Enter the inductance in Henry and the resistance in Ohms into the calculator to determine the L/R time constant in seconds.

L/R Time Constant Formula

The following formula is used to calculate the L/R time constant.

τ = L / R


  • τ (tau) is the L/R time constant (seconds)
  • L is the inductance (Henry)
  • R is the resistance (Ohms)

To calculate the L/R time constant, divide the inductance by the resistance.

What is an L/R Time Constant?

The L/R time constant, represented by the Greek letter τ (tau), is a measure of the time it takes for the current in an inductive circuit to reach approximately 63.2% of its maximum value after the voltage is applied. It is a critical parameter in the analysis of transient responses in electrical circuits containing inductors and resistors.

How to Calculate an L/R Time Constant?

The following steps outline how to calculate the L/R Time Constant.

  1. First, determine the inductance (L) in Henry.
  2. Next, determine the resistance (R) in Ohms.
  3. Use the formula τ = L / R to calculate the time constant (τ) in seconds.
  4. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem :

Use the following variables as an example problem to test your knowledge.

Inductance (L) = 0.5 Henry

Resistance (R) = 200 Ohms