〰️ RC Time Constant & Filter Calculator
Enter a resistance and capacitance to get the RC time constant and the filter's cutoff frequency.
Cutoff frequency (−3 dB)
159.2 Hz
Time constant τ
1 ms
Time constant τ = R × C (the capacitor reaches ~63% of its final voltage in one τ, and is essentially fully charged after 5τ). The −3 dB cutoff of an RC filter is fₒ = 1 ÷ (2π·R·C) — below it a low-pass passes signals, above it an RC high-pass does. 🔒 In your browser.
How the rc time constant & filter calculator works
A resistor and capacitor together set a characteristic time and frequency. The time constant is τ = R × C — the capacitor charges to about 63% of the final voltage in one τ and is essentially full after five. The −3 dB cutoff frequency of an RC filter is fₒ = 1 ÷ (2π·R·C): a low-pass passes frequencies below it and attenuates above; a high-pass does the reverse.
The same RC pair describes both a timing circuit (how fast a capacitor charges through a resistor) and a filter (where it starts rolling off signals). Units carry through automatically here — pick kΩ and nF or Ω and µF as convenient.
Frequently asked questions
What is the RC time constant?
τ = R × C, the time for a capacitor charging through a resistor to reach about 63% of its final voltage. After five time constants it's over 99% charged. A 1 kΩ resistor and 1 µF capacitor give τ = 1 ms.
How do I calculate an RC filter's cutoff frequency?
fₒ = 1 ÷ (2π·R·C). For 1 kΩ and 1 µF that's about 159 Hz. Below the cutoff a low-pass filter passes the signal; above it, the signal is progressively attenuated.
What is the −3 dB point?
The cutoff frequency, where the output power has dropped to half (the voltage to about 70.7%) of the passband — a fall of 3 decibels. It marks the edge between the passed and attenuated bands.
What's the difference between a low-pass and high-pass RC filter?
Both use the same R and C and share the same cutoff; the difference is where you take the output. Across the capacitor gives a low-pass (passes low frequencies); across the resistor gives a high-pass (passes high frequencies).
How long until a capacitor is fully charged?
Practically, about five time constants (5·R·C) — it reaches ~63% after one τ, ~86% after two, ~95% after three and >99% after five. The tool shows τ so you can multiply.