RC Time Constant Calculator

Find the RC time constant from resistance and capacitance, plus the charge/discharge level at any time and the time to reach a target. Calculates in your browser.

Time constant τ

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5τ (≈ settled)-
Cutoff frequency-

Show the math

The time constant sets how fast the circuit charges or decays:

RC: τ = R × C  ·  RL: τ = L ÷ R

cutoff f = 1 ÷ (2π × τ)

After 5τ a circuit is within ~1% of its final value. All math runs in your browser.

What this does

An RC time constant calculator finds the time constant from resistance and capacitance, plus the charge or discharge level at any time and the time to reach a target.

How to use it

  1. Enter the resistance.
  2. Enter the capacitance.
  3. Read the time constant τ.
  4. Enter a time to see the charge level.

How it works

One time constant charges a capacitor to about 63% (or discharges to 37%) of the change. After five time constants it is over 99% charged.

τ = R × C; fully charged after ~5τ

Understanding your result

One time constant only gets you to about 63 percent of the change, and the practical rule is that a capacitor is effectively fully charged or discharged after about five time constants, so use 5 tau, not 1, when you need it settled. This sets timing for filters, debounce, and delays, and tolerances on real resistors and capacitors mean the actual time will vary around the estimate.

Example

10 kΩ with 100 µF gives τ = 1 second, fully charged in ~5 s.

Sources & methodology

Last updated .

Frequently asked questions

What is the RC time constant?

It is τ = R × C, the time for a capacitor to charge to about 63% (or discharge to 37%) of the change. A capacitor is treated as fully charged after about 5τ.

How long to fully charge?

Practically, after five time constants (5 × R × C) the capacitor is over 99% charged.

Is my data stored?

No. Everything is calculated locally in your browser.