Capacitor Charge Calculator
Chart the charge or discharge curve of an RC circuit with time constant and energy.
Results update as you type.
About this calculator
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.
Frequently asked questions
What is the time constant of an RC circuit?
The time constant τ = R × C is the time to charge to about 63% of the supply voltage. After 5τ the capacitor is regarded as fully charged (>99%).
How much energy does a capacitor store?
Energy is E = ½ × C × V². A 1000 µF capacitor at 5 V stores ½ × 0.001 × 25 = 0.0125 joules.
What is the difference between charging and discharging?
Charging rises toward the supply as V(1 − e^(−t/τ)); discharging falls from the starting voltage as V·e^(−t/τ). Both reach ~99% of the change after 5 time constants.
How much charge does a capacitor hold?
The charge is Q = C × V in coulombs. A 1,000 µF capacitor at 5 V holds 0.001 × 5 = 0.005 C. Larger capacitance or higher voltage both store more charge, in direct proportion.
How long until the capacitor is fully charged?
Practically, after five time constants (5τ = 5RC) it is within about 1% of the supply and treated as full. With a 1 kΩ resistor and 1,000 µF capacitor τ is 1 s, so it is essentially charged after roughly 5 seconds.
What percentage is reached after each time constant?
Charging reaches about 63% after 1τ, 86% after 2τ, 95% after 3τ, 98% after 4τ and 99% after 5τ. Discharging follows the mirror image: 37%, 14%, 5%, 2% and 1% remaining at the same points.
Why add a series resistor at all?
The resistor sets the charging speed. Without it the current spike at switch-on is limited only by stray resistance and can be huge; the resistor makes the timing predictable, which is exactly what RC timers, filters and debounce circuits rely on.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/capacitor-charge/curl
curl "https://calculator.free/api/v1/capacitor-charge/?mode=charge&voltage=5&capacitance=1000&resistance=1000"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/capacitor-charge/?" + new URLSearchParams({
"mode": "charge",
"voltage": "5",
"capacitance": "1000",
"resistance": "1000"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.