RC Passive Filter Calculator
Design a low-pass or high-pass RC filter and inspect its gain and phase response.
fc = 1 / (2πRC) τ = RCCalculations stay in your browser. Share links contain the input values you choose to share.
How to use this calculator
- Choose low-pass or high-pass. The diagram updates to show the correct component order.
- Calculate cutoff from R and C, or solve for one component at a target cutoff.
- Enter an evaluation frequency to inspect the magnitude and phase at that point.
- Use the plots to compare behavior below and above cutoff.
A quick worked example
With R = 10 kΩ and C = 10 nF, the cutoff is 1.5915 kHz and the time constant is 100 µs. At 1 kHz, the low-pass gain is approximately −1.445 dB with phase −32.14°. At cutoff, both filter types have magnitude 0.7071 (−3.0103 dB); phase is −45° for low-pass or +45° for high-pass.
A few useful answers
What is the difference between low-pass and high-pass?
The low-pass circuit has a series resistor and shunt capacitor, with output across the capacitor. The high-pass circuit has a series capacitor and shunt resistor, with output across the resistor. Both share fc = 1/(2πRC).
Does the filter stop all signals beyond cutoff?
No. A first-order RC filter transitions gradually. Far into its stopband, the magnitude falls at approximately 20 dB per decade. At cutoff, output magnitude is about 70.71% of input.
What happens when I connect a load?
A finite load changes the transfer function. This calculator assumes an ideal zero-impedance source and an effectively infinite-impedance output load. Account for the next circuit’s input impedance when selecting real components.
Can I cascade two stages for a second-order filter?
Two unbuffered RC stages load each other. Their response cannot generally be found by multiplying two independent unloaded responses. This tool models one RC stage.
Model & assumptions: Ideal first-order RC network, zero source impedance, infinite load impedance, and positive component values. Component tolerances, capacitor ESR, leakage, and parasitics are not modeled.
Reference: Analog Devices — An Introduction to Electrical Filters ↗
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