RC Low-Pass Cutoff Calculator
An RC low-pass filter passes low-frequency signals while attenuating high-frequency ones, and its defining characteristic is the cutoff frequency, the point at which the output power falls to half the input, three decibels down. That cutoff is set by just two components: a resistor and a capacitor. The cutoff frequency equals one divided by two pi times the resistance times the capacitance, so a larger resistor or capacitor lowers the cutoff and lets through a narrower band of low frequencies. This calculator takes the resistance in ohms and the capacitance in farads and returns the cutoff frequency in hertz to two decimal places. Use the matching units, for example a 100 nanofarad capacitor entered as 0.0000001 farads. The same formula describes the corner frequency of a first-order RC stage whether it is used as a simple filter, a smoothing network or part of a timing circuit. The definitions of the ohm, the farad and the hertz are maintained by the National Institute of Standards and Technology. Every figure here is computed deterministically from the standard cutoff formula, shown in full below, with a worked example that reconciles exactly to the calculator so you can verify each step yourself and check the arithmetic.
The RC low-pass cutoff frequency is f = 1 / (2 pi R C). With 1,000 ohms and 100 nF (0.0000001 F), the cutoff is 1,591.55 Hz. A larger R or C lowers the cutoff.
RC low-pass cutoff formula
f = 1 / (2 pi R C)
f = cutoff (corner) frequency (Hz)
R = resistance (ohms)
C = capacitance (farads)
Multiply the resistance by the capacitance to get the RC time constant, multiply by two pi, then take the reciprocal. The result is the frequency at which the filter's output power has dropped to half (3 dB down). Increasing either component lowers the cutoff.
Worked example
An RC low-pass filter uses a 1,000 ohm resistor and a 100 nF capacitor, which is 0.0000001 farads.
- Multiply R by C: 1,000 x 0.0000001 = 0.0001
- Multiply by 2 pi: 2 x 3.14159265 x 0.0001 = 0.00062832
- Take the reciprocal: 1 / 0.00062832 = 1,591.55
- The cutoff frequency is 1,591.55 Hz
So the filter's cutoff frequency is about 1,591.55 Hz. These are the calculator's default inputs, so the result above matches the widget exactly.
RC Low-Pass Cutoff Calculator: frequently asked questions
How do you calculate an RC filter cutoff frequency?
The cutoff is f = 1 / (2 pi R C), where R is the resistance in ohms and C the capacitance in farads. With R = 1,000 ohms and C = 100 nF, the cutoff is 1 / (2 pi x 1,000 x 0.0000001) = 1,591.55 Hz.
What does the cutoff frequency mean?
At the cutoff frequency the filter's output power has fallen to half the input, which is 3 dB down in amplitude terms. Below it, low frequencies pass nearly unchanged; above it, signals are increasingly attenuated at 20 dB per decade for a first-order filter.
Is the formula the same for a high-pass filter?
Yes, a first-order RC high-pass filter has the identical cutoff formula, f = 1 / (2 pi R C). The difference is in how R and C are arranged, which determines whether low or high frequencies pass.
How do I enter nanofarads or microfarads?
Convert to farads first. One nanofarad is 0.000000001 F and one microfarad is 0.000001 F. So 100 nF is 0.0000001 F, which is what this example uses.
What is the RC cutoff frequency formula?
The corner frequency of a first-order RC filter is f = 1 / (2 pi R C).
Official sources
- Electrical units and measurement standards: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.