Calculate electric motor torque, power, or speed from known values, including AC full-load torque and DC voltage, current, and efficiency inputs.

Electric Motor Torque Calculator

Enter any 2 values to calculate the third.

Electric Motor Torque Formula

The basic motor torque relationship connects mechanical power, shaft speed, and torque. The calculator converts all entered units to watts, RPM, and newton-meters before calculating the missing value.

T = (60*P)/(2*pi*N)
  • T = torque, in newton-meters (N-m)
  • P = mechanical power, in watts (W)
  • N = shaft speed, in revolutions per minute (RPM)
  • pi = 3.14159265...
P = T*2*pi*N/60
  • P = mechanical power, in watts (W)
  • T = torque, in newton-meters (N-m)
  • N = shaft speed, in RPM
N = (60*P)/(2*pi*T)
  • N = shaft speed, in RPM
  • P = mechanical power, in watts (W)
  • T = torque, in newton-meters (N-m)

For AC motor mode, the full-load torque uses the same power, speed, and torque relationship. Optional multipliers estimate starting torque, breakdown torque, and service-factor torque.

T_start = M_start*T_FL
T_breakdown = M_breakdown*T_FL
T_SF = SF*T_FL
  • T_FL = full-load torque
  • T_start = starting torque
  • T_breakdown = breakdown torque
  • T_SF = service-factor torque
  • M_start = starting torque multiplier
  • M_breakdown = breakdown torque multiplier
  • SF = service factor

For DC motor mode, the calculator uses voltage, current, efficiency, speed, and torque. Efficiency is entered as a percent, so it is divided by 100 in the formula.

P_out = V*I*(eta/100)
T = (60*V*I*(eta/100))/(2*pi*N)
  • P_out = mechanical output power, in watts
  • V = voltage, in volts
  • I = current, in amps
  • eta = efficiency, in percent
  • T = torque, in newton-meters
  • N = speed, in RPM
T_est = T_stall*(1 - N/N_no_load)
N_est = N_no_load*(1 - T/T_stall)
  • T_est = estimated DC motor torque from the linear torque-speed model
  • N_est = estimated DC motor speed from the linear torque-speed model
  • T_stall = stall torque
  • N_no_load = no-load speed

In Basic mode, you enter any two of power, speed, and torque to calculate the third. In AC Motor mode, you enter any two of rated power, rated speed, and full-load torque, with optional multiplier results. In DC Motor mode, you enter any four of voltage, current, efficiency, speed, and torque to solve the missing value, with optional stall torque and no-load speed estimates.

Common Motor Torque Unit Conversions

Quantity Conversion Use in the calculator
Power 1 kW = 1000 W Used for W and kW entries
Power 1 HP = 745.699872 W Used for horsepower entries
Torque 1 lb-ft = 1.355817948 N-m Used for lb-ft torque entries
Torque 1 lb-in = 0.112984829 N-m Used for lb-in torque entries
Speed rad/s = RPM*2*pi/60 Used for RPM and rad/s speed entries

Typical AC Motor Torque Multipliers

Torque value Typical range Meaning
Starting torque 1.5 to 2.5 x full-load torque Torque available when the motor starts from rest
Breakdown torque 2.0 to 3.5 x full-load torque Maximum torque before the motor pulls out or stalls
Service factor 1.0 to 1.15 is common Allowable short-term load margin when conditions permit

Example Problems

Example 1: Calculate torque from power and speed

You have a 5 HP motor running at 1750 RPM. Find the shaft torque.

P = 5*745.699872 = 3728.49936 W
T = (60*3728.49936)/(2*pi*1750)

The torque is about 20.35 N-m, which is about 15.01 lb-ft.

Example 2: Calculate DC motor torque

A DC motor runs at 24 V and 10 A with 80% efficiency at 3000 RPM. Find the output torque.

P_out = 24*10*(80/100) = 192 W
T = (60*192)/(2*pi*3000)

The torque is about 0.611 N-m, which is about 5.41 lb-in.

FAQ

What is motor torque?

Motor torque is the turning force available at the shaft. Higher torque means the motor can apply more twisting force to a load. Torque is commonly shown in newton-meters, pound-feet, or pound-inches.

Why does torque decrease when speed increases for the same power?

Power is torque multiplied by angular speed. If power stays the same and speed increases, torque must decrease. If speed decreases while power stays the same, torque increases.

What is the difference between full-load torque and starting torque?

Full-load torque is the torque the motor produces at rated power and rated speed. Starting torque is the torque available at zero speed when the motor first starts. For many AC motors, starting torque is expressed as a multiplier of full-load torque.