RC Cutoff Frequency Calculator
Enter resistance and capacitance to calculate the -3 dB cutoff frequency for a first-order RC filter.
The RC filter is one of the most important building blocks in electronics. Whether you are smoothing sensor noise, building an audio crossover, or shaping a control signal, knowing the cutoff frequency helps you design circuits that behave as expected.
What this RC filter frequency calculator gives you
- Cutoff frequency (fc) in Hz, kHz, MHz, etc.
- Time constant (τ) to understand transient response speed
- Angular cutoff frequency (ωc) in rad/s
The same equation applies to both first-order RC low-pass and high-pass filters. What changes is where you measure output and how the circuit is arranged.
RC cutoff frequency formula
The fundamental relationship is:
fc = 1 / (2πRC)
Where:
- R is resistance in ohms (Ω)
- C is capacitance in farads (F)
- fc is cutoff frequency in hertz (Hz)
At cutoff, the output magnitude is 0.707 of passband level (about -3 dB), and phase shift is ±45° for a first-order RC filter.
How to use this calculator correctly
1) Enter effective resistance
Use the resistance that actually interacts with the capacitor. In real circuits, source and load impedances can change the effective value.
2) Enter capacitor value with the right unit
Most mistakes come from unit conversion: µF vs nF vs pF. Double-check before calculating.
3) Interpret the result in context
Cutoff is a transition point, not a brick-wall edge. Signals above or below cutoff are attenuated gradually at about 20 dB/decade for first-order filters.
Worked example
Suppose you choose R = 10 kΩ and C = 100 nF.
- R = 10,000 Ω
- C = 100 × 10-9 F
- RC = 0.001 s
- fc = 1 / (2π × 0.001) ≈ 159.15 Hz
This is a common value for smoothing low-frequency noise in control or sensor front ends.
Practical design tips
- Component tolerance matters: a 5% resistor and 10% capacitor can shift cutoff noticeably.
- Loading changes response: if the next stage has low input impedance, effective R may be lower.
- Pick realistic values: very large R can increase noise and leakage sensitivity; very large C can be bulky.
- Account for parasitics at high frequency: lead inductance and capacitor ESR become relevant.
Low-pass vs high-pass RC filters
RC Low-pass
Passes low frequencies and attenuates high frequencies. Usually output is measured across the capacitor.
RC High-pass
Passes high frequencies and attenuates low frequencies. Usually output is measured across the resistor.
Both use the same cutoff equation, which is why this calculator works for either type.
Quick FAQ
Is this calculator for active filters?
It is for first-order passive RC sections. Active filters can still use RC sections, but amplifier topology affects overall transfer function.
What happens at exactly cutoff frequency?
Output magnitude is 70.7% of passband level and power is half, corresponding to -3 dB.
Can I solve for R or C instead?
Yes. Rearranging gives R = 1/(2πfcC) or C = 1/(2πfcR).
Use the calculator above to iterate quickly, then validate with simulation or bench measurement for final designs.