Capacitor Charge Calculator

RC circuit transient time constant solver, charge/discharge curves, stored energy, and total charge.

Component Specifications

Resistance

Capacitance

Supply Voltage (V)

Solve at Specific Time

Time elapsed

Voltage:3.1606 V

% Charged:63.2%

RC Constant (τ)

1000.000 ms

Stored Energy:1.250 mJ

Total Charge (Q):500.000 µC

Charging curve steps

Time Interval	Voltage (V)	% Charged
0 (0τ)	0.0000 V	0.0%
1000.000 ms (1τ)	3.1606 V	63.2%
2.000 s (2τ)	4.3233 V	86.5%
3.000 s (3τ)	4.7511 V	95.0%
4.000 s (4τ)	4.9084 V	98.2%
5.000 s (5τ)	4.9663 V	99.3%

RC Time Constants & Physics Formulas

The RC time constant, denoted by the Greek letter $\tau$ (tau), is the timeframe required to charge a capacitor through a series resistor to approximately 63.2% of the source voltage. Formula:\tau = R \times C.

During discharge, the remaining voltage decays exponentially: V(t) = V_{supply} \times e^-t/ au. Total stored electrical charge is defined by $Q = C \times V$ in Coulombs, while electrostatic potential energy is computed as $E = \frac12 C V^2$ in Joules.

Built and maintained by Meet Shah · Last updated 1 August 2026

What this tool is used for

Working out an RC time constant for a timing circuit.
Finding how long a capacitor takes to reach a target voltage.
Sizing a resistor and capacitor for a desired delay.
Checking a measured discharge time against the design.
Understanding why a circuit settles more slowly than expected.

Frequently Asked Questions

What is the RC time constant?: τ = R × C, in seconds when resistance is in ohms and capacitance in farads. It is the time for the capacitor to reach 63.2% of the supply voltage — 1 − 1/e — and it is the single number that characterises the whole charging curve.
Why 63.2% and not something rounder?: Because charging is exponential: V(t) = V(1 − e^(−t/τ)). At t = τ the exponent is −1, so the remaining gap is 1/e ≈ 36.8% and the reached fraction is its complement. The number is a consequence of e, not a convention.
When is a capacitor considered fully charged?: At five time constants, by convention — 99.3%. Mathematically it never finishes, since the exponential only approaches the supply asymptotically, so engineering picks a threshold where the remainder is below measurement noise. Three τ is 95%, which is often enough.
Does discharging follow the same curve?: The mirror image: V(t) = V₀ × e^(−t/τ), falling to 36.8% after one time constant. Same τ, same shape, opposite direction — which is why a symmetric RC circuit charges and discharges in the same time.
How much energy does a charged capacitor store?: E = ½CV², so the energy scales with the SQUARE of voltage — doubling the voltage stores four times the energy. It is also why a large capacitor at high voltage is genuinely dangerous long after the supply is disconnected, and why bleeder resistors exist.

Common errors and gotchas

Assuming full charge at one time constant, when it is about 63% — five constants is the usual practical figure.
Mixing units, where microfarads with ohms give microseconds and it is easy to be out by a million.
Ignoring the capacitor's tolerance, which on electrolytics can be very wide.
Overlooking leakage and ESR, which matter at long time constants.
Treating charge and discharge curves as linear, when both are exponential.