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RC Filter Calculator

Low-pass & high-pass filter calculator

Ω
Result
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Formulas

ττ = R × C
fcfc = 1 / (2πRC)
XCXC = 1 / (2πfC)

Time Constant Response

TimeChargeDischarge
1τ63.2%36.8%
2τ86.5%13.5%
3τ95.0%5.0%
5τ99.3%0.7%

Related

RL FilterOhm's LawVoltage Divider

Understanding RC Filters

RC filters are fundamental electronic circuits that use a resistor and capacitor to selectively pass or block certain frequencies. They're used in audio systems, power supplies, signal processing, and countless other applications.

Low-Pass Filter

In a low-pass configuration, the output is taken across the capacitor. At low frequencies, the capacitor's high impedance allows signals to pass. At high frequencies, the capacitor's low impedance shunts signals to ground. Used for: noise filtering, smoothing DC supplies, audio bass filters.

High-Pass Filter

In a high-pass configuration, the output is taken across the resistor. The capacitor blocks DC and low frequencies while passing high frequencies. Used for: DC blocking, audio treble filters, coupling stages in amplifiers.

Cutoff Frequency

The cutoff frequency (fc) is where output power is half the input power (-3dB point). At fc, the capacitive reactance equals the resistance (Xc = R). The filter's roll-off is -20dB/decade (first-order filter).

Time Constant

The time constant τ = RC determines how fast the circuit responds to changes. After one time constant, a charging capacitor reaches 63.2% of final voltage. After 5τ, it reaches 99.3% - essentially complete.

Frequently Asked Questions

The cutoff frequency fc = 1/(2πRC) is the -3dB point where output is 70.7% of input voltage (50% power). At this frequency, capacitive reactance equals resistance. It marks the boundary between the passband and stopband of the filter.

Time constant τ = R × C (in seconds when R is ohms and C is farads). It's the time for capacitor voltage to reach 63.2% of final value during charging or 36.8% during discharging. Five time constants (5τ) gives 99.3% completion.

The capacitor is in parallel with the output. At low frequencies, capacitor impedance is high (Xc = 1/2πfC), so signal passes through. At high frequencies, impedance drops, shunting signal to ground. The resistor limits current flow.

Low-pass: Output across capacitor, passes DC and low frequencies, attenuates high frequencies. High-pass: Output across resistor, blocks DC and low frequencies, passes high frequencies. Same R and C values give same fc for both configurations.

A single RC filter (first-order) has a roll-off of -20 dB/decade or -6 dB/octave. This means for every 10× increase in frequency beyond fc, the output drops by 20 dB. For steeper roll-off, cascade multiple filter stages or use active filters.

After 5 time constants, the capacitor reaches 99.3% of final voltage - close enough to 100% for most practical purposes. The charging curve is exponential and technically never reaches 100%, but 5τ is the engineering convention for "complete" charging/discharging.

Rearrange the formula: RC = 1/(2πfc). Choose a practical capacitor value first (they have limited standard values), then calculate R = 1/(2πfcC). Keep R between 1kΩ and 100kΩ for best results. Use standard component values closest to calculated values.

Low-pass: Power supply ripple filtering, anti-aliasing before ADC, audio tone controls, noise reduction. High-pass: DC blocking in audio, coupling between amplifier stages, removing DC offset, bass cut filters.