About this tool
RC / RL Time Constant Calculator is a free, in-browser tool that finds the time constant of a resistor-capacitor or resistor-inductor circuit and shows how far it has charged after any interval. Everything is computed locally in your browser.
The time constant is τ = R·C for an RC circuit and τ = L/R for an RL circuit. After one time constant a charging circuit reaches 63.2% of its final value, then 86.5% at 2τ, 95.0% at 3τ, 98.2% at 4τ and 99.3% at 5τ — which is why circuits are treated as fully settled after five time constants. If you enter a source voltage and a time, the tool evaluates the charging curve V(t) = Vs·(1 − e^(−t/τ)) and the discharging curve V(t) = Vs·e^(−t/τ). Inputs accept Ω/kΩ/MΩ, µF/nF/pF and mH/µH/H through the unit selectors.
Use it to size a timing or filter network, to estimate how long a capacitor takes to charge, or to check when an RL circuit's current has essentially settled. The time constant is auto-scaled to s, ms or µs.