RC Time Constant Calculator

Compute the RC time constant τ = R·C and the charging/discharging curves of a series RC circuit. Enter the resistance and capacitance (plus a source or initial voltage) to get τ, the cutoff frequency f_c = 1/(2πRC), the energy stored in the capacitor, and a sampled table of voltage versus time. A "time to reach a target voltage" mode solves the exponential for the exact elapsed time. Pairs with the Resistor, Capacitor, Inductor, and LC Resonance calculators. Everything runs locally in your browser.

Resistance R Capacitance C Direction Source V_s V

Voltage vs. time

Time constant τ = R·C seconds. Charging toward V_s: V(t) = V_s(1 − e^−t/τ); discharging from V₀: V(t) = V₀·e^−t/τ. After one τ a charging capacitor reaches 63.2% of the source, a discharging one falls to 36.8% of its initial value; after 5τ the transient is ~99.3% complete. Time to reach a target voltage: charging t = −τ·ln(1 − V/V_s) (target must be below V_s), discharging t = −τ·ln(V/V₀) (target must be below V₀). Energy stored E = ½·C·V² joules; low-pass cutoff frequency f_c = 1/(2πRC) hertz. Everything runs locally — nothing leaves your browser.