Case 1
- Resistance R (Ω)
- 1000Ω
- Capacitance C (µF)
- 1µF
159.1549Hz
Open with these valuesRC Low-Pass Filter Calculator
159.1549Hz
Result: 159.1549 HzA resistor in series with a capacitor to ground passes the slow part of a signal and holds back the fast part. The boundary is the −3 dB cutoff, where the output has fallen to about 70.7 % of the input. Enter ohms and microfarads: 100 nF is 0.1 µF, 1 nF is 0.001 µF.
Worked examples
Case 2
- Resistance R (Ω)
- 4700Ω
- Capacitance C (µF)
- 0.022µF
1,539.2161Hz
Open with these valuesCase 3
- Resistance R (Ω)
- 100000Ω
- Capacitance C (µF)
- 0.001µF
1,591.5494Hz
Open with these valuesHow it's calculated
f_c = 1 / (2π × R × C)
- StepEnter the resistor value in ohms — 4.7 kΩ is 4700.
- StepEnter the capacitor value in microfarads — 22 nF is 0.022.
- ResultRead the cutoff frequency; the time constant is 1 / (2π × f_c).
Reference table
| R (Ω), C (µF) | Time constant τ | Cutoff (Hz) |
|---|---|---|
| 1000, 1 | 1 ms | 159.1549 |
| 10000, 0.1 | 1 ms | 159.1549 |
| 100000, 0.001 | 100 µs | 1591.5494 |
| 4700, 0.022 | 103.4 µs | 1539.2161 |
| 2200, 10 | 22 ms | 7.2343 |
Questions
How do I calculate the cutoff frequency of an RC low-pass filter?
Divide 1 by 2π times the resistance times the capacitance in farads: f_c = 1 / (2π × R × C). For a 1 kΩ resistor and a 1 µF capacitor that is 159.1549 Hz. Convert the capacitance from microfarads to farads first — the calculator does it for you.
What does the cutoff frequency mean?
The cutoff is the −3 dB point of the filter. Exactly there the output amplitude has dropped to about 70.7 % of the input, and the power to half. Below it signals pass almost untouched; above it they roll off at 6 dB per octave.
Do I enter the capacitance in farads or microfarads?
In microfarads (µF), and this is the one mistake that ruins the answer without looking wrong. A 100 nF capacitor is 0.1 µF, a 10 nF capacitor is 0.01 µF and a 1 nF capacitor is 0.001 µF. Entering raw farads gives a cutoff a million times too high.
What is the time constant of an RC filter?
The time constant is τ = R × C in seconds, and it is the same fact seen in the time domain: f_c = 1 / (2πτ). After one time constant the capacitor voltage reaches about 63 % of a step change, after five it is within 1 %. The reference table below lists τ next to each cutoff.
What is an RC low-pass filter used for?
It passes low frequencies and attenuates high ones. It smooths a noisy DC supply, debounces switches and sensors, strips high-frequency noise and rolls off treble in audio. One resistor in series with one capacitor to ground is the whole circuit.
Sources and last check
Information, not professional advice.