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Capacitor Charge Calculator

Find a capacitor's charge and stored energy.

How it is calculated

The engine works in SI units, so enter capacitance C in farads (F) and voltage V in volts (V). It returns two values: Q = C × V in coulombs (C) and E = ½ × C × V² in joules (J). Both are printed with six decimal places.

Capacitance must be greater than zero, otherwise there is no result. The voltage is not checked for sign: a negative voltage gives a negative charge, but the energy stays positive because the voltage is squared. The tool describes an ideal capacitor and ignores leakage, ESR and voltage derating.

Worked example

A 4700 µF capacitor is 0.0047 F, charged to 12 V.

Charge: 0.0047 × 12 = 0.0564, shown as 0.056400 C. Energy: ½ × 0.0047 × 12² = 0.5 × 0.0047 × 144 = 0.3384, shown as 0.338400 J. Doubling the voltage to 24 V would double the charge but quadruple the energy.

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Common mistakes

  • Typing microfarads as if they were farads. 4700 µF must be entered as 0.0047, not 4700; the result would be a million times too large.
  • Using the capacitor's rated voltage instead of the voltage actually applied. Energy depends on the square of V, so the difference is large.
  • Expecting energy to double when the voltage doubles. It quadruples, while charge doubles.
  • Treating the joule result as usable energy. A real circuit cannot drain the capacitor to zero volts, so only part of it is available.

About this calculator

This calculator finds the charge stored on a capacitor, Q = C·V, and the energy stored in its electric field, E = ½·C·V², from its capacitance C and the voltage V applied across it. Charge scales linearly with voltage, but stored energy scales with the square of voltage, since energy is the integral of voltage times the incremental charge added as the capacitor charges up from zero.

Electronics engineers use these formulas to design power-supply filtering and smoothing circuits, camera-flash circuits (which charge a capacitor slowly then discharge it almost instantly for a bright flash), and defibrillators, which store a precise, life-critical amount of energy in a capacitor bank before delivering a shock. Electrical engineering students, electronics hobbyists building circuits, and technicians troubleshooting power-supply capacitors use this calculator to check charge and energy figures against a component's rated capacitance and voltage.

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Comments (1)

  • Editorial team

    The energy shown assumes the capacitor goes all the way to zero volts, which real circuits rarely allow. For the usable part between two voltages, run the tool at each voltage and subtract the energies. Our 4700 µF example falling from 12 V to 9 V yields about 0.148 J, less than half of the 0.338 J total. Check that before sizing a hold-up or flash capacitor. What are you powering, and how low can its supply voltage drop before it stops working?

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