What is the -3 dB cutoff frequency?

The cutoff frequency is the frequency at which the output voltage drops to 1/√2 (about 0.707) of the input voltage, corresponding to a power reduction of one half (-3 dB). For a simple RC filter it is f = 1 / (2π·R·C).

Does this formula apply to both low-pass and high-pass RC filters?

Yes. Both RC low-pass (R in series, C to ground) and RC high-pass (C in series, R to ground) share the same cutoff formula f = 1 / (2π·R·C). The difference is which side of the corner is passed and which is attenuated.

What is the RC time constant?

The time constant τ = R·C (in seconds) describes how quickly the capacitor charges or discharges. At τ seconds the voltage reaches about 63.2% of its final value. The cutoff frequency relates to τ as f = 1 / (2π·τ).

What roll-off does a single-stage RC filter provide?

A single RC stage is a first-order filter with a -20 dB/decade (or -6 dB/octave) roll-off beyond the cutoff frequency. For steeper roll-off, multiple RC stages or active filter designs are needed.

Can I use this for RLC or active filter designs?

No — this tool covers the passive single-pole RC case only. RLC and active (op-amp) filters have additional resonance or gain factors that require separate calculations.

Why does the reactance equal R at the cutoff frequency?

At f_c, the capacitive reactance X_C = 1/(2π·f·C) equals R. This makes the voltage divider ratio 1/√2, which is the definition of the -3 dB point.

RC Filter Cutoff Frequency Calculator

Name: RC Filter Cutoff Frequency Calculator
Availability: InStock
Author: Nham Vu

Enter R and C values to get the -3 dB cutoff frequency, or enter frequency to solve for a missing component.

Solve for

Resistance (R)

Capacitance (C)

Quick examples

Enter values and click Calculate

Summary

Enter R and C values to get the -3 dB cutoff frequency, or enter frequency to solve for a missing component.

How it works

Select solve mode: find frequency from R and C, find C from frequency and R, or find R from frequency and C.
Enter the known values with appropriate unit prefixes (kΩ, MΩ, nF, pF, etc.).
The calculator applies f = 1 / (2π·R·C) and solves for the unknown quantity.
The time constant τ = R·C is displayed in seconds (and milliseconds / microseconds for readability).
Reactance X_C at the cutoff frequency equals R, confirming the -3 dB (0.707 voltage) point.
Results update instantly as you type.

Use cases

Design an audio low-pass anti-aliasing filter before an ADC.
Calculate the high-pass cutoff for a DC-blocking capacitor in an audio preamp.
Find the capacitor value needed to hit a specific corner frequency with a given resistor.
Verify the -3 dB point of an existing RC stage on a schematic.
Estimate the time constant of an RC charging or discharging circuit.
Prototype a simple tone-control or treble-cut filter for audio work.
Cross-check SPICE simulation cutoff frequencies against the hand-calculation formula.
Teach or learn the relationship between R, C, and bandwidth interactively.

Frequently Asked Questions

Last updated: 2026-07-22 · Reviewed by Nham Vu