Capacitor Charge Calculator

How capacitor charge and energy are calculated

A capacitor stores electrical charge on two conductive plates separated by a dielectric. The amount of charge it holds at a given voltage depends only on its capacitance — larger capacitance stores more charge at the same voltage. Energy, unlike charge, scales with the square of voltage, since work must be done against an increasing electric field as the capacitor fills.

When a series resistance is provided, the calculator also finds the RC time constant and — if an elapsed time is given — the instantaneous voltage and percentage charge at that moment using the standard exponential charging curve for a capacitor charging through a resistor from a fixed supply voltage.

Formula steps

Safety guidance: inrush, derating, and stored energy

• Inrush current: an uncharged capacitor briefly behaves like a short circuit, so charging directly from a low-impedance source without a series resistor (or other current limiting) can produce a large inrush current spike capable of damaging switches, connectors, or the capacitor itself.
• Voltage derating: keeping working voltage at or below roughly 80% of the capacitor's rated voltage — more conservatively for electrolytics in hot or high-ripple environments — is a common practice to extend service life and reduce failure risk. This calculator flags whether your entered voltage is within that margin if you provide a rated voltage.
• Stored energy hazard: a charged capacitor retains its energy after the source is removed. Larger capacitance or higher voltage capacitors can retain a shock hazard; verifying a capacitor is discharged (and using bleeder resistors in permanent designs) before handling is standard practice.

Code cautions

Power factor correction capacitors and capacitor banks used in electrical installations are addressed by dedicated code sections — for example, NFPA 70 (NEC) Article 460 in the United States — which include requirements for discharge devices, enclosures, disconnecting means, and overcurrent protection. This calculator covers basic component-level charge and energy math only; it does not verify code compliance for capacitor installations, and any capacitor bank, power supply filter capacitor, or high-voltage/high-energy capacitor should be installed, serviced, and discharged according to the applicable code and manufacturer guidance by a qualified person.

Design limitations of this calculator

• Ideal capacitor assumed: equivalent series resistance (ESR), equivalent series inductance (ESL), dielectric absorption, and leakage current are not modeled.
• No capacitance tolerance or voltage/temperature drift: real capacitors (especially ceramic types) can vary significantly in effective capacitance with applied voltage and temperature; a single nominal capacitance value is used here.
• Single time-constant RC charging assumed: more complex circuits with multiple capacitors, parallel paths, or non-constant source voltage are not modeled.
• DC analysis only: AC behavior, capacitive reactance, and impedance at specific frequencies are not covered by this calculator.
• No ripple current or thermal modeling: heating from ripple current in filter capacitor applications is not calculated.

Frequently asked questions

• How do you calculate the charge stored in a capacitor?
  Charge stored equals capacitance multiplied by voltage: Q = C × V, where Q is in coulombs, C is in farads, and V is in volts. For example, a 1000µF capacitor charged to 12V stores Q = 0.001 × 12 = 0.012 coulombs (12 millicoulombs).
• How do you calculate the energy stored in a capacitor?
  Energy stored is E = ½ × C × V², measured in joules. This is different from charge — energy grows with the square of voltage, so doubling the voltage across a capacitor quadruples the stored energy, while charge only doubles.
• What is the RC time constant?
  The RC time constant, τ (tau), equals resistance multiplied by capacitance: τ = R × C, measured in seconds. It represents the time for a capacitor charging through a resistor to reach about 63.2% of the final voltage. After about 5 time constants (5τ), a capacitor is considered fully charged (about 99.3%).
• How long does it take a capacitor to charge?
  In an RC circuit, the capacitor voltage follows V(t) = Vfinal × (1 − e^(−t/RC)). It never mathematically reaches exactly 100%, but common practical milestones are 63.2% after 1τ, 86.5% after 2τ, 95.0% after 3τ, 98.2% after 4τ, and 99.3% after 5τ — so 5 time constants is the usual rule of thumb for "fully charged."
• Why does a capacitor need a series resistor when charging?
  Without a limiting resistor (or other series impedance), a capacitor charging from a low-impedance source draws a very high inrush current in the first instant, since an uncharged capacitor briefly looks like a short circuit. This inrush can exceed the current rating of a power supply, switch, or the capacitor's own terminals, so a series resistor (or inductor, in some designs) is commonly used to limit it.
• Is stored capacitor charge dangerous after power is removed?
  Yes, potentially. A charged capacitor retains its stored energy even after the circuit is powered off or disconnected, and larger or higher-voltage capacitors — such as those in power supplies, motor drives, camera flashes, or power factor correction banks — can retain enough energy to cause a painful or dangerous shock. Bleeder/discharge resistors, and verifying a capacitor is discharged with a meter before handling, are standard safety practice.
• How much should I derate a capacitor's voltage rating?
  A common guideline, especially for electrolytic capacitors, is to keep the actual working voltage at or below about 80% of the capacitor's rated voltage, with more conservative margins (50–70%) sometimes used in high-reliability or high-temperature designs. Running a capacitor close to its rated voltage — especially combined with high ripple current or elevated temperature — reduces its service life.
• Does temperature affect capacitor life?
  Yes, particularly for electrolytic capacitors, which have a chemical electrolyte that degrades faster at higher temperatures. A widely used approximation is that electrolytic capacitor life roughly halves for every 10°C rise in operating temperature above its rated temperature, which is why derating both voltage and operating temperature extends service life.