Calculators
RC Cutoff Frequency Calculator
Calculate RC filter cutoff frequency or solve R or C.
RC Cutoff Frequency Calculator tool
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About This Tool
A first-order resistor-capacitor network has a characteristic cutoff frequency that marks the transition between its passband and attenuation region. This calculator uses the ideal RC relationship to find that -3 dB frequency from resistance and capacitance, or rearranges the same equation to choose a resistor or capacitor for a target cutoff. It is useful for simple low-pass and high-pass filters, signal conditioning, audio experiments, sensor smoothing, and electronics study. All calculations run locally in your browser.
How To Use It
- Choose whether to solve for cutoff frequency, resistance, or capacitance.
- Enter the two known values and select the engineering units that match your circuit.
- Read the calculated cutoff or component value. When solving cutoff frequency, the RC time constant is also shown.
- Treat the result as an ideal starting point and account for component tolerance and surrounding circuit impedances in a real design.
Examples
1 kΩ and 100 nF
An ideal RC network with R = 1 kΩ and C = 100 nF has a time constant of 100 µs and a cutoff frequency of about 1.592 kHz.
Choose R for 1 kHz with 100 nF
For a 1 kHz target cutoff and a 100 nF capacitor, R = 1/(2πfC) gives about 1.592 kΩ.
Choose C for 1 kHz with 10 kΩ
For a 1 kHz target cutoff and a 10 kΩ resistor, C = 1/(2πfR) gives about 15.92 nF; a nearby standard capacitor value will shift the actual nominal cutoff.
Useful Notes
RC cutoff frequency formula
For an ideal first-order RC low-pass or high-pass network, fc = 1/(2πRC), where R is resistance in ohms, C is capacitance in farads, and fc is frequency in hertz. At this frequency the magnitude is 1/√2 of the passband value, corresponding to approximately -3.01 dB.
Solving for resistance or capacitance
The formula can be rearranged as R = 1/(2πfcC) or C = 1/(2πfcR). These forms help choose a nominal component value when the desired cutoff frequency and the other component are already known.
Relationship to the RC time constant
The time constant is τ = RC seconds, so fc = 1/(2πτ). A larger time constant means a lower cutoff frequency. The time constant also describes the exponential charging or discharging rate of a simple RC circuit.
Low-pass and high-pass use the same cutoff equation
A basic passive RC low-pass and high-pass filter use the same ideal cutoff formula. What changes is where the output is taken: across the capacitor for the common low-pass arrangement and across the resistor for the common high-pass arrangement.
Real circuit loading matters
The simple formula assumes the intended R and C dominate the network. Source resistance and load impedance can effectively change the resistance seen by the capacitor. Component tolerances, capacitor ESR, parasitics, and frequency-dependent behavior can also move the measured cutoff.
Choosing practical component values
The exact calculated resistor or capacitor may not be a standard stocked value. Choose a nearby standard value, recalculate the resulting cutoff, and consider tolerance. Very large resistances can become sensitive to leakage and noise, while very small resistances can unnecessarily load the source.
FAQ
Is the RC cutoff always exactly -3 dB?
For an ideal isolated first-order RC transfer function, the magnitude at fc is 1/√2 of the passband value, about -3.01 dB. Loading and non-ideal components can change a real circuit's response.
Does this work for both RC low-pass and high-pass filters?
Yes for the ideal first-order cutoff frequency. Both use fc = 1/(2πRC), although their frequency responses and output connections are different.
Can I use this for an active filter?
Only when the active circuit topology reduces to the same first-order RC pole and its surrounding impedances do not alter the effective R or C. Higher-order active filters require topology-specific equations.
Why is my measured cutoff different?
Common causes include resistor and capacitor tolerance, source and load impedance, capacitor ESR or parasitics, measurement setup, and using a circuit topology whose effective resistance differs from the entered resistor value.