Filter Calculator
RC Filter Calculator
Calculate the cutoff frequency and time constant (τ) for RC low-pass and high-pass filter circuits. Enter resistance and capacitance values in any unit.
Common Design Presets
Quick-fill standard RC filter values used in real projects.
Enter R and C Values
Enter resistance in Ohms and capacitance in Farads (use scientific notation, e.g. 1e-6 for 1µF).
Ohms (Ω) — e.g. 10000 for 10kΩ
Farads — e.g. 0.000001 for 1µF
Cutoff Frequency (fc)
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Time Constant (τ)
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Formulas & Theory
Cutoff Frequency:
f_c = 1 / (2π × R × C)
Time Constant:
τ (tau) = R × C (seconds)
Key Behaviour:
At f_c → signal is −3 dB (≈70.7% amplitude)
At 5τ → capacitor is ~99.3% fully charged
At 10τ → considered fully charged
Low-Pass Filter: passes DC and low frequencies, attenuates high frequencies above f_c
High-Pass Filter: same R and C, output taken across R instead of C — passes high frequencies
Unit conversions:
1 µF = 1×10⁻⁶ F · 1 nF = 1×10⁻⁹ F · 1 pF = 1×10⁻¹² F
1 kΩ = 1000 Ω · 1 MΩ = 1,000,000 Ω
f_c = 1 / (2π × R × C)
Time Constant:
τ (tau) = R × C (seconds)
Key Behaviour:
At f_c → signal is −3 dB (≈70.7% amplitude)
At 5τ → capacitor is ~99.3% fully charged
At 10τ → considered fully charged
Low-Pass Filter: passes DC and low frequencies, attenuates high frequencies above f_c
High-Pass Filter: same R and C, output taken across R instead of C — passes high frequencies
Unit conversions:
1 µF = 1×10⁻⁶ F · 1 nF = 1×10⁻⁹ F · 1 pF = 1×10⁻¹² F
1 kΩ = 1000 Ω · 1 MΩ = 1,000,000 Ω