Find an RC circuit's time constant and the capacitor's voltage at any moment during charging or discharging.

How to Use the Resistor Capacitor Circuit Calculator

Choose Charging or Discharging, then enter resistance, capacitance (with its own µF/nF/pF unit dropdown), supply voltage, and how much time has elapsed since the circuit started that way. The capacitor's voltage at that instant, plus the circuit's time constant, appear immediately.

Capacitors don't charge or discharge instantly — connect one to a resistor and a voltage source and it approaches its final voltage on a curve, fast at first and progressively slower, never quite arriving in a strict mathematical sense. Three equations describe the whole curve:

τ = R × C
Charging:    Vc(t) = V × (1 − e^(−t ÷ τ))
Discharging: Vc(t) = V × e^(−t ÷ τ)

τ (tau), the time constant, is how long the circuit takes to close about 63% of the remaining gap to its final voltage — not the time to fully charge, a common mix-up. R in ohms times C in farads gives τ in seconds directly, which is why the widget converts whatever capacitance unit you pick back to farads internally before running the math.

Five Time Constants Is "Basically Done"

After one τ, a charging capacitor has covered about 63% of the distance to full voltage; after two τ, roughly 86%; by five τ, it's past 99% and close enough to call it finished for most practical purposes, even though the curve technically never touches zero remaining gap. Discharging follows the identical shape in reverse — 37% of the starting voltage remains after one τ, and by five τ there's essentially nothing left.

This is why RC circuits show up everywhere from camera flash timing to debounce circuits on a mechanical switch: picking R and C together sets a predictable delay, and that delay scales with both — double either one and every stage of the curve takes twice as long to reach.