Cutoff Frequency Calculator — RC, RL & LC Filters
Find the −3 dB cutoff frequency of a passive RC, RL or LC filter. Enter the component values and get the cutoff frequency in Hz, angular frequency in rad/s, and the time constant — plus a simplified frequency-response view.
Filter type
Ω
F
Frequency at which output power drops to −3 dB (≈ 70.7 % of input amplitude)
- 1
RC product
1,000 × 0.000001 = 0.001 - 2
2π × RC
2π × 0.001 = 0.006283 - 3
Cutoff frequency (Hz)
1 ÷ 0.006283 = 159.155
How does this calculator work?
The −3 dB cutoff frequency marks where a filter's output drops to 70.7 % of its input. For an RC filter, f = 1/(2πRC); for RL, f = R/(2πL); for LC, f = 1/(2π√(LC)). Enter component values to get the cutoff in Hz, angular frequency in rad/s, and the time constant τ.
Formula
How this is calculated
Every passive filter has a characteristic frequency — the cutoff (or corner) frequency — at which the output power falls to half of its pass-band value, equivalent to a −3 dB reduction in signal level or an amplitude of about 70.7 % (1/√2) of the input. Below this frequency a low-pass filter passes the signal largely intact; above it the output rolls off at 20 dB per decade for a first-order filter.
For an RC filter, the cutoff is set by 1/(2πRC): a larger resistor or capacitor shifts it lower. For an RL filter, increasing inductance lowers it while increasing resistance raises it, giving f = R/(2πL). An LC circuit (often called a resonant tank circuit) has no resistor; its natural resonant frequency is 1/(2π√(LC)) — lower inductance or capacitance pushes the frequency up. The time constant τ (RC or L/R) is the reciprocal of ω = 2πf and governs how fast the circuit reacts to a step input.
These formulas assume ideal components: no parasitic resistance, inductance or capacitance. Real inductors have DC resistance; real capacitors have equivalent series resistance (ESR); both shift the actual cutoff from the theoretical value. For precision work, a SPICE simulation or impedance analyser measurement is recommended.
Frequently asked questions
At the cutoff frequency the circuit output power is exactly half of the input power (−3 dB). In terms of voltage or current amplitude, it is 1/√2 ≈ 0.707 of the input. Frequencies below the cutoff pass through a low-pass filter; frequencies above are attenuated.
Enter values in the base SI unit: 1 µF = 0.000001 F, 1 nF = 0.000000001 F, 1 mH = 0.001 H, 1 µH = 0.000001 H. You can paste the decimal directly into the field.
The cutoff frequency is the same for both; only the output tap changes. In a series RC circuit, taking the voltage across the capacitor gives a low-pass response; taking it across the resistor gives a high-pass response. Both share the same f = 1/(2πRC).
Also known as
TG we-Calculate Editorial Team. (2026). Cutoff Frequency Calculator — RC, RL & LC Filters [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/cutoff-frequency-calculator
TG we-Calculate Editorial Team. "Cutoff Frequency Calculator — RC, RL & LC Filters." TG we-Calculate. 2026. https://we-calculate.com/calculator/cutoff-frequency-calculator.
TG we-Calculate Editorial Team, "Cutoff Frequency Calculator — RC, RL & LC Filters," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/cutoff-frequency-calculator
@misc{wecalculate_cutoff_frequency_calculator, title = {Cutoff Frequency Calculator — RC, RL & LC Filters}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/cutoff-frequency-calculator}}, year = {2026}, note = {TG we-Calculate} }
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