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Capacitor charge is the electric charge a capacitor holds in the field between its plates, Q = C × V in coulombs, and the energy stored is E = ½ × C × V² in joules, so doubling the voltage quadruples the stored energy. When you charge a capacitor through a series resistor, the voltage does not jump instantly — it rises along the curve V(t) = V × (1 − e^(−t/τ)); discharging, it falls along V(t) = V × e^(−t/τ). Both are governed by the time constant τ = R × C, the natural timescale of the RC circuit.

After one time constant the capacitor reaches about 63% of the supply voltage when charging (or falls to 37% when discharging); after 5τ it is within 1% of its final value and is treated as fully charged or discharged. For example, a 1,000 µF capacitor charged through a 1 kΩ resistor has τ = 1,000 × 10⁻⁶ × 1,000 = 1 second, so it is 63% charged after 1 s and essentially full after 5 s. At 5 V that same capacitor stores Q = 0.001 × 5 = 0.005 C and E = ½ × 0.001 × 25 = 0.0125 J.

Enter the supply voltage, capacitance and series resistance and the calculator returns the full charge, the stored energy, the time constant, the settling time (5τ), the voltage at a chosen instant, and the whole charge or discharge curve. These numbers are the everyday basis for RC timing circuits, power-supply smoothing, debounce delays, flash-photography energy storage and the turn-on delays in analogue electronics.