RC Time Constant Calculator

The RC time constant τ characterizes how quickly a resistor-capacitor network responds to voltage changes. It governs low-pass filter cutoff frequency, timing oscillator periods, debounce intervals, and camera shutter delays. One number — tau — predicts behavior across charging, discharging, and AC frequency response, making it among the most reused quantities in analog electronics.

Time constant definition:

τ = R × C

Charge and discharge milestones (from zero or toward zero):

V(t) = V_max × (1 − e^−t/τ) (charging toward supply)

Standard milestones: 1τ → 63.2%, 2τ → 86.5%, 3τ → 95.0%, 4τ → 98.2%, 5τ → 99.3%. The calculator lists these times for your R and C values so you can set firmware timeouts, choose debounce intervals, or spec power-supply hold-up without plotting exponentials by hand.

AC low-pass −3 dB cutoff frequency: f_c = 1 / (2π × R × C). A 1 kΩ / 100 nF pair gives τ = 100 µs and f_c ≈ 1.59 kHz — audio treble rolloff territory. High-pass configurations swap R and C roles in the network but use the same τ product for equivalent timing.

Worked example: Debouncing a mechanical switch with R = 10 kΩ and C = 100 nF: τ = 10,000 × 0.0000001 = 1 ms. Contact bounce under 5 ms clears by 3τ (3 ms) at 95% settled. For a 555 monostable timer, pulse width ≈ 1.1 × R × C = 1.1 ms with the same components. A 47 kΩ / 10 µF RC gives τ = 0.47 s — suitable for a power-on reset hold of roughly 1.4 s (3τ) before releasing reset.

See the capacitor charge time calculator for arbitrary target percentages and the series/parallel resistor calculator when multiple resistors set the effective R.