Capacitor Charge and Energy Calculator
A capacitor stores electrical energy in an electric field between two conducting plates separated by a dielectric. The amount of charge stored is directly proportional to both the capacitance value and the applied voltage, given by Q = C * V, where Q is charge in coulombs, C is capacitance in farads, and V is voltage in volts. The energy stored is E = 0.5 * C * V^2 in joules. Because energy increases with the square of voltage, small voltage increases can significantly increase stored energy. When a capacitor charges or discharges through a resistance, the process follows an exponential curve with a time constant tau = R * C. After one time constant the voltage reaches about 63.2% of the final value; after five time constants the capacitor is considered fully charged. This calculator accepts capacitance in farads, microfarads (uF), nanofarads (nF), or picofarads (pF). Enter an optional resistance to compute the RC time constant and the five-tau full-charge time. Results update instantly as you type.
Capacitor formulas
Q = C × V (charge in coulombs)
E = 0.5 × C × V² (energy in joules)
τ = R × C (time constant in seconds)
Full charge ≈ 5 × τ
Worked example
C = 100 uF, V = 10 V, R = 1,000 Ω:
- C in farads = 100 × 10^-6 = 0.0001 F
- Q = 0.0001 * 10 = 0.001 C (1.00 mC)
- E = 0.5 * 0.0001 * 100 = 0.005 J (5.00 mJ)
- tau = 1000 * 0.0001 = 0.10 s (100 ms)
- Full charge = 5 * 0.10 = 0.50 s
Frequently asked questions
What is the formula for charge stored in a capacitor?
The charge stored is Q = C * V, where Q is charge in coulombs, C is capacitance in farads, and V is voltage in volts. A 100 microfarad capacitor charged to 10 V stores Q = 100e-6 * 10 = 0.001 coulombs (1 millicoulomb).
How is the energy stored in a capacitor calculated?
Energy stored is E = 0.5 * C * V^2, in joules. The energy grows with the square of voltage, so doubling the voltage quadruples the stored energy. A 100 microfarad capacitor at 10 V stores 0.5 * 100e-6 * 100 = 0.005 J = 5 mJ.
What is the RC time constant?
The RC time constant (tau) equals resistance (R) multiplied by capacitance (C), expressed in seconds. It represents the time for a capacitor to charge to about 63.2% of the supply voltage (or discharge to about 36.8%). After five time constants, the capacitor is considered fully charged or discharged (greater than 99%).
What units does capacitance use?
The SI unit of capacitance is the farad (F). Practical capacitors are usually rated in microfarads (1 uF = 1e-6 F), nanofarads (1 nF = 1e-9 F), or picofarads (1 pF = 1e-12 F). This calculator accepts any of these units via the unit selector.
Why does capacitor energy matter?
Energy stored determines how much power a capacitor can deliver in a burst. This matters for camera flash circuits, power supply smoothing, defibrillators, and motor start applications. It also determines the potential safety hazard of a charged capacitor at high voltages.
Sources
- NIST: SI Units (farad definition).
Reviewed by the CalculatorHub team, edited by James Graham. 14 June 2026.