At the cutoff frequency, a first-order filter passes half the power (−3 dB, or 70.7% of the voltage). Below it a low-pass filter passes signals; above it a high-pass filter does. Audio crossovers, anti-aliasing filters and tone controls are built this way.
How to use the Cutoff Frequency Calculator
- Circuit
- Resistance R
- Capacitance C
- Inductance L
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose RC or RL and enter the component values.
Inputs explained
| Field | What to enter |
|---|---|
| Circuit | Choose one: RC, RL |
| Resistance R | Units: kΩ, Ω, MΩ |
| Capacitance C | Units: nF, µF, pF, F |
| Inductance L | Units: mH, µH, nH, H |
Example values
How the main result changes when Resistance R changes and everything else stays at the example values above:
| Resistance R (kΩ) | Cutoff frequency f_c (Hz) |
|---|---|
| 5 | 318.3099 |
| 7.5 | 212.2066 |
| 10 | 159.1549 |
| 12.5 | 127.324 |
| 15 | 106.1033 |
| 20 | 79.5775 |
Formula
RC: f_c = 1/(2πRC), τ = RC · RL: f_c = R/(2πL), τ = L/R
Worked example
Inputs — Circuit: RC; Resistance R: 10 kΩ; Capacitance C: 100 nF.
Results — Cutoff frequency f_c = 159.15494 Hz; Time constant τ = 1 ms; Angular cutoff ω_c = 1,000 rad/s.
Key terms
- Cutoff frequency
- Frequency at which output falls by 3 dB.
Frequently asked questions
What does −3 dB mean?
Half the power, or 70.7% of the amplitude.
Is it the same for low- and high-pass?
Yes – the same formula, different arrangement.
How steep is a first-order filter?
20 dB per decade (6 dB per octave).
How do I get a sharper filter?
Use higher-order filters (more poles).