RC Circuit Frequency Calculator
Calculate cutoff frequency (fc), required resistor, required capacitor, or time constant for RC low-pass and high-pass filters.
What is RC cutoff frequency?
In a basic resistor-capacitor (RC) filter, the cutoff frequency (also called corner frequency or break frequency) is the point where the output amplitude drops to about 70.7% of the input, which is -3 dB. This is the key frequency where filter behavior transitions:
- Low-pass RC filter: frequencies below cutoff pass more easily, while higher frequencies are attenuated.
- High-pass RC filter: frequencies above cutoff pass more easily, while lower frequencies are attenuated.
Core RC frequency formula
The standard cutoff equation is:
Where:
- fc = cutoff frequency in hertz (Hz)
- R = resistance in ohms (Ω)
- C = capacitance in farads (F)
- π ≈ 3.14159
The time constant is also useful:
And therefore:
How to use this RC circuit calculator
1) Pick a calculation mode
Choose one of four modes:
- Find cutoff frequency from R and C
- Find R from target frequency and C
- Find C from target frequency and R
- Find time constant from R and C
2) Enter values with units
The calculator supports practical engineering units like kΩ, MΩ, µF, nF, and pF. It converts everything internally to SI units before calculating.
3) Read the result
Results are shown in engineering notation (for example mHz, kHz, µF, kΩ), plus the corresponding time constant.
Example calculations
Example 1: Find cutoff frequency
Given R = 10 kΩ and C = 100 nF:
This is a common value for simple audio or sensor smoothing circuits.
Example 2: Find resistor for a target cutoff
Suppose you need fc = 1 kHz using C = 10 nF:
You’d select the nearest standard resistor value (for instance 16 kΩ or 15.8 kΩ depending on series availability).
Example 3: Find capacitor for debounce timing
If R = 100 kΩ and your target corner is around 20 Hz, the required capacitor is:
A standard 82 nF capacitor is a practical choice.
Design tips for RC filters
- Use realistic tolerances: resistors and capacitors have tolerance (1%, 5%, 10%), so real cutoff frequency shifts.
- Mind source/load impedance: connected stages can alter effective R and change fc.
- Check capacitor type: temperature and voltage coefficients affect capacitance stability.
- Pick standard values: choose nearest E12/E24 series parts and verify resulting cutoff.
- Simulate when needed: SPICE simulation can confirm response before hardware build.
Common mistakes
- Mixing units (for example entering µF as F).
- Forgetting that frequency is in Hz while formulas use SI base units.
- Assuming high-pass and low-pass have different cutoff equations in first-order RC (they do not).
- Ignoring component tolerances and parasitic effects at high frequency.
RC frequency calculator FAQ
Is this calculator valid for both low-pass and high-pass RC circuits?
Yes. For first-order RC filters, cutoff frequency is given by the same equation in both cases.
What is the -3 dB point physically?
It is where output power is half of passband power, and voltage amplitude is 1/√2 (about 0.707) of the input.
Can I use this for timing circuits too?
Yes. The time constant τ = RC is directly useful for charging/discharging behavior and simple delay estimates.
Final thoughts
A reliable RC circuit frequency calculator saves time and reduces design errors. Whether you’re building an analog sensor front end, shaping audio response, or creating a clean digital edge, getting the RC cutoff right is foundational. Use the calculator above to iterate quickly and then refine with real component values and measured results.