Capacitor Charge/Discharge Calculator

Analyze RC charging and discharging with live results for capacitor voltage, current, charge, energy, and time constant.

How to Use

  1. Select Charge or Discharge.
  2. Enter resistance, capacitance, and the source or initial voltage values.
  3. Choose whether you want to solve for voltage at time or time to reach target voltage.
  4. Review live outputs, time constant, and Show Work for the formulas used.
RC Circuit View
Visual overview of resistor-capacitor timing behavior and charge state.
τ
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Vc
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I
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E
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Status: —
Vs SW Resistor (R) Capacitor (C) RC timing view: state, voltage ratio, and energy update with the live calculation. Ideal first-order RC model. No ESR, leakage, dielectric absorption, or source resistance unless included in R.
Inputs & Settings
Use realistic units and choose the solve mode that matches your workflow.
Charge uses a source voltage. Discharge decays from an initial capacitor voltage.
Switch between direct voltage lookup and target-voltage timing.
The resistor controls how fast the capacitor charges or discharges.
Examples: 100nF timing cap, 100µF filter cap, 1F supercap.
For charge mode this is the supply. For discharge mode this is the starting capacitor voltage.
Used for target timing or as the comparison point in the live readout.
Used when solving capacitor voltage at a specific time.
Quick Adjust
1.00 τ

The slider represents elapsed time in multiples of the time constant, where τ = R × C.

Show Work (step-by-step)
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Work is shown in base units: volts, ohms, farads, seconds, coulombs, amps, and joules.

RC Charge/Discharge Formulas

Quick answer: A resistor-capacitor circuit changes exponentially with time constant τ = R × C.

This tool uses the ideal first-order RC equations below.

  • Time Constant: τ = R × C
  • Charging Voltage: Vc(t) = Vs × (1 − e−t/RC)
  • Discharging Voltage: Vc(t) = V0 × e−t/RC
  • Charge: Q = C × V
  • Stored Energy: E = ½ × C × V²
  • Current (magnitude): I = (ΔV / R) × e−t/RC for ideal RC response
Where R = resistance, C = capacitance, τ = time constant, V = voltage, Q = charge, and E = energy.

FAQ

What does one time constant mean?

After one time constant, a charging capacitor reaches about 63.2% of its final voltage, and a discharging capacitor falls to about 36.8% of its starting voltage.

When is a capacitor considered fully charged?

In practical terms, many engineers treat 5τ as effectively complete because the capacitor is above 99% of its final value in an ideal RC model.

Why is the voltage change exponential?

Because the resistor limits current, and that current falls as the capacitor voltage approaches its final value, producing the classic exponential response.

Does this include ESR or leakage?

No. This page uses an ideal first-order RC model. Real capacitors can have equivalent series resistance, leakage, tolerance spread, and temperature effects.

Tool Info

Last updated:

Updates may include improved unit handling, edge-case validation, UI refinements, and additional electronics workflow support.