RC time constant calculator
Enter resistance and capacitance in convenient units to see how fast the capacitor charges or discharges in delay and timer circuits.
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How to use it
- Enter resistance, resistance unit, capacitance and capacitance unit.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
τ = R × C. The charge level after time t is 1 − e^(−t ÷ τ): 63.2% after τ and 99.3% after 5τ.
Example
Inputs: Resistance = 10, Resistance unit = kilohm (kΩ), Capacitance = 100, Capacitance unit = microfarad (µF). Result: Time constant τ: 1 s; Near-full charge time 5τ: 5 s; τ in short form: 1 s.
Limits and caveats
- Assumes an ideal capacitor and a stiff voltage source; leakage and source resistance change the real timing.
- Electrolytic capacitors can be ±20% off, which shifts the timing by the same amount.
Questions about this tool
Why is a capacitor called fully charged after 5τ?
Because it reaches 99.3% after 5τ, which is close enough to full for most practical purposes.
What is the time constant of 10 kΩ and 100 µF?
Exactly one second, because 10000 × 0.0001 = 1.
Is discharge just as fast?
Yes, through the same resistor: 36.8% of the voltage remains after τ and 0.7% after 5τ.