What a capacitor calculation tells you
A capacitor stores energy in an electric field between two plates. Two numbers describe its behavior in a circuit: how much charge it holds at a given voltage, and how much energy is stored. Both come directly from its capacitance, measured in farads. Practical capacitors are usually microfarads or smaller, so this calculator takes capacitance in microfarads (µF) and the voltage across the capacitor in volts.
Use it to size a capacitor for a flash circuit, a power-supply filter, or a timing network, to estimate how much energy a bank of capacitors can deliver, or to judge how long a resistor-capacitor (RC) circuit takes to charge. If you also enter a series resistance, the calculator gives the RC time constant and the time to reach about 99% of the supply voltage. Leave resistance blank if you only need charge and energy.
Formulas and variables
- Q = C × V: charge in coulombs. C is capacitance in farads, V is voltage in volts.
- E = ½ × C × V²: stored energy in joules. Energy grows with the square of voltage, so doubling the voltage quadruples the energy.
- τ = R × C: the RC time constant in seconds, with R in ohms. After one time constant a charging capacitor reaches about 63.2% of the supply voltage.
- 5τ: after five time constants the capacitor is within about 0.7% of full charge, so it is reported as roughly 99% charged.
Worked example
A 100 µF capacitor is charged to 12 V through a 1000 Ω resistor. Convert the capacitance: 100 µF = 0.0001 F. Charge is Q = 0.0001 × 12 = 0.0012 C, shown as 1.200 mC. Energy is E = 0.5 × 0.0001 × 12² = 0.5 × 0.0001 × 144 = 0.0072 J, shown as 7.200 mJ. The time constant is τ = 1000 × 0.0001 = 0.1 s, displayed as 100.0 ms, and 5τ is 500.0 ms. So the capacitor is nearly full half a second after the supply is connected.
Common mistakes and how to interpret the result
- Entering farads instead of microfarads. The field expects µF. Typing 0.0001 for a 100 µF part will give a result a million times too small.
- Exceeding the voltage rating. The calculator will happily compute charge at any voltage, but real capacitors fail above their rated voltage. Leave a safety margin.
- Forgetting that the energy can be dangerous. A large capacitor charged to a high voltage stores serious energy even after power is removed; discharge it safely.
- Applying the time constant to non-ideal parts. Leakage, equivalent series resistance and source impedance change real charging times. Treat 5τ as an estimate.