Calculates the time constant of a resistor-capacitor (RC) circuit — the characteristic time it takes to charge or discharge.
How it works
Resistance is multiplied directly by capacitance to give the time constant.
What this does not include
This does not include the full charge or discharge curve over time — the time constant is one characteristic value; the actual voltage at any given moment follows an exponential curve based on it.
How to use this calculator
- Enter the resistance and capacitance values.
A worked example
A 1,000Ω resistor with a 1 µF (0.000001 F) capacitor: time constant τ = R×C = 0.001 seconds (1 millisecond).
A 4,700Ω resistor with a 0.1 µF capacitor: τ = 0.00047 seconds.
What the variables mean
| Variable | Meaning |
|---|---|
| Resistance | Resistor value, in ohms |
| Capacitance | Capacitor value, in farads |
Edge cases worth knowing
The time constant is the time to reach about 63% of full charge (or discharge) — not the time to fully charge, which theoretically takes infinite time but is practically complete after about 5 time constants.
A negative resistance has no physical meaning, so the calculator declines to show a result for one.
Frequently asked questions
What does the time constant actually represent?
The time it takes a charging capacitor to reach about 63.2% of its final voltage, or a discharging capacitor to drop to about 36.8% of its starting voltage.
How long until a capacitor is considered “fully” charged?
By convention, about 5 time constants is considered close enough to fully charged (over 99%) for most practical purposes.
Why does a bigger resistor slow down charging?
A larger resistance limits the current that can flow into the capacitor at any moment, so it takes longer to deliver the same total charge.
