RC Time Constant Calculator

Calculate RC time constant, 20–80% and 10–90% rise time, and the 3 dB cutoff frequency from resistance and capacitance.

Solved value–

Time constant (τ = R·C)–

Rise time 20% → 80%–

Rise time 10% → 90%–

3 dB cutoff frequency–

About RC timing

One time constant (τ = R × C) is the time a first-order RC circuit needs to move about 63.2% of the way from its starting voltage toward its final value. After five time constants the response is within 1% of settled.

Rise time and bandwidth

20–80% rise time = ln(4) × τ ≈ 1.386 τ

10–90% rise time = ln(9) × τ ≈ 2.197 τ

3 dB cutoff frequency f_c = 1 / (2π·R·C)

The rise-time figures describe how quickly an edge settles; the cutoff frequency is where a simple RC low-pass filter attenuates a signal by 3 dB.