Capacitor Charge Calculator

Capacitor Charge & Energy Calculator

RC Time Constant Calculator

Capacitor Charge, Energy & RC Time Constant Calculator

Capacitors are fundamental components in nearly every electronic circuit. They store electrical charge and energy, filter signals, smooth power supplies, and control timing in RC (resistor-capacitor) circuits. This free calculator computes the charge stored (Q), energy stored (E), and the RC time constant (τ) for any capacitor and voltage combination.

Charge Stored in a Capacitor (Q = CV)

The charge stored on a capacitor is proportional to both its capacitance and the applied voltage: Q = C × V, where Q is in Coulombs, C is in Farads, and V is in Volts. A 100 μF capacitor charged to 12 V stores 0.0012 C (1.2 mC) of charge.

Energy Stored in a Capacitor (E = ½CV²)

The energy stored in a capacitor's electric field is given by: E = ½ × C × V². This energy is in Joules. Notice that energy scales with the square of voltage — doubling the voltage quadruples the stored energy. This is why high-voltage capacitors can be extremely dangerous even after power is disconnected.

The RC Time Constant (τ = RC)

When a capacitor charges through a resistor, it does not charge instantly. The RC time constant τ = R × C defines the charging rate. After one time constant (τ), a capacitor reaches about 63.2% of the supply voltage. After 5τ it is considered fully charged (99.3%). The time constant is critical in timer circuits, filter design, and signal conditioning.

Capacitance Unit Prefixes

  • F (Farad) — very large; used in supercapacitors.
  • mF (millifarad) — 10⁻³ F; large electrolytic capacitors.
  • μF (microfarad) — 10⁻⁶ F; common in power supplies.
  • nF (nanofarad) — 10⁻⁹ F; ceramic capacitors for filtering.
  • pF (picofarad) — 10⁻¹² F; RF and precision circuits.

Frequently Asked Questions

What is capacitance?
Capacitance is the ability of a component to store electrical charge per unit voltage. It is measured in Farads (F). A capacitor with higher capacitance stores more charge at the same voltage.
Why does energy depend on V² and not just V?
As a capacitor charges, each additional unit of charge must work against the voltage already built up on the plates. This cumulative resistance means energy scales as V², not linearly with V.
What happens after 5 time constants?
After 5τ the capacitor is at 99.3% of the supply voltage, which is considered fully charged for practical purposes. Theoretically it never reaches exactly 100% — it approaches asymptotically.
How is the RC time constant used in circuit design?
RC time constants set the cutoff frequency of low-pass and high-pass filters, determine the period of 555 timer circuits, control debounce timing in switch inputs, and set attack/decay times in audio circuits.
Are capacitors dangerous?
Large capacitors charged to high voltages store significant energy and can deliver dangerous shocks even when power is off. Always discharge capacitors safely before touching circuits, especially in power supplies, amplifiers, and flash equipment.