Capacitor & RC Time Constant Calculator

Energy stored, charge held, and the RC charge-and-discharge timing of a capacitor — plus the voltage at any moment as it charges or discharges through a resistor.

Capacitance C

C unit

Voltage V (V)

Series resistance R

R unit

Elapsed time t (optional)

t unit

Energy stored—

What a Capacitor Holds, and How Fast

A capacitor stores energy in an electric field. Two numbers describe how much: the charge it holds and the energy in it. A third — the time constant — tells you how quickly it fills or empties through a resistor. Together they cover most of what you need for timing circuits, filters, power-hold-up and energy storage.

E = ½ C V² · Q = C V · τ = R C

Energy rises with the square of voltage, so doubling the voltage quadruples the stored energy — which is why voltage rating matters so much. Charge is simply capacitance times voltage. And the product of resistance and capacitance sets the timing.

The 63% Rule

Charge a capacitor through a resistor and it does not jump to full — it follows a curve. After one time constant τ = RC it reaches about 63% of the supply voltage; after two, 86%; after five, over 99%, which engineers treat as fully charged. Discharging mirrors it: down to 37% after one τ, near zero after five. That single curve underlies RC timers, debounce circuits and filter roll-off.

Frequently Asked Questions

Why five time constants for full charge?

The approach is exponential, so it never mathematically reaches 100%. At 5 RC it is at 99.3%, close enough for practical purposes – hence the rule of thumb.

Charge or energy – which matters?

Depends on the job. Timing and current draw care about charge (Q = C V); how much work the capacitor can deliver, or how much it dumps in a fault, depends on energy (half C V squared).

What sets the inrush current?

At the instant of connection the capacitor looks like a short, so the peak current is just V divided by R. A small series resistor limits that inrush.

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AC Impedance Calculator — capacitive reactance and resonance in AC circuits.
Ohm’s Law Calculator — voltage, current and resistance.
Power Converter — watts, horsepower and more.
Engineering Unit Converter — electrical and other units.

For education and preliminary design. Real capacitors have equivalent series resistance, leakage, tolerance and strict voltage and temperature ratings – never exceed the rated voltage. A charged capacitor can hold a dangerous shock or deliver a large fault current; discharge and verify before handling.