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RC High-Pass Cutoff Calculator

Enter the resistance and capacitance to get the high-pass cutoff frequency.

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.

Worked example

Take a 10 kΩ resistor with a 100 nF capacitor, entered as 10000 ohms and 0.0000001 farads. Multiply: R × C = 10000 × 0.0000001 = 0.001 s. Then 2π × 0.001 = 0.0062832, and 1 / 0.0062832 = 159.155. The tool shows 159.15 Hz. Signals well above that frequency pass almost unchanged, while those well below are progressively attenuated. A 1 kHz tone is therefore passed, but a 50 Hz hum is reduced.

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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.

About this calculator

An RC high-pass filter — a resistor and capacitor arranged so the output is taken across the resistor — attenuates low frequencies while letting high ones pass. Its −3 dB cutoff frequency, the point where output power drops to half (voltage to ~70.7%), is given by fc = 1 ⁄ (2πRC), where R is resistance in ohms and C is capacitance in farads.

This single-pole filter is one of the most common building blocks in analog electronics: it's used to strip DC offset and low-frequency hum from audio signals, form the “treble” stage in tone-control circuits, and AC-couple stages in amplifier and sensor circuits. Electronics designers use this calculation to pick R and C values that place the cutoff exactly where unwanted low frequencies should start being rejected.

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  • Editorial team

    A typical use is an AC-coupling capacitor in front of an input whose resistance you already know. Work backwards from the lowest frequency you want to keep: with a 10000 ohm input and a cutoff near 10 Hz, 0.0000016 F gives about 9.95 Hz. Real capacitors have tolerances, and electrolytics can be loose by 20 percent, so the actual corner moves with them. What kind of circuit are you coupling, and which part values did you land on?

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