High Pass Filter Cutoff Frequency Calculator

This calculator finds the cutoff frequency of a simple RC high-pass filter. You enter the resistance (R) and capacitance (C), and it returns the frequency (in hertz) where the output signal drops to 70.7% of the input (the −3 dB point).

Formula

The cutoff frequency of a first-order RC high-pass filter is given by:

f_c = 1 / (2πRC)

• f_c – cutoff frequency in hertz (Hz)
• R – resistance in ohms (Ω)
• C – capacitance in farads (F)

Note that the formula uses capacitance in farads. If you enter microfarads (µF), you must multiply by 10⁻⁶ to convert to farads, as done in the calculator.

Worked Example

Using the default values: R = 1000 Ω, C = 0.1 µF.

1. Convert capacitance: 0.1 µF = 0.1 × 10⁻⁶ F = 1 × 10⁻⁷ F
2. Compute the denominator: 2π × 1000 × 1×10⁻⁷ = 6.2832 × 10⁻⁴
3. Take the reciprocal: 1 / (6.2832 × 10⁻⁴) ≈ 1591.5 Hz

The calculator shows 1591.5 Hz, which rounds to 1591.5 Hz (or about 1.59 kHz).

What Does Cutoff Frequency Mean?

In a high-pass filter, frequencies above the cutoff pass through with little attenuation, while frequencies below it are reduced. At the cutoff frequency, the output power is half the input power (−3 dB), which corresponds to a voltage amplitude of about 70.7% of the input. This is a standard definition used in audio, signal processing, and electronics.

How to Choose R and C Values

Selecting component values depends on your desired cutoff frequency and the impedance of your circuit.

• Determine the target cutoff frequency – e.g., 100 Hz to block mains hum, or 20 kHz for an audio crossover.
• Pick a reasonable resistance – typically between 1 kΩ and 100 kΩ for audio circuits to avoid loading effects.
• Calculate the required capacitance using C = 1 / (2πRf_c).
• Use standard component values – e.g., E12 series (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82) and adjust R slightly if needed.

For example, if you want f_c = 1 kHz and choose R = 10 kΩ, then C = 1 / (2π × 10000 × 1000) ≈ 15.9 nF. You might use a 16 nF capacitor (standard value) which gives a cutoff of about 995 Hz – close enough for most applications.

Common Mistakes and How to Avoid Them

• Unit mix-ups – Forgetting to convert µF to F is the most common error. Always multiply microfarads by 10⁻⁶.
• Using the wrong formula – A low-pass filter has the same formula, but the circuit is different. Make sure you are working with the correct topology.
• Ignoring load impedance – The formula assumes no load. If you connect a load, the cutoff shifts. For accurate results, include the load resistance in parallel with R.
• Rounding too early – Keep intermediate values to several decimal places to avoid significant rounding errors.

Limitations of the Formula

The simple RC formula applies only to first-order filters. Real components have parasitic inductance and resistance, and at very high frequencies those effects become noticeable. For higher-order filters (e.g., Butterworth, Chebyshev), the cutoff frequency definition and calculation are different. Also, the formula assumes an ideal voltage source with zero output impedance – in practice, the source impedance adds to R.

Frequently Asked Questions

What does −3 dB mean exactly?

−3 dB corresponds to a power ratio of 0.5. For voltage, that is a ratio of 1/√2 ≈ 0.707. So at the cutoff, the output voltage is about 70.7% of the input.

Can I use this calculator for a low-pass filter?

Yes, the formula is the same: f_c = 1/(2πRC). The difference is the circuit configuration – a low-pass filter has the resistor in series and the capacitor in parallel with the output, while a high-pass filter swaps them.

What if I have the frequency and want R or C?

Rearrange the formula: R = 1/(2πf_cC) or C = 1/(2πf_cR). You can also use this calculator by entering a trial value and adjusting until the output matches your target.

Does the cutoff frequency change if I use a different capacitor type?

In theory, no – the value of capacitance is what matters. In practice, electrolytic capacitors have wide tolerances (±20% is common), so the actual cutoff may differ from the calculated value. Ceramic capacitors are more stable but can have voltage dependence.

Why is my measured cutoff different from the calculated one?

Common reasons include component tolerances, stray capacitance, load impedance, and the output impedance of the signal source. Measure the actual R and C values with a multimeter if you need high accuracy.

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in r | Resistance | Ω | 1000 | 0
in c | Capacitance | µF | 0.1 | 0
out fc | Cutoff frequency | Hz | 1 / (2 * pi * r * c * 1e-6) | 2