Motor Torque Calculator: Power, Speed, and Torque Relationships

Torque and power are the two defining characteristics of any motor, and they're directly related through speed. It's not obvious which you care about until you're trying to select a motor for a specific application — a conveyor belt needs high torque at low speed; a drill needs moderate torque at high speed; a fan needs low torque but often at high speed.

The CalcHub Motor Torque Calculator converts between torque, power, and rotational speed in any direction.

The Core Equations

Power = Torque × Angular Velocity

In practical units:

• Metric: P (W) = T (N·m) × ω (rad/s) = T × (2π × RPM ÷ 60)


• Imperial: P (HP) = T (lb·ft) × RPM ÷ 5,252

Rearranging:

• Torque from Power: T = P × 5,252 ÷ RPM (imperial: lb·ft)


• Torque from Power: T = P × 9,550 ÷ RPM (metric: N·m, P in kW)


• RPM from Power and Torque: RPM = P × 5,252 ÷ T

Unit Conversions

Torque Unit	Equivalent
1 N·m	0.7376 lb·ft
1 N·m	8.851 lb·in
1 lb·ft	1.356 N·m
1 lb·in	0.1130 N·m
1 kgf·m	9.807 N·m

Power Unit	Equivalent
1 HP (mechanical)	745.7 W = 0.7457 kW
1 kW	1.341 HP

Worked Examples

Example 1: What torque does a 5HP motor produce at 1,750 RPM? T = 5 × 5,252 ÷ 1,750 = 15.0 lb·ft = 20.4 N·m Example 2: A motor produces 50 N·m at 3,000 RPM. What is its power? P = T × RPM ÷ 9,550 = 50 × 3,000 ÷ 9,550 = 15.7 kW = 21.0 HP Example 3: I need 80 N·m at 1,200 RPM. What motor power do I need? P = 80 × 1,200 ÷ 9,550 = 10.05 kW — choose a 11kW standard motor size

Torque-Speed Characteristics by Motor Type

Motor Type	Torque at Low Speed	Torque at High Speed	Best For
DC series motor	Very high (startup torque)	Decreases with RPM	Hoists, traction, hard starts
DC shunt motor	Moderate, nearly constant	Slight decrease	Constant speed loads
AC induction (squirrel cage)	Low at zero speed, peaks near sync	Drops off sharply	Pumps, fans, compressors
Permanent magnet DC	Good low-speed torque	Drops with RPM	Robots, EV drive motors
Stepper motor	High holding torque	Significant torque reduction at speed	CNC, precision positioning
Servo motor	High torque at all speeds	Constant with feedback	Industrial automation

Selecting a Motor for a Load

Step 1: Calculate the torque your load requires.
Step 2: Determine the operating speed range.
Step 3: Calculate required power = T × RPM ÷ 9,550.
Step 4: Apply a service factor (1.15–1.5 for variable loads, shock loads, or continuous duty).
Step 5: Select the next standard motor size above your calculated requirement.

Example: Conveyor belt load

• Belt force required: 500N
• Pulley radius: 0.15m
• Belt speed: 0.5 m/s → pulley RPM = (0.5 / (2π × 0.15)) × 60 = 31.8 RPM
• Torque needed: 500 × 0.15 = 75 N·m
• Power: 75 × 31.8 ÷ 9,550 = 0.25 kW minimum
• With 1.5× service factor: 0.37 kW → select a 0.4 kW motor

What's the difference between rated torque and stall torque?

Stall torque (locked-rotor torque) is the maximum torque a motor develops at zero RPM. Rated torque is the torque at the motor's rated speed and power. DC motors typically have stall torques 5–10× their rated torque; AC induction motors typically have 150–300% rated torque at stall.

Why do electric motors have a service factor?

Service factor (SF) is the motor's built-in overload capacity. An SF of 1.15 means the motor can run continuously at 115% of rated load without damage. It's a safety margin for intermittent overloads, not a reason to chronically overload the motor.

How does gear reduction affect torque?

A gearbox with ratio n:1 multiplies output torque by n while dividing output speed by n (minus gearbox losses). A 10:1 gearbox on a 20 N·m motor at 3,000 RPM gives approximately 200 N·m at 300 RPM (at ~90% gearbox efficiency: 180 N·m).

Related Tools

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• Power Consumption Calculator — electrical power draw for motor selection
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