values for reference.

Horsepower (HP)

0.00 HP

Based on torque and RPM

Formula Used

HP = (Torque × RPM) / 5252

Electrical Input (if applicable)

Equivalent kW

What Problem Does This Calculator Solve?

Selecting the right motor for an application requires understanding the relationship between horsepower, torque, and speed. This calculator solves:

Mechanical horsepower – from torque (lb-ft or Nm) and rotational speed (RPM). Essential for pumps, conveyors, and industrial machinery.
Electrical horsepower – from voltage, current, efficiency, and power factor. Helps size motors and verify power consumption.
Missing variable – find torque or RPM when HP and the other variable are known.

Whether you're a plant engineer, mechanic, or student, this tool gives accurate motor power calculations instantly.

How to Use This Motor Horsepower Calculator

Select mode from tabs: Mechanical HP, Electrical HP, Find Torque, or Find RPM.
Enter known values – torque and RPM for mechanical, or voltage, current, efficiency, PF for electrical.
For three-phase motors, check the box to multiply by √3.
Use preset examples to load typical motor parameters.
Click Calculate to see horsepower, equivalent kW, and formula details.

Note: 1 HP = 746 W = 0.746 kW. Mechanical HP uses constants 5252 (lb-ft) or 7121 (Nm).

Motor Horsepower Formulas

HP_mech = (Torque × RPM) / 5252 (lb-ft)

HP_mech = (Torque × RPM) / 7121 (Nm)

HP_elec = (V × I × η × PF) / 746

HP: horsepower (mechanical or electrical output)

Torque: lb-ft or Nm (Newton-meters)

RPM: revolutions per minute

V: voltage (volts)

I: current (amperes)

η: efficiency (decimal, 0-1)

PF: power factor (0-1, for AC motors)

For three-phase: multiply V × I by √3 (≈1.732)

People Also Ask

🤔 How do you calculate horsepower from torque and RPM?

Use HP = (Torque × RPM) / 5252 for torque in lb-ft, or /7121 for Nm. Example: 100 lb-ft at 1800 RPM = (100×1800)/5252 ≈ 34.3 HP.

🔍 What is the difference between mechanical and electrical horsepower?

Mechanical HP is the output power at the shaft. Electrical HP is the input electrical power multiplied by efficiency. For an ideal motor, electrical HP > mechanical HP due to losses.

⚡ How to calculate three-phase motor horsepower?

HP = (√3 × V × I × η × PF) / 746. Use our checkbox to include √3 automatically.

📏 What is typical motor efficiency?

Small motors: 50-70%, large industrial motors: 85-95%, premium efficiency motors: up to 96%.

🎯 Why is 5252 used in the horsepower formula?

James Watt defined 1 HP as 33,000 ft·lbf/min. Since 1 revolution = 2π radians, the constant becomes 33,000/(2π) ≈ 5252.

🔥 Real-world applications?

Sizing motors for pumps, fans, conveyors, electric vehicles, and industrial machinery.

Typical Motor HP & Torque Examples

Motor Type	HP	Typical RPM	Torque (lb-ft)
Fractional HP (fan)	0.25	1550	0.85
Small AC motor (conveyor)	1	1725	3.0
Industrial motor (pump)	10	1750	30
Large motor (compressor)	50	1780	148
Electric vehicle motor	150	6000	131

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Common Questions

Can I use this calculator for DC motors?

Yes, for DC motors, use the electrical HP mode with PF=1. The formula becomes HP = (V × I × η) / 746.

What about motor service factor?

Service factor indicates temporary overload capacity. Our calculator gives nominal HP. Multiply by service factor (e.g., 1.15) for maximum allowable load.

How accurate are these formulas?

They are exact for ideal conditions. Real motors have losses not captured by simple electrical formula, but efficiency accounts for most.