RL Time Constant

How to use this calculator

Enter the inductance and resistance; the time constant τ = L / R is shown immediately, along with the practical settling times. After one τ the current has reached about 63.2% of its final value; 3τ reaches roughly 95% and 5τ about 99.3%. These multiples are the rule of thumb for "when is the current steady?" in any RL circuit, so the calculator reports them all at once.

What the value means in a circuit

The RL time constant sets how fast current builds up (or decays) through an inductive load — it governs inrush and turn-on delay in relays, solenoids and motors, the corner frequency of first-order filters (f = R / 2πL), and how quickly stored magnetic energy dissipates when a circuit is opened. A larger inductance or smaller resistance gives a slower response. It is the inductive twin of the RC constant.