How to use the High Pass Filter Calculator
A first-order RC high-pass filter passes signals above a cutoff frequency and attenuates signals below it — the opposite of a low-pass filter. It is built from a single resistor and capacitor: the capacitor sits in series with the signal path, and the output is taken across the resistor. At low frequencies the capacitor's reactance (1/2πfC) is large, so most of the signal drops across the capacitor and little reaches the output; at high frequencies the reactance shrinks and the signal passes through nearly unattenuated. This calculator finds the cutoff frequency from your R and C values, then evaluates the filter's gain, output amplitude, and phase shift at any signal frequency you specify.
The cutoff frequency formula
The -3 dB cutoff (or "corner") frequency of an RC high-pass filter is f_c = 1 / (2πRC), where R is resistance in ohms and C is capacitance in farads. At f = f_c, the output amplitude is 1/√2 ≈ 70.7% of the input (a drop of about 3 dB) and the output leads the input by exactly 45°. For any signal frequency f, the filter's linear gain is Vout/Vin = f / √(f² + f_c²), which converts to decibels as 20·log₁₀(Vout/Vin). The phase shift is φ = arctan(f_c / f) — the output leads the input, approaching 90° well below cutoff and 0° well above it.
Reading the roll-off and choosing components
Below the cutoff frequency, a first-order (single-stage) RC high-pass filter attenuates the signal at roughly 20 dB per decade (6 dB per octave) — each tenfold drop in frequency cuts the output by a further factor of about 10. To push the cutoff lower, increase R or C (or both); to push it higher, decrease them. Common uses include blocking DC offset before an amplifier stage, removing low-frequency rumble or hum from audio, and AC-coupling signals between circuit stages. A real op-amp or transistor stage adds its own input impedance and bandwidth limits, so the practical cutoff can shift slightly from the ideal RC calculation — check the datasheet for high-gain or high-frequency designs.