RC Filter Calculator

How to use this calculator

1. Enter the resistance R in ohms.
2. Enter the capacitance C and pick its unit.
3. Click Calculate for the cutoff (−3 dB) frequency and time constant.

Quick reference: cutoff frequency

The RC filter formula

The cutoff frequency (where the output has fallen by 3 dB, to about 70.7% of the input) is:

The time constant τ (tau) is simply R × C, the time the capacitor takes to charge to about 63% of the applied voltage. The same R and C give the same cutoff whether the circuit is wired as a low-pass or high-pass filter; only the position of R and C relative to the output changes which frequencies pass.

Frequently asked questions

What is the RC filter cutoff frequency formula?

f_c = 1 / (2π × R × C). For example, 1 kΩ with 100 nF gives about 1.59 kHz.

What is the time constant?

τ = R × C, the time the capacitor takes to charge to about 63% of the applied voltage.

What is the difference between low-pass and high-pass?

The same R and C give the same cutoff; whether it is low-pass or high-pass depends on where the output is taken relative to R and C.