RC Filter Calculator (Low-Pass / High-Pass)
Enter any two values and leave the third blank. The calculator will solve for the missing value using the first-order RC formula.
Formula used: fc = 1 / (2πRC). This is the -3 dB cutoff for a first-order passive RC network.
What this RC filter calculator does
This online RC filter calculator helps you quickly design first-order low-pass and high-pass filters. In practical terms, it tells you how resistor value, capacitor value, and cutoff frequency are related. Whether you are cleaning sensor noise, shaping audio tone, or building simple analog front ends, this saves manual recalculation and unit-conversion mistakes.
RC filter formula quick reference
- Cutoff frequency:
fc = 1 / (2πRC) - Resistance:
R = 1 / (2πfcC) - Capacitance:
C = 1 / (2πfcR) - Time constant:
τ = RC - Angular cutoff frequency:
ωc = 2πfc
Low-pass and high-pass first-order RC filters share the same cutoff equation. The difference is where you take the output node in the circuit.
How to use this calculator
Step-by-step
- Select your filter type (low-pass or high-pass).
- Enter any two values: R, C, or cutoff frequency.
- Choose the unit for each field (for example kΩ, µF, kHz).
- Leave the value you want solved as blank.
- Click Calculate to get the missing value plus extra design data.
Example
If you set R = 10 kΩ and C = 100 nF, the cutoff frequency is about 159.15 Hz. This is a common value for basic signal smoothing in microcontroller circuits.
Low-pass vs high-pass in plain language
Low-pass RC filter: passes low frequencies and attenuates higher ones. Great for noise reduction and smoothing.
High-pass RC filter: passes higher frequencies and attenuates lower ones. Useful for AC coupling and removing DC offset.
At the cutoff frequency, output magnitude is approximately 70.7% of input (or -3 dB), regardless of whether it is low-pass or high-pass.
Practical design tips
- Use standard component values (E12/E24 series) and re-check the resulting cutoff frequency.
- Consider component tolerances. A 5% resistor and 10% capacitor can shift cutoff noticeably.
- Large resistors increase susceptibility to noise and bias-current effects.
- Very small capacitors can be sensitive to PCB parasitics and probe capacitance.
- Account for source and load impedance; ideal formulas assume no loading effects.
Common use cases
Sensor signal smoothing
For slow-changing sensor data, a low-pass RC filter can suppress high-frequency noise before ADC sampling.
Audio input coupling
A high-pass RC stage can block DC while allowing audio frequencies through, protecting downstream amplifier bias points.
Debounce and edge shaping
In digital interfaces, RC networks can reduce switch bounce and soften aggressive edges that cause EMI.
FAQ
Does this work for active filters?
This tool is for the basic first-order RC corner frequency relationship. Active filters add op-amp behavior and may require additional equations.
Why is my measured cutoff different from the calculation?
Real-world variation comes from component tolerance, loading, source impedance, and measurement setup. Simulate and prototype if precision matters.
Can I use Hz, kHz, and MHz directly?
Yes. Unit selectors are built in, and conversions are handled automatically by the calculator.