RC High-Pass Cutoff Calculator

How it is calculated

The cutoff frequency comes from the time constant of the resistor and capacitor: fc = 1 / (2 × π × R × C). R is in ohms and C in farads, the base SI units, and fc is returned in hertz with two decimals. Both values must be greater than zero.

The tool takes plain numbers only, so a prefix such as k or n is not understood: 10 kΩ is 10000 and 100 nF is 0.0000001 (1e-7). Doubling either R or C halves the cutoff. The result is the ideal single-pole value at the -3 dB point, where the output has fallen to about 70.7% of the input voltage. It assumes the source has no resistance of its own and that the next stage draws no current, so real circuits with a load can land lower or higher.

Common mistakes

• Typing the capacitance in microfarads or nanofarads (100 for 100 nF). The field expects farads, so entering 100 gives a cutoff near 0.00 Hz; write 0.0000001.
• Entering kilo-ohms as ohms, for example 10 for a 10 kΩ resistor. The cutoff then comes out a thousand times too high.
• Treating fc as a wall. At the cutoff the signal is still at about 70.7% of its voltage; attenuation grows gradually below it rather than stopping.
• Using this for a low-pass filter. The formula is the same, but the output is taken across the capacitor and the behaviour is reversed: low frequencies pass.
• Ignoring the load. The calculation does not include source or load impedance, so a low-impedance next stage shifts the real cutoff upward.