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Calixo

RC Charge Time Calculator

Find how long a resistor-capacitor circuit takes to charge to a given percentage.

Inputs

1000 µF = 0.001 F.

Saved Scenarios

— select 2+ to compare

Charge Time (s)

2.303

Time Constant τ (s)

1.0000

Spark says

How it's calculated

Formula

t=−RCln⁡(1−Target%100)t = -RC \ln\left(1 - \dfrac{Target\%}{100}\right)
RC
— The circuit's time constant τ

What is the RC Charge Time Calculator?

A capacitor charging through a resistor follows an exponential curve — it never mathematically reaches 100%, so charge time is normally expressed as time to reach a target percentage (63.2% at one time constant, ~99.3% at five).

How to use it

  1. 1 Enter the resistance in the charging path.
  2. 2 Enter the capacitance.
  3. 3 Enter what percentage of full charge you're timing to.

Worked examples

1kΩ, 1000µF, charging to 90%

About 2.303 seconds to reach 90% charge.

Try it

Limitations

  • •Assumes an ideal RC circuit charged from a constant voltage source with no initial charge — real components have tolerances and leakage that shift this somewhat.

Frequently asked questions

What's special about 5 time constants?

After 5τ, a capacitor is charged to about 99.3% — commonly treated as 'fully charged' for practical circuit design purposes.