Calculators
RC Filter Cutoff Calculator
Calculate the RC filter cutoff frequency fc = 1 / (2πRC), or solve for the resistance or capacitance needed for a target cutoff, locally in your browser.
Use the RC filter cutoff formula fc = 1 ÷ (2 × π × R × C) to find the -3 dB cutoff frequency of a resistor-capacitor low-pass or high-pass filter, or to solve for the resistance or capacitance needed for a target cutoff. Enter positive values with matching unit families, then choose which quantity to solve for. Calculations run locally in your browser; your values are not uploaded or stored.
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Find the -3 dB cutoff of an RC filter.
The cutoff frequency of a resistor-capacitor low-pass or high-pass filter is fc = 1 ÷ (2 × π × R × C), the frequency where the output power drops to half (-3 dB) and the output voltage falls to about 70.7% of the input. Enter resistance and capacitance in any convenient units to find the cutoff, or enter a target cutoff with one component to solve for the other. For example, 10 kΩ with 100 nF gives fc ≈ 159.15 Hz, and a 1 kHz cutoff with 10 kΩ needs C ≈ 15.92 nF. The same cutoff applies to both low-pass and high-pass RC topologies; only the output tap changes. Results are reported in automatically scaled units. This assumes ideal components: no tolerance error, parasitic inductance or capacitance, or source and load impedance effects. All calculations run locally in your browser; your values are not uploaded or stored.
Frequently Asked Questions
Everything you need to know about this tool, how it works, and privacy.
What is the RC filter cutoff frequency?
The cutoff frequency fc of a resistor-capacitor low-pass or high-pass filter is fc = 1 ÷ (2 × π × R × C), where R is resistance in ohms and C is capacitance in farads. It is the frequency at which the output power drops to half (-3 dB) and the output voltage falls to about 70.7% of the input voltage. Frequencies pass relatively unchanged on one side of the cutoff and are attenuated on the other.
How do I solve for resistance or capacitance at a target cutoff?
Rearrange fc = 1 ÷ (2 × π × R × C). For resistance use R = 1 ÷ (2 × π × fc × C), and for capacitance use C = 1 ÷ (2 × π × fc × R). For example, a 1 kHz cutoff with 10 kΩ needs C = 1 ÷ (2 × π × 1000 × 10000) ≈ 15.92 nF, and a 1 kHz cutoff with 100 nF needs R ≈ 1.59 kΩ.
Does the formula differ for low-pass and high-pass filters?
No. A first-order RC low-pass filter (resistor in series, capacitor to ground) and a first-order RC high-pass filter (capacitor in series, resistor to ground) have the same cutoff frequency fc = 1 ÷ (2 × π × R × C). Only the output tap changes: the low-pass takes the output across the capacitor, and the high-pass takes it across the resistor.
Which units can I use?
Resistance accepts ohms (Ω), kilo-ohms (kΩ), mega-ohms (MΩ), and giga-ohms (GΩ); capacitance accepts picofarads (pF), nanofarads (nF), microfarads (µF), millifarads (mF), and farads (F); the cutoff frequency accepts hertz (Hz), kilohertz (kHz), megahertz (MHz), and gigahertz (GHz). Results are shown in automatically scaled units.
What are the limitations of this model?
The calculator assumes ideal components with no tolerance error, no parasitic inductance or capacitance, and no source or load impedance effects. Real filters shift cutoff with component tolerances, loading, and parasitics, and higher-order filters have different responses. It is an educational estimate for design intuition, not a substitute for measurement or simulation, and all calculations run locally in your browser.