Calculate stepper motor torque from load and lead screw, belt/pulley, or motor constant and current for stepper-driven systems, with N·m conversion.

Mechanism modes estimate steady load torque, excluding acceleration, bearing/drive losses and design margin. Mass units assume vertical weight at standard gravity. Available motor torque must be checked at operating speed.

For horizontal motion, enter the actual resisting force, not mass alone.

35% is an editable assumption, not a universal screw-type rating.

Stepper Motor Torque Formula

The following formula can be used to estimate stepper motor torque from a torque constant and phase current, assuming the motor is operating in its approximately linear region and the torque constant is defined for the same excitation method.

Tₛₘ ≈ KT × Iₚₕ
  • Where Tsm is the estimated stepper motor torque (N·m)
  • KT is the torque constant for your definition (N·m/A)
  • Iph is the phase current (A, per phase)

To estimate torque, multiply the torque constant by the phase current. If you only have datasheet holding torque (Thold) at a rated phase current (Irated), a common approximation is KT ≈ Thold/Irated using the same wiring and excitation mode. Real available torque depends strongly on speed and drive conditions, so use the motor’s torque-speed curve for design work.

How to Calculate Stepper Motor Torque?

The following two example problems outline how to estimate the Stepper Motor Torque using the relationship above.

Example Problem #1:

  1. First, determine the torque constant (N·m/A). In this example, the torque constant is 0.25 N·m/A.
  2. Next, determine the phase current (A, per phase). For this problem, the phase current is 2.0 A.
  3. Finally, calculate the estimated torque using the formula above: 

Tsm ≈ KT·Iph

Inserting the values from above and solving the equation with the input values gives: 

Tsm ≈ 0.25*2.0 = 0.50 (N·m)

Example Problem #2: 

Using the same process as example problem 1, we first define the variables outlined by the formula. In this case, the values are:

torque constant (N·m/A) = 0.18

phase current (A, per phase) = 1.5

Entering these values yields:

Tsm ≈ 0.18*1.5 = 0.27 (N·m)