RC Cutoff Frequency Calculator
Use this tool to calculate cutoff frequency (fc), resistance (R), or capacitance (C) in a first-order RC circuit.
Tip: For a 10 kΩ resistor and 100 nF capacitor, the cutoff frequency is about 159.15 Hz.
What is RC frequency?
In a basic resistor-capacitor (RC) circuit, the “RC frequency” usually means the cutoff frequency (also called corner frequency or break frequency). At this frequency, the output amplitude drops to about 70.7% of the input (−3 dB point).
This matters because RC circuits are the foundation of many analog electronics tasks: filtering noise, shaping signals, timing delays, and conditioning sensor outputs.
Core formula
For a first-order RC network, the cutoff frequency is:
- fc = cutoff frequency in hertz (Hz)
- R = resistance in ohms (Ω)
- C = capacitance in farads (F)
- π ≈ 3.14159
Related concept: the time constant is τ = RC. A larger RC product means slower response and lower cutoff frequency.
How to use this calculator
1) Calculate cutoff frequency
Select Cutoff Frequency (f), enter R and C, then click Calculate.
2) Solve for resistance
Select Resistance (R), enter target frequency and capacitor value, then calculate the required resistor.
3) Solve for capacitance
Select Capacitance (C), enter target frequency and resistor value, then calculate the required capacitor.
Quick unit reference
| Quantity | Unit | Multiplier |
|---|---|---|
| kΩ | kilo-ohm | 1,000 Ω |
| MΩ | mega-ohm | 1,000,000 Ω |
| µF | microfarad | 10−6 F |
| nF | nanofarad | 10−9 F |
| pF | picofarad | 10−12 F |
Practical design notes
- Low-pass and high-pass: both use the same cutoff formula; only component arrangement changes output behavior.
- Tolerance matters: real resistors and capacitors have tolerances (e.g., ±1%, ±5%, ±10%), shifting actual cutoff frequency.
- Loading effects: following stages can alter the effective resistance and move the true corner frequency.
- Temperature drift: component values can drift with temperature, especially certain capacitor types.
Example calculations
Example A: Find cutoff frequency
If R = 4.7 kΩ and C = 47 nF:
fc = 1 / (2π × 4700 × 47×10−9) ≈ 720.5 Hz
Example B: Pick R for a target cutoff
For f = 1 kHz and C = 10 nF:
R = 1 / (2πfC) ≈ 15.9 kΩ
Common mistakes to avoid
- Mixing units (for example, entering nF as if it were µF).
- Using zero or negative values for R, C, or f.
- Ignoring source and load impedance in real circuits.
- Expecting a “brick-wall” filter from a single RC stage (roll-off is only 20 dB/decade).
When to use an RC filter
RC filters are ideal for lightweight analog tasks: sensor smoothing, simple anti-aliasing, signal coupling/decoupling, and audio tone shaping. For sharper cutoff characteristics, designers typically cascade stages or move to active filter topologies.