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RC Circuit Calculator

Calculate the RC time constant and cutoff frequency for any resistor–capacitor circuit.

Time constant τ
1 ms
τ = R × C = 1,000 Ω × 1 µF. The capacitor charges to ~63% of the supply voltage in one τ.
τ (seconds)
0.001 s
Cutoff frequency
159.15 Hz
5τ settling time
5 ms

How it works

  1. 1Enter the resistance in ohms and the capacitance in microfarads (µF).
  2. 2The calculator computes τ = R × C — the time to charge to ~63% of the supply.
  3. 3It also shows τ in seconds, the cutoff frequency fc = 1/(2πRC), and the 5τ settling time.

Use cases

  • Designing RC low-pass or high-pass filters for audio and signals.
  • Estimating capacitor charge and discharge times in timing circuits.
  • Choosing values for debounce circuits, oscillators, and sensor interfaces.

Frequently asked questions

What is the RC time constant?

τ = R × C (seconds) sets how fast a capacitor charges or discharges. After one τ it reaches ~63.2% of the final voltage; after 5τ it is effectively full (>99%).

How is the cutoff frequency calculated?

The −3 dB cutoff of an RC filter is fc = 1 ÷ (2π × R × C). For 1 kΩ and 1 µF, fc ≈ 159 Hz.

What does 5τ settling time mean?

After 5τ the capacitor is within 0.67% of its final value — considered fully settled. For 1 kΩ and 1 µF, 5τ = 5 ms.

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