What this calculator does
A high pass filter is a circuit that lets frequencies above a cutoff point through largely unchanged while attenuating everything below it, the opposite job to a low pass filter. In a simple RC high pass filter, the capacitor sits in series with the signal and the resistor sits across the output to ground, which is exactly the low pass arrangement with the resistor and capacitor swapped.
Because the two circuits share the same components, the cutoff frequency formula is identical: f = 1 / (2πRC). What differs is the topology and the purpose. A low pass filter is used to strip high-frequency noise or alias frequencies from a signal; a high pass filter is used to block DC offset or slow drift and pass the faster-changing part of a signal, which is why the two are usually built, and searched for, separately even though the maths is the same.
The formula
The cutoff frequency, also called the -3 dB point, is where the output has dropped to about 70.7% of the input amplitude. Below this frequency, an RC high pass filter attenuates the signal at roughly 6 dB per octave; above it, the signal passes through largely intact. Rearranging f = 1 / (2πRC) for R or C lets you work backwards from a target cutoff to the component values needed.
| Term | Meaning |
|---|---|
| f | Cutoff frequency (the -3 dB point), in hertz. |
| R | Resistance, in ohms. |
| C | Capacitance, in farads (entered here in nanofarads for convenient values). |
The inputs explained
| Field | What to enter |
|---|---|
| Solve for | Choose whether to calculate the cutoff frequency from known R and C, or work backwards to the resistance or capacitance needed for a target cutoff. |
| Resistance (Ω) | The resistor value in the circuit, used when solving for frequency or capacitance. |
| Capacitance (nF) | The capacitor value in the circuit, in nanofarads, used when solving for frequency or resistance. |
| Target cutoff frequency (Hz) | The cutoff frequency you are aiming for, used when solving for resistance or capacitance. |
When to use it
Blocking DC offset before an amplifier stage
A high pass filter with a cutoff well below the lowest signal frequency of interest removes DC bias or slow drift from a sensor or microphone signal before it reaches an amplifier, without touching the frequencies that actually matter.
Designing an audio crossover
A speaker crossover splits an audio signal by frequency, sending highs to a tweeter through a high pass filter and lows to a woofer through a low pass filter, both built from the same RC cutoff formula.
Choosing standard component values
Working backwards from a target cutoff frequency to the resistance or capacitance needed helps pick from standard component ranges, since resistors and capacitors are only manufactured at specific preferred values.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How cutoff frequency changes with resistance at a fixed 100 nF capacitor
A fixed 100 nF capacitor, across a range of common resistor values.
| Resistance | Cutoff frequency (-3 dB point) |
|---|---|
| 1,000 Ω | 1,591.55 Hz |
| 2,200 Ω | 723.43 Hz |
| 4,700 Ω | 338.63 Hz |
| 10,000 Ω | 159.15 Hz |
| 22,000 Ω | 72.34 Hz |
| 47,000 Ω | 33.86 Hz |
Questions
What is the difference between a high pass and a low pass filter?
A low pass filter passes frequencies below its cutoff and attenuates frequencies above it. A high pass filter does the opposite: it passes frequencies above its cutoff and attenuates frequencies below it. A simple RC version of each uses the same two components, just wired the other way round.
Why is the cutoff called the -3 dB point?
At the cutoff frequency, the output amplitude has fallen to about 70.7% of the input, which corresponds to a power drop of 3 decibels. It is the conventional reference point for describing where a filter starts to take effect, not the frequency at which the signal disappears entirely.
What is a second-order high pass filter?
A second-order filter cascades two RC stages (or uses an active op-amp design) to attenuate more steeply beyond the cutoff, roughly 12 dB per octave instead of 6 dB. This calculator covers the simple first-order RC case, which is the building block for those higher-order designs.
Can I use this for an RC high pass filter driven by a signal source with its own resistance?
The formula assumes an ideal signal source. In practice, the source and load impedance interact with the filter, which can shift the effective cutoff, so treat the result as a starting point to verify against the actual circuit.
For the low pass version of this same circuit, which attenuates frequencies above its cutoff instead of below it, see the low pass filter calculator.