Enter the steady state current and the peak inrush time into the calculator to determine the inrush current. This calculator helps to estimate the initial surge of current when an electrical device is turned on.

Inrush Current Formula

The following formula is used to calculate the inrush current:

I_{inrush} = I_{steady} \times sqrt{2} \times sqrt{\frac{T_{peak}}{10}}


  • I_{inrush} is the inrush current (Amps)
  • I_{steady} is the steady state current (Amps)
  • T_{peak} is the peak inrush time (milliseconds)

To calculate the inrush current, multiply the steady state current by the square root of 2, and then multiply by the square root of the peak inrush time divided by 10.

What is Inrush Current?

Inrush current is the maximum, instantaneous input current drawn by an electrical device when first turned on. Inrush current is higher than the steady state current due to the initial surge of current required to charge capacitors and inductors or to start motors. It is a critical parameter in the design of circuits, as it can cause circuit breakers to trip or fuses to blow if not properly managed.

How to Calculate Inrush Current?

The following steps outline how to calculate the Inrush Current.

  1. First, determine the steady state current (I_{steady}) in Amps.
  2. Next, determine the peak inrush time (T_{peak}) in milliseconds.
  3. Next, gather the formula from above = I_{inrush} = I_{steady} times sqrt{2} times sqrt{frac{T_{peak}}{10}}.
  4. Finally, calculate the Inrush Current (I_{inrush}) in Amps.
  5. 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.

steady state current (I_{steady}) = 5 Amps

peak inrush time (T_{peak}) = 20 milliseconds