RC Filter Cutoff

In a simple RC low-pass or high-pass filter, the cutoff frequency is the point where the output power has dropped to half its passband value, which corresponds to an output amplitude of about 70.7% of the low-frequency or high-frequency level. It comes from comparing the resistor and capacitor impedances: at the cutoff, the magnitude of the capacitive reactance equals the resistance, so the circuit’s voltage divider response reaches its transition point. Because capacitive reactance decreases as frequency rises, this formula shows that larger R or C shifts the cutoff lower.

Use this formula when designing or analyzing first-order RC filters in audio, sensor conditioning, timing circuits, or basic signal smoothing. It is also useful whenever you need the frequency at which a capacitor starts to significantly pass or block alternating signals.

• Forgetting to convert kΩ, nF, or µF into ohms and farads before calculating
• Using this formula for higher-order filters, where the cutoff is not set by a single RC pair alone
• Confusing cutoff frequency with the exact point where the signal becomes zero instead of the -3 dB transition point