RL Time Constant Calculator

About this calculator

The time constant of an RL (resistor-inductor) circuit is τ = L/R, where L is inductance and R is resistance. It defines how quickly current rises or decays in the circuit: after one time constant, current reaches about 63.2% of its final value; after five time constants, it's essentially fully settled (>99%) — the same exponential charging pattern seen in RC circuits, but governed by inductance instead of capacitance.

Electrical engineers use the RL time constant to predict switching behavior in circuits with inductive loads — relays, solenoids, motor windings, and transformer coils — where a sudden current interruption can generate a damaging voltage spike, which is why techniques like flyback diodes are sized with this time constant in mind. Enter L and R to get τ.