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Capacitor Charge Time Calculator

Work out capacitor charge time instantly with clear inputs, formula shown and shareable results.

About Capacitor Charge Time Calculator

An RC circuit charges exponentially: V(t) = Vs x (1 - e^(-t/RC)). One time constant reaches 63.2%, three reach 95% and five reach over 99%. Rearranging gives the time to any target voltage as -RC x ln(1 - Vt/Vs).

Formula

RC charging

V(t) = Vs (1 - e^(-t/tau)), tau = R x C; t = -tau x ln(1 - Vt/Vs)

Worked example

With the default values below, Capacitor Charge Time Calculator returns Time to reach the target of 1.39. Change any input above and every figure here is recalculated in your browser.

Inputs used

Series resistance
10,000 Ω
Capacitance
100 µF
Supply voltage
12 V
Target voltage
9 V

Results

Time to reach the target
1.39 s
Time constant
1.00 s
Time to 99% (5 tau)
5.00 s
Target as multiples of tau
1.386 x

Step by step

  1. 1Values used
  2. 2RC charging
  3. 3Time to reach the target= 1.39 s
  4. 4Time constant= 1.00 s
  5. 5Time to 99% (5 tau)= 5.00 s
  6. 6Target as multiples of tau= 1.386 x

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

Frequently Asked Questions

Why does it never quite finish?

The charging current is proportional to the remaining voltage difference, so it decays asymptotically. Five time constants is treated as complete in practice.

What if I need a linear ramp instead?

Use a constant-current source rather than a resistor: dV/dt = I/C gives a straight line instead of an exponential.

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