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DIY Cost Tools

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Capacitor Energy Calculator

Calculate energy stored in an ideal capacitor at a stated terminal voltage. Enter capacitance in microfarads, rather than farads, and the voltage across that single capacitor. The energy is always nonnegative because voltage is squared. The second result is the magnitude of charge on either plate. Doubling voltage quadruples stored energy in this ideal model, while doubling capacitance doubles it.

Enter every field, including explicit zeros when permitted. Decimal and scientific notation are accepted; commas, fractions and blanks are rejected. Each input must be finite and no larger than 1 billion in absolute value. Observe the units and positive-value requirements below.

Stored energy (J)0.0072
Charge magnitude (C)0.0012

How to use this calculator

  1. Match your inputs to the stated circuit model and field labels.
  2. Convert your quantities to the units printed next to each field.
  3. Select Calculate, or press Enter, and read the labeled results.

Formula and method

U = ½CV²; |Q| = C|V|. Capacitance in microfarads is multiplied by 10⁻⁶ before calculation.

The calculation runs in your browser. After changing an input, select Calculate again. Invalid inputs or undefined mathematical conditions hide the previous result so it cannot be mistaken for an answer to the new values.

Worked example

A 100 µF capacitor at 12 V stores 0.5×0.0001×12² = 0.0072 J and has charge magnitude 0.0001×12 = 0.0012 C.

Limitations and safety

Ideal-component calculation only. Component tolerance, heating, parasitic effects and frequency limits are not modeled. This output does not establish component ratings, wiring compliance or a safe circuit design. Have consequential designs checked by a qualified professional; do not use this tool as instructions for working on energized equipment. A capacitor can retain dangerous energy after power is removed. This calculator is not a discharge procedure, and an entered voltage does not prove that a capacitor is discharged.

The fields accept signed numerical input, but positive-only component values and nonnegative time or frequency requirements are enforced by the calculation. Read each error message before changing a value. Results use floating-point arithmetic and are shown to approximately 10 significant digits; component tolerances generally do not justify using all those digits in a physical design.

Frequently asked questions

Does a negative voltage create negative energy?

No. Voltage polarity changes charge sign, but stored energy depends on V². This page reports charge magnitude.

Why are my units important?

The calculator uses exactly the units printed in the input labels. For example, a micro-unit is one millionth of a base unit, and a milli-unit is one thousandth. Entering a base-unit value into a prefixed-unit field changes the meaning of the input.

Does the answer establish a safe component rating?

No. The formula evaluates an ideal model. Ratings, tolerances, transients, installation requirements and environmental limits need a separate design assessment.

Review notes and reference

Reviewed October 3, 2026. The default worked example and an additional boundary or scaling check were tested against expected values. Validation checks included blanks, nonnumeric text and inputs outside the supported range. OpenStax University Physics formula reference provides the circuit theory behind this model. The text and worked example on this page are original.

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