What is the RC cutoff frequency formula?

The cutoff frequency is fc = 1 / (2π × R × C), where R is in ohms and C is in farads. At this frequency the output voltage is 70.7% (−3 dB) of the input.

What is the difference between a low-pass and high-pass RC filter?

In a low-pass RC filter the capacitor is in parallel with the output, so high-frequency signals are shunted to ground. In a high-pass filter the capacitor is in series, blocking low frequencies and passing high ones. Both share the same fc = 1 / (2πRC) formula.

What is the RC time constant?

The time constant τ (tau) = R × C, in seconds. It describes how quickly the capacitor charges or discharges and equals 1 / (2π × fc). After one time constant, a step response reaches about 63% of its final value.

What does −3 dB mean?

At the −3 dB point, output power drops to half the input power. Equivalently, the output voltage is 1/√2 ≈ 70.7% of the input voltage. This is the standard definition of a filter's cutoff or corner frequency.

What is capacitive reactance at the cutoff frequency?

At fc, the capacitive reactance Xc = 1 / (2π × fc × C) equals the resistance R. This equality is why the output is exactly −3 dB at the cutoff frequency.

Can I solve for R or C given a target frequency?

Yes — rearrange the formula: R = 1 / (2π × fc × C) or C = 1 / (2π × fc × R). Enter your target frequency and one known component, then solve for the other.

RC Filter Cutoff Frequency Calculator

Enter resistance and capacitance to instantly calculate the -3 dB cutoff frequency for an RC low-pass or high-pass filter.

Filter Configuration

Filter Type

Low-Pass High-Pass

Low-pass: passes signals below fc, attenuates above.

Resistance (R)

Capacitance (C)

Signal Frequency — optional, for gain & phase

Results

Enter R and C values and click Calculate

Cutoff Frequency (fc)

—

−3 dB point

Time Constant (τ)

—

τ = R × C

Angular Frequency (ω)

—

ω = 2π × fc

Capacitive Reactance at fc (Xc)

—

Xc = R at cutoff — this equality defines the −3 dB point

Formula

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Summary

An RC filter uses a resistor and capacitor to pass or block signals based on frequency. This calculator computes the -3 dB cutoff frequency (fc = 1 / 2πRC), time constant (τ = RC), and capacitive reactance at the cutoff. Optionally enter a signal frequency to see the gain in dB, voltage ratio, and phase shift for either low-pass or high-pass topology.

How it works

Select the filter type: low-pass (passes low frequencies, attenuates high) or high-pass (passes high frequencies, attenuates low).
Enter the resistance (R) and select the unit (Ω, kΩ, or MΩ).
Enter the capacitance (C) and select the unit (pF, nF, µF, or mF).
Optionally enter a signal frequency to compute gain and phase shift at that point.
Click Calculate to get the cutoff frequency, time constant, reactance, and more.
Use the Copy button to copy the cutoff frequency value for use in your design.

Use cases

Design audio tone-control circuits and speaker crossovers.
Verify component values for power supply noise filters.
Calculate the cutoff frequency for signal-conditioning RC networks.
Determine how much attenuation an RC filter provides at a given frequency.
Analyze the phase shift introduced by an RC filter stage.
Choose R and C values to hit a specific -3 dB corner frequency.
Teach or learn passive RC filter theory with instant calculation.
Cross-check SPICE simulation results against hand calculations.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu