RC Filter Cutoff Frequency Calculator

How to use the RC filter cutoff frequency calculator

Enter R and C to get the cutoff frequency and time constant. Add a test frequency to see how much a low-pass or high-pass filter attenuates it. To design a filter, enter the target cutoff and one component, then press × on the other.

For more background, read RC and RL Time Constant Explained.

RC filter cutoff frequency formulas

f_c = 1 ÷ (2πRC)

Low-pass gain = −10 log₁₀[1 + (f/f_c)²] dB  ·  High-pass gain = −10 log₁₀[1 + (f_c/f)²] dB

Common RC combinations

R	C	Cutoff
100 kΩ	10 µF	0.16 Hz
10 kΩ	10 µF	1.59 Hz
10 kΩ	1 µF	15.92 Hz
10 kΩ	100 nF	159.15 Hz
1 kΩ	100 nF	1.59 kHz
1 kΩ	10 nF	15.92 kHz
100 Ω	1 nF	1591.55 kHz

Low-pass response

Frequency	Gain	Output (of input)
0.1 fc	−0.04 dB	99.5%
0.5 fc	−0.97 dB	89.4%
fc	−3.01 dB	70.7%
2 fc	−6.99 dB	44.7%
10 fc	−20.0 dB	9.95%
100 fc	−40.0 dB	1.0%

Practical tips

• The source impedance adds to R; the load impedance appears in parallel with C. Keep the load at least ten times R.
• For ADC anti-aliasing, set the cutoff below half the sampling rate.
• Use C0G/NP0 or film capacitors for stable filters; ceramic X7R values change with voltage and temperature.

Frequently asked questions

What is the cutoff frequency of an RC filter?

fc = 1 ÷ (2πRC). At this frequency the output is 70.7% of the input (−3 dB). 10 kΩ and 100 nF give 159 Hz.

What is the difference between low-pass and high-pass?

A low-pass filter (R in series, C to ground) passes low frequencies; a high-pass filter (C in series, R to ground) passes high frequencies. Both have the same cutoff formula.

How steep is an RC filter?

First order — 20 dB per decade (6 dB per octave) beyond the cutoff. For steeper roll-off, cascade stages or use an active filter.

How do I filter noise on a sensor signal?

Choose a cutoff well above the signal bandwidth but below the noise frequency — for slow process signals, a few hertz is common.

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