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Low Pass Filter Cutoff Calculator

Work out low pass filter cutoff instantly with clear inputs, formula shown and shareable results.

About Low Pass Filter Cutoff Calculator

A single-pole RC low-pass filter has cutoff at 1/(2 pi RC), where output is 0.707 of input (-3 dB) and phase lag is 45 degrees. Above cutoff the response falls at 6 dB per octave, and the step response rise time is 2.2 RC.

Formula

RC low pass

fc = 1 / (2 pi R C); gain = 1 / sqrt(1 + (f/fc)^2); rise time = 2.2 R C

Worked example

With the default values below, Low Pass Filter Cutoff Calculator returns Cutoff frequency of 1,591.55. Change any input above and every figure here is recalculated in your browser.

Inputs used

Resistance
10,000 Ω
Capacitance
10 nF
Frequency of interest
5,000 Hz

Results

Cutoff frequency
1,591.55 Hz
Attenuation at that frequency
-10.36 dB
Phase shift
-72.34 °
10-90% rise time
220.00 µs

Step by step

  1. 1Values used
  2. 2RC low pass
  3. 3Cutoff frequency= 1,591.55 Hz
  4. 4Attenuation at that frequency= -10.36 dB
  5. 5Phase shift= -72.34 °
  6. 610-90% rise time= 220.00 µs

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or fees or charges into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

Frequently Asked Questions

Why -3 dB at cutoff?

At fc the capacitive reactance equals the resistance, so the output is 1/sqrt(2) of the input — half the power, which is -3 dB.

Does the filter load the source?

Yes. The input impedance falls toward R at high frequency, so drive it from a low-impedance source and load it lightly.

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