RC Filter Calculator — Cutoff Frequency fc = 1/(2πRC)

RC low pass & high pass — solve for R, C, or fc

Operating Mode

Target Variable

Required Parameters

Resistance

Ohm

Capacitance

Waiting for input data...

Ad Placement

Sidebar Adaptive Ad Slot

Quick Answer

RC filter cutoff: fc = 1/(2πRC). At fc the output is -3 dB (70.7% voltage). A first-order RC filter rolls off at -20 dB/decade. Enter R and C to find fc, or solve backwards for component values.

Documentation

RC Filter Calculator

Design first-order RC filters and compute the -3 dB cutoff frequency from resistor and capacitor values — or solve for R, C, or fc when you know any two.

Quick start: Select RC Low Pass or RC High Pass, enter R and C → instant fc. Use Target Variable to find C for a desired cutoff.

RC filter formula

fc = 1 / (2π × R × C)

Symbol	Meaning
fc	Cutoff frequency (Hz)
R	Resistance (Ω)
C	Capacitance (F)

Rearranged:

C = 1 / (2π fc R)
R = 1 / (2π fc C)
τ = RC = 1 / (2π fc) (time constant)

RC low pass vs RC high pass

Type	Passes	Blocks	Typical use
RC Low Pass	Below fc	Above fc	Noise filtering, ADC anti-aliasing, debounce
RC High Pass	Above fc	Below fc	DC blocking, coupling capacitors

Both use fc = 1/(2πRC) — only the circuit topology changes.

Worked example — 1 kHz RC low pass

Target fc = 1 kHz, choose C = 47 nF:

R = 1 / (2π × 1000 × 47×10⁻⁹) ≈ 3.39 kΩ
Nearest E24: 3.3 kΩ → actual fc ≈ 1.03 kHz

Common applications

Anti-aliasing before ADC — set fc ≤ fs/2
Power-supply filtering — e.g. 10 Ω + 100 nF → fc ≈ 159 kHz
Switch debounce — τ = 5–10 ms (10 kΩ + 1 µF)

Related tools

Low pass filter calculator — RC & RL low pass focused
RC/LC filter cutoff calculator — includes RL modes
RC time constant calculator
Reactance calculator

Design Notes

First-order passive filters only roll off at -20dB per decade. They are great for simple noise filtering, anti-aliasing before a slow ADC, or debounce circuits, but inadequate for separating close frequencies. When designing RC filters, avoid using extremely large resistors (e.g., >1MΩ) because the input bias current of the next stage (like an Op-Amp or ADC) will cause a DC voltage error. Conversely, avoiding extremely small resistors (<100Ω) will save power and prevent overloading the driving source.

Common Mistakes

1
Expecting a sharp cutoff from a simple RC filter. It's a gentle curve, not a brick wall.
2
Not balancing the source impedance. If your signal source already has 10kΩ output impedance, and you use a 1kΩ series resistor for your filter, your effective R is 11kΩ.
3
Ignoring capacitor tolerances. Cheap Y5V or Z5U ceramic capacitors can vary by +20%/-80% with temperature and DC bias, completely ruining your cutoff frequency.

Engineering Handbox

1. Identify variables: R = 10,000, C = 0.0000001 2. Multiply R × C = 0.001 seconds (Time Constant τ) 3. Multiply by 2π: 0.001 × 6.283 = 0.006283 4. Divide 1 by result: 1 / 0.006283 = 159.15 Hz

VerificationThe cutoff frequency is approximately 159.15 Hz.

Knowledge Base

What is an RC filter calculator?

An RC filter calculator computes the -3 dB cutoff frequency of resistor-capacitor filters using fc = 1/(2πRC). It helps you design RC low pass filters (block high frequencies) and RC high pass filters (block low frequencies).

What is the RC filter formula?

fc = 1 / (2π × R × C), where R is in ohms, C in farads, fc in hertz. Time constant τ = RC = 1/(2πfc). Rearranged: C = 1/(2πfcR) or R = 1/(2πfcC).

How do I calculate RC low pass filter cutoff?

For RC low pass: fc = 1/(2πRC). Example: R = 10 kΩ, C = 100 nF → fc = 159 Hz. High frequencies above fc are attenuated at -20 dB/decade.

What is the difference between RC low pass and RC high pass?

Same formula fc = 1/(2πRC) — only the component topology differs. Low pass passes frequencies below fc; high pass passes frequencies above fc. Select the mode above before entering R and C.

How do I pick C for a target cutoff frequency?

C = 1/(2πfcR). Example: fc = 1 kHz, R = 3.3 kΩ → C ≈ 48 nF (use 47 nF standard value). Prefer C0G/NP0 capacitors for stable cutoff.

Can I use this RC filter calculator for debouncing?

Yes. A switch debounce RC with τ = 5–10 ms (e.g. 10 kΩ + 1 µF) filters contact bounce. Cutoff fc = 1/(2πτ) ≈ 16 Hz for τ = 10 ms.

Related Engineering Tools

capacitors filters