RC High-Pass Filter Calculator
Enter resistor and capacitor values to calculate cutoff frequency. Optionally add signal frequency and input voltage to estimate gain, phase shift, and output voltage.
Formula: fc = 1 / (2πRC). At cutoff, output is 70.7% of input (-3 dB).
What this RC high-pass filter calculator does
An RC high-pass filter allows high frequencies to pass while attenuating low frequencies. This calculator helps you quickly determine the cutoff frequency for a resistor-capacitor pair and, when frequency is provided, the expected gain and phase response at that point.
If you are designing an audio coupling stage, sensor conditioning front-end, or a simple analog pre-filter before an ADC, this tool gives you fast, practical numbers without manual equation work every time.
Core equations used
Time constant: τ = RC
Angular frequency: ω = 2πf
Magnitude (gain ratio): |H(jω)| = (ωRC) / √(1 + (ωRC)²)
Gain in dB: 20 log10(|H|)
Phase shift (degrees): φ = arctan(1 / (ωRC))
Where:
- R is resistance in ohms (Ω)
- C is capacitance in farads (F)
- f is signal frequency in hertz (Hz)
- τ is the RC time constant in seconds
How to use the calculator
Step 1: Enter R and C
Type resistor and capacitor values and choose units (kΩ, µF, etc.). The script converts everything to base SI units automatically.
Step 2: (Optional) Enter signal frequency
If you add a frequency, you’ll get gain ratio, decibel gain, phase shift, and capacitor reactance at that point.
Step 3: (Optional) Enter input voltage
When input voltage is supplied along with frequency, the calculator estimates output amplitude using Vout = Vin × |H|.
Worked example
Suppose you choose:
- R = 10 kΩ
- C = 100 nF
The time constant is:
The cutoff frequency is:
So frequencies well above 159 Hz pass with little attenuation, while low-frequency content (including DC) is suppressed.
Practical component selection tips
- Pick C first if you are constrained by available capacitor types (film, ceramic, electrolytic).
- Pick R first if your stage has input impedance limits.
- Keep resistor values moderate (often 1 kΩ to 100 kΩ) to avoid excessive noise or loading issues.
- For precision filters, use tighter-tolerance parts (1% resistors, stable dielectric capacitors).
- Remember source and load impedances can shift real-world cutoff frequency.
Where RC high-pass filters are commonly used
- Audio coupling between amplifier stages
- Removing DC offset before measurement
- Conditioning sensor outputs
- Simple edge-detection and transient emphasis circuits
- Front-end filtering in mixed-signal systems
Quick FAQ
Why is output lower at cutoff?
At cutoff, the output amplitude is exactly 1/√2 of input (about 0.707), which corresponds to -3 dB.
Can I cascade multiple RC high-pass stages?
Yes. Cascading increases filter order and steepens roll-off, but total response depends on interaction between stages and loading effects.
Is this tool good for precision analog design?
It is excellent for first-pass sizing and verification. Final design should include tolerances, impedance interactions, and simulation or lab measurement.
Final note
This RC high-pass filter calculator is designed to be quick and practical: enter values, get cutoff and response, then iterate component choices until your design target is met.