RC Time Constant Calculator
Multiply resistance by capacitance and you get the time constant — the single number that governs how fast a capacitor charges. After one time constant it reaches 63.2% of the supply voltage, and that figure is not a convention anyone chose: it is one minus one over e, and it falls out of the exponential. This computes the curve and checks that figure rather than printing it from memory.
What this generator does
Multiplies resistance by capacitance to get the time constant, then evaluates the charging curve at one through five time constants, giving the elapsed seconds, the voltage and the percentage of full charge at each. The 63.2% figure at one time constant is checked, not assumed.
How to use this tool
- Enter the resistance in ohms, the capacitance in microfarads and the supply voltage.
- Read the time constant, then the voltage at one through five time constants.
- Check the first row is at 63.2% — it always is, whatever the components.
- Note that five time constants gets to 99.3%, which is why that is the usual rule for 'charged'.
Understanding the controls
- Resistance
- The series resistor in ohms. Doubling it doubles the time constant, exactly as doubling the capacitance does.
- Capacitance
- The capacitor in microfarads, converted to farads internally so the time constant comes out in seconds.
- Supply
- The voltage the capacitor charges towards. It scales the curve but has no effect at all on the time constant.
Common use cases
- Sizing an RC delay or a debounce circuit
- Teaching the exponential charging curve with real component values
- Checking a timing calculation against the standard figures
- Electronics coursework and worked examples
- Understanding why a capacitor never quite finishes charging
How this generator works
The charging curve is the supply voltage times one minus e to the minus t over tau. Both the shape and the familiar percentages come from that single expression: 63.2% at one time constant, 86.5% at two, 99.3% at five. The check confirms the time constant really is resistance times capacitance, that no point on the curve exceeds the supply, and that the one-time-constant figure lands on 63.2%.
Randomness and fairness
No randomness at all. The time constant and the whole curve follow deterministically from the component values you enter.
For how randomness is produced across the whole site, see how Generate Random works.
Limitations and good to know
- Charging from empty towards the supply; discharge is the mirror curve and is not shown.
- An ideal resistor and capacitor with no leakage, tolerance or equivalent series resistance.
- Real components have tolerances of 5 to 20%, so a measured time constant will differ from this figure.
- Strictly the capacitor never reaches the supply voltage — past about 37 time constants the arithmetic underflows and the two become indistinguishable in floating point.
Privacy and your data
The curve is computed in your browser. The component values you enter are never transmitted or stored.
Related generators
- Resistor Colour Code CalculatorTurns a resistance into its colour bands and reads them straight back, with the nearest value actually manufactured shown alongside.
- Ohm's Law Question GeneratorVoltage, current and resistance questions with whole-number answers, chaining into power at the stretch level.
- Gear Ratio CalculatorMulti-stage gear trains with the overall ratio, output speed and torque factor, checked by multiplying the answer back to the input.
- Lever Balance CalculatorBalanced lever problems where the final weight's position is solved so the moments cancel exactly, with the sum shown as zero.