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RC filter cutoff calculator

Cutoff frequency from R and C — and reverse: pick the resistor for the cutoff you want.

Cutoff frequency (−3 dB)1 / (2π · R · C)159.15 Hz
At the cutoffsignal is at 70.7%, not gone — rolloff is 20 dB/decade after−3 dB · 45° phase
Reverse: pick R for a target cutoff (choose C first — fewer standard values)
Exact R1.59 kΩ
Nearest standard (E24)1.6 kΩ
Actual cutoff with it-0.5% off target994.72 Hz

The circuit this computes

RC Low-Pass Filter — fully explained →

How it works

The resistor and capacitor form a frequency-dependent divider: the cap's impedance falls as frequency rises, so high frequencies divide away. The knee sits where the two impedances are equal: fc = 1 / (2π·R·C).

Common questions

What actually happens at the cutoff frequency?

The signal is at 70.7% (−3 dB) and shifted 45° — NOT blocked. An RC filter is a gentle slope (20 dB per decade), not a wall: content an octave above the cutoff still gets through at half amplitude. If you need a wall, you need more poles (or an active filter).

Filter before or after the amplifier?

Both have costs: filtering first attenuates signal you then amplify (noise too); amplifying first means the amp must handle the unfiltered interference. The usual answer for sensor chains: gentle RC right at the ADC pin, real filtering earlier in the chain.

Why choose C first in the reverse mode?

Capacitors come in far fewer standard values than resistors, and odd capacitances are annoying to buy. Pick a friendly C (1n, 10n, 100n), let R absorb the precision — E24 resistors get you within a few percent of any cutoff.

Design it in the editor — freeLive electrical checks, automatic BOM, KiCad export.