RC Low-Pass Filter Calculator

This is a small-signal educational model; source/load impedance and non-ideal component behavior can change a real circuit.

Resistance

Capacitance

Probe frequency

—Low-pass cutoff frequency (−3 dB point)

—Gain at probe

—Voltage ratio

—Phase

—Time constant

Electrical model check

Verify the circuit relation, units, and result

ideal first-order RC low-pass

fc = 1/(2πRC) · |H| = 1/√(1+x²)

Reading the current calculator inputs…

At f = fc the ideal magnitude is 1/√2 = −3.0103 dB.

Low-voltage educational scope. This review does not provide mains wiring, live-work, conductor-sizing, protection-device, or safety-certification instructions.

RC Low-Pass Filter Calculator: Bode Plot, Gain & Phase

Calculate RC low-pass cutoff, gain and phase, then inspect time constant, rise time and a log-frequency Bode response table.

Bode span

Response rows

Ideal first-order RC model

The calculator uses the ideal cutoff relation f_c = 1/(2πRC) plus the standard first-order transfer magnitude and phase.

Real loading changes the response

Source resistance, load impedance, capacitor ESR/tolerance and parasitics can move the measured response. Treat this as educational small-signal analysis, not safety guidance.

Governing electrical relation

fc = 1/(2πRC) · |H| = 1/√(1+x²) — ideal first-order RC low-pass.

How to cross-check the result

Set probe frequency to cutoff and verify magnitude is 0.7071 and −3.0103 dB; far below cutoff approaches unity gain.

Real-component boundary

At f = fc the ideal magnitude is 1/√2 = −3.0103 dB. Component tolerance, temperature, parasitics, datasheet limits, installation rules, and hazardous-energy safety are not inferred by this idealized calculator.

Practical guide and verification

Cutoff is the minus-three-decibel point for the ideal first-order model

At fc = 1/(2πRC), magnitude is about 0.707 of the low-frequency voltage ratio and phase is about -45 degrees. Checking those two properties is a fast independent verification of a first-order RC result.

Source and load impedance can change the effective filter

The simple formula assumes the source is sufficiently stiff and the following load is sufficiently high impedance. Real source resistance or load resistance can alter the effective R and the transfer function, so include them in a circuit model when they are not negligible.

Component tolerances move the actual cutoff

Resistors and capacitors are manufactured with tolerance and capacitors may vary with temperature, bias, age and frequency. Use the tolerance envelope when the cutoff must stay inside a required band instead of treating the nominal value as exact.

Read the Bode plot as a trend, not a sampled guarantee

A first-order low-pass approaches a -20 dB/decade slope well above cutoff. The table and plot sample selected frequencies; a narrow resonance or additional circuit pole outside this ideal model will not appear unless it is part of the model.

Verify one point directly from the transfer function

For a frequency f, compute |H| = 1/sqrt(1+(f/fc)^2). Comparing that value with the displayed gain is a useful round-trip check before exporting a table or using the result in a design note.