Calculate an RC circuit’s cutoff frequency and time constant (τ = RC) from resistance and capacitance, or solve for R, C, or charge and discharge time.
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RC Circuit Formula
The following equation can be used to calculate the cutoff (corner) frequency of a first‑order RC network (such as a simple low‑pass or high‑pass RC filter).
- Where fc is the cutoff frequency (Hz)
- R is the effective resistance seen by the capacitor (ohms, Ω)
- C is the capacitance (farads, F)
This equation applies to first‑order (single‑pole) RC networks, where the relevant time constant is τ = RC. In many practical circuits, R is not simply the “sum of all resistors,” but the equivalent (Thevenin) resistance seen by the capacitor in the operating condition being analyzed. Common uses of RC networks include filtering (setting a cutoff frequency) and timing/delay functions (using the time constant to shape charge and discharge behavior). The energy stored in a capacitor depends on both capacitance and voltage: E = ½ C V².
RC Circuit Definition
An RC circuit contains resistance and capacitance; it may also include a voltage source, switches and other components.
How to calculate cutoff frequency of an RC filter?
How to calculate the cutoff frequency of an RC filter?
- First, measure the resistance.
Measure the effective resistance seen by the capacitor (often the Thevenin resistance).
- Next, measure the capacitance.
Measure the capacitance of the capacitor.
- Finally, calculate the cutoff frequency.
Use the equation above to calculate the cutoff frequency.
FAQ
An RC circuit contains resistance and capacitance; it may also include a voltage source, switches and other components.
