ElectroCalc
AC/Reactive Tool · 012

R times C.
One time constant.

Enter resistance and capacitance to find the RC time constant — how long it takes a capacitor to charge or discharge through a resistor.

RC Time Constant Calculator
100% 0% 1τ
Time Constant (τ = RC) —
63.2% charged at —
~99.3% (5τ, considered "full") —
Formula: τ = R × C, in seconds. After 1τ, a charging capacitor reaches ~63.2% of full voltage; after 5τ it's considered fully charged (~99.3%). Discharging follows the same timing, just in reverse — voltage drops to ~36.8% after 1τ.

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Discussion

RC Time Constant: Formula and Practical Meaning

The RC time constant describes how quickly a resistor-capacitor circuit charges or discharges. It is represented by the Greek letter tau (τ).

Formula

τ = R × C, where R is resistance in ohms (Ω) and C is capacitance in farads (F). After one time constant, a charging capacitor reaches about 63.2% of its final voltage; during discharge, about 36.8% remains.

Example

For a 10 kΩ resistor and a 100 µF capacitor: τ = 10,000 × 0.0001 = 1 second. After about 5 seconds, the capacitor is approximately 99.3% charged.

How to Use This Calculator

  1. Enter the resistance value.
  2. Enter the capacitance and select its unit.
  3. Choose charging or discharging to view the curve.

Important Notes

The calculation assumes an ideal first-order RC circuit. Real circuits can have leakage, source resistance, component tolerances and other effects that change the measured response.

FAQs

What happens at 1τ? A charging capacitor reaches about 63.2% of its final value.

How long is a practical full charge? About 5τ is commonly used as a practical approximation for nearly complete charging.