MasterTool

RC Filter Calculator

Find the −3 dB cutoff frequency and time constant of a first-order RC low-pass or high-pass filter.

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What this tool does

Find the −3 dB cutoff frequency and time constant of a first-order RC low-pass or high-pass filter. Unloaded, ideal components. Source resistance adds to R and load resistance modifies the effective values — see the FAQs.

How to use the RC Filter Calculator

  1. Enter or select filter type.
  2. Enter or select resistance r (ohms).
  3. Enter or select capacitance c (µf).
  4. Read the calculated result; change any measurement to compare alternatives.

Formula

cutoff fc = 1 ÷ (2πRC); time constant τ = RC
type
Filter type
r
Resistance R (ohms)
c
Capacitance C (µF)

Unloaded, ideal components. Source resistance adds to R and load resistance modifies the effective values — see the FAQs.

Worked example

For rc filter calculator, the following measurements illustrate the exact method: Filter type: low; Resistance R (ohms): 1000; Capacitance C (µF): 0.1.

Inputs

  • Filter typeLow-pass
  • Resistance R (ohms)1000
  • Capacitance C (µF)0.1

Result

  • Cutoff frequency (Hz)1,591.55
  • Time constant τ (ms)0.1
  • Angular cutoff frequency (rad/s)10,000

Results explained

Cutoff frequency (Hz)
Cutoff frequency (Hz) from the formula above. Unloaded, ideal components. Source resistance adds to R and load resistance modifies the effective values — see the FAQs.
Time constant τ (ms)
Time constant τ (ms) from the formula above. Unloaded, ideal components. Source resistance adds to R and load resistance modifies the effective values — see the FAQs.
Angular cutoff frequency (rad/s)
Angular cutoff frequency (rad/s) from the formula above. Unloaded, ideal components. Source resistance adds to R and load resistance modifies the effective values — see the FAQs.

Frequently asked questions

fc = 1 ÷ (2πRC). With R = 1 kΩ and C = 0.1 µF, fc = 1 ÷ (2π × 1,000 × 0.0000001) ≈ 1,592 Hz.

The output is 3 dB down (about 70.7% of the passband voltage) and shifted 45° in phase. Above (low-pass) or below (high-pass) cutoff the response falls at 20 dB per decade for this single-pole filter.

Yes — swapping the positions of R and C turns a low-pass into a high-pass with the same cutoff frequency for the same component values.

τ = R × C, the time to charge to about 63% of a step input. It is the reciprocal of the angular cutoff frequency: fc = 1 ÷ (2πτ).

Yes. A load resistance in parallel with the capacitor (low-pass) lowers the effective resistance and raises fc; keep the load much larger than R for the nominal value.