SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/eng-rc-time-constant

An empty capacitor charges through a resistor. About how full is it after one time constant (RC)?

About 63 percent of its final charge.

Start playing free

  1. About 63 percent ✔
  2. About 50 percent
  3. 100 percent

Why: Charging slows down as the capacitor fills, following an exponential curve. After one time constant, tau = R times C, it reaches 1 minus 1/e, about 0.632 of full.

Huh, didn't know that: Engineers treat a capacitor as basically full after about five time constants, over 99 percent.

The dad joke
My capacitor only charged 63 percent. It said it needed more time, constantly.
Like this card?
Play the game

References

Answer: About 63 percent. Charging slows down as the capacitor fills, following an exponential curve. After one time constant, tau = R times C, it reaches 1 minus 1/e, about 0.632 of full.

  1. OpenStax University Physics Vol. 2, 10.5 RC Circuits openstax.org/books/university-physics-volume-2/pages/10-5-rc-circuits
    the charge is equal to 1 - e^-1 = 1 - 0.368 = 0.632 of the maximum charge
    Checked 2026-10-10.

Think this answer is wrong? Tap "Challenge this answer" on the card in the game and tell us why.

Test yourself in the game

EngineeringAfter it charges, how does a capacitor behave in a steady DC circuit?EngineeringIn an AC circuit with only a capacitor, how are current and voltage timed?EngineeringHow many base units does the International System of Units (SI) have?

That's stuff everyone should know.