Capacitor Charge Calculator

Capacitor Charge Calculator
Calculate Charge, Energy, and RC Timing
C Capacitance
V Voltage
V
R Series Resistance (optional — for RC time constant and charge/discharge timing)
Capacitor Charge and Energy
Charge (Q)
—
Q = C × V
Energy (E)
—
E = ½CV²
Capacitance
—

RC Charging Circuit

When a voltage source is applied through a series resistor, the capacitor charges exponentially. The time constant τ = RC determines how fast it charges. After 1τ the capacitor reaches 63.2% of V. After 5τ it is effectively fully charged (99.3%).

V + + – SW R I(t) C + – Vc(t) Vc(t) = V(1 – e-t/τ)   |   τ = RC   |   Q = CV

Capacitor Charge Calculator

Three questions: how much charge does a capacitor store at a given voltage, how much energy is in that charge, and how long does it take to charge or discharge through a resistor. Enter capacitance and voltage to get charge and energy. Add a series resistance to get the full RC timing analysis — time constant, peak current, and specific charge/discharge times.

Core Formulas

Q = C × V — charge stored (coulombs)
E = ½ × C × V² — energy stored (joules)

τ = R × C — time constant (seconds)
Ipeak = V / R — peak charging current at t = 0

Vc(t) = V × (1 − e−t/τ) — charging voltage over time
Vc(t) = V × e−t/τ — discharging voltage over time

Charge (Q) tells you how many coulombs of electrons sit on the capacitor plates. Energy (E) tells you how much work they can do when released. Energy scales with V squared — doubling the voltage quadruples the stored energy. The underlying V/R relationship follows Ohm’s law.

The Time Constant (τ = RC)

The time constant is the single number that characterises how fast the circuit responds. During charging, τ is the time to reach 63.2% of the supply voltage. During discharging, τ is the time to fall to 36.8% of the initial voltage. Key milestones:

Charging: 1τ → 63.2%  |  3τ → 95.0%  |  4.6τ → 99.0%

Discharging: 1τ → 36.8%  |  3τ → 5.0%  |  4.6τ → 1.0%

For practical purposes, a capacitor is “fully charged” at 5τ (99.3%) and “fully discharged” at 5τ (0.7%). Most designs use 3τ (95%) as the working threshold and 4.6τ (99%) for safety-critical timing. For a dedicated timing analysis, see the RC Time Constant Calculator.

Worked Example — RC Timer (100 µF, 10 kΩ, 5 V)

Classic timing circuit. C = 100 µF, R = 10 kΩ, V = 5 V.

Q = 100×10−6 × 5 = 500 µC
E = 0.5 × 100×10−6 × 25 = 1.25 mJ

τ = 10000 × 100×10−6 = 1.0 second
Ipeak = 5 / 10000 = 0.5 mA

Charge to 63.2% (3.16 V): 1.0 s
Charge to 95% (4.75 V): 3.0 s
Charge to 99% (4.95 V): 4.6 s

Discharge to 36.8% (1.84 V): 1.0 s
Discharge to 5% (0.25 V): 3.0 s
Discharge to 1% (0.05 V): 4.6 s

A 1-second time constant makes the maths easy and the behaviour visible on an oscilloscope. This is the standard RC combination for demonstrating exponential charging and for basic delay circuits (555 timer, comparator-based timing, microcontroller analog input timing).

Worked Example — Camera Flash (330 µF, 300 V)

C = 330 µF, V = 300 V. No series resistance (the flash tube is the discharge path).

Q = 330×10−6 × 300 = 99 mC (99000 µC)
E = 0.5 × 330×10−6 × 90000 = 14.85 J

14.85 joules released in a few milliseconds produces a bright flash. This is enough energy to be dangerous — 300 V across 330 µF can deliver a lethal shock. Camera flash capacitors are the reason bleeder resistors and discharge procedures matter.

Worked Example — Decoupling Capacitor (100 nF, 50 Ω, 3.3 V)

C = 100 nF, R = 50 Ω (source impedance), V = 3.3 V.

Q = 100×10−9 × 3.3 = 330 nC
E = 0.5 × 100×10−9 × 10.89 = 544.5 nJ

τ = 50 × 100×10−9 = 5 µs
Ipeak = 3.3 / 50 = 66 mA

Charge to 95%: 15 µs
Charge to 99%: 23 µs

Nanosecond-scale time constant. This is why 100 nF ceramic capacitors are placed next to every IC — they respond fast enough to supply transient current demands before the bulk power supply can react.

Charge and Energy — How They Scale

Charge Scales Linearly

Q = C × V. Double the capacitance or double the voltage and charge doubles. A 1000 µF capacitor at 5 V stores 5 mC. The same capacitor at 10 V stores 10 mC.

Energy Scales with Voltage Squared

E = ½CV². Double the voltage and energy quadruples. A 1000 µF capacitor at 5 V stores 12.5 mJ. At 10 V: 50 mJ. At 50 V: 1.25 J. At 400 V: 80 J. This is why high-voltage capacitors are dangerous even at small capacitance values — and why energy storage applications (supercapacitors, capacitor banks) focus on maximising both C and V. For a focused energy analysis, use the Capacitor Energy Calculator.

Practical Applications

Timing Circuits

555 timers, monostable multivibrators, and microcontroller delay loops all rely on RC charge/discharge timing. The time constant τ = RC sets the pulse width or oscillation frequency. Adjust R or C to change the timing.

Power Supply Filtering

Bulk capacitors after a rectifier smooth the DC output. The RC time constant (where R is the load resistance) determines how much voltage ripple remains between charging pulses. Larger C or larger R (lighter load) = less ripple.

Energy Storage

Supercapacitors (1 F to 3000 F) store enough energy to power backup systems, hold memory during power loss, or provide burst current. E = ½CV² quantifies exactly how much energy is available.

Decoupling and Bypass

Small ceramic capacitors (100 nF, 1 µF) placed near IC power pins absorb high-frequency transient demands. Their fast RC response (nanoseconds to microseconds) is the key — they supply current faster than the power supply traces can.

Safety — Stored Energy Hazards

Any capacitor charged above ~50 V with more than ~0.25 J of stored energy is a shock and burn hazard. Use the calculator to check E = ½CV² before working on a circuit. If the number is significant, discharge the capacitor through a bleeder resistor first.

Frequently Asked Questions

How long does it take to fully charge a capacitor?
Theoretically, never — the exponential curve asymptotically approaches the supply voltage. Practically, 5τ gets you to 99.3%, which is “fully charged” for any real application. Multiply R × C × 5 to get the time in seconds.
Does a capacitor charge faster with a smaller resistor?
Yes. τ = RC, so halving R halves the time constant. But the peak current doubles (I = V/R), which may exceed what the power supply or wiring can handle. There is always a trade-off between speed and peak current.
Why does energy scale with V² but charge scales with V?
Charge is the quantity of electrons moved onto the plates: Q = CV, linear with voltage. Energy is the work done moving those electrons against an increasing voltage: the first electrons are easy (low voltage), the last are hard (full voltage). Integrating gives E = ½CV² — the squared term reflects this increasing difficulty.
What is the difference between charge (Q) and energy (E)?
Charge (coulombs) is how many electrons are stored. Energy (joules) is how much work they can do. A supercapacitor at 2.7 V might store 1000 C of charge but only 3645 J of energy. A small film capacitor at 400 V stores far less charge but the energy per coulomb is much higher.
Can I use this calculator for supercapacitors?
Yes. Select F (farads) as the unit. A 10 F supercapacitor at 2.7 V stores Q = 27 C and E = 36.45 J. Add the ESR (equivalent series resistance, typically 10–100 mΩ) as the series resistance to see the RC time constant — which can be seconds to minutes for supercapacitors.
How do I calculate charge time without knowing R?
You cannot — without a defined resistance in the circuit, the charge time is undefined. In practice, every circuit has some resistance (wire, trace, source impedance). If you only need charge and energy, leave R empty and the calculator gives you Q and E without the timing analysis.

Related Capacitor Calculators

More tools from the capacitor family:

Browse all Electronics Calculators →

Last updated: March 2026