How long until a capacitor is fully charged?
Never, exactly — an RC charge only ever gets closer to the supply, not all the way there. What matters in practice is how close, and how long that takes. Put in your own R, C, and supply, and see. Everything runs in this page; nothing is uploaded.
Your circuit's settling table
Default 100 nF, written as 0.0000001 F.
Time to a target percent
Not one of the rows above? Pick any percent below 100 and get the exact time for your own R and C.
Common questions
How long until a capacitor is fully charged?
There is no exact answer, because an RC charge never mathematically reaches 100% — it only gets closer and closer, forever. What people usually mean is one of a few conventions stated in time constants (τ = RC). On this page's defaults (10 kΩ, 100 nF, 5 V supply, so τ = 1 ms): 4τ reaches 98.17%, 5τ reaches 99.33%, and 10τ reaches 99.995%, indistinguishable from full for almost any purpose. Pick your own R, C, and supply above to see the real numbers for your circuit.
Why doesn't a charging capacitor ever reach exactly 100%?
Because the rate it charges at is proportional to how far it still has to go. The voltage follows V(t) = Vs(1 − e^−t/RC): every additional time constant closes 63.2% of whatever gap is left, not a fixed amount of voltage. Closing 63.2% of a gap can shrink that gap forever without ever making it zero, the same way repeatedly halving a distance never quite reaches the wall — it is arithmetic, not a limitation of any particular circuit.
Is 5 time constants really "fully charged"? How close is that?
5τ reaches 1 − e^−5 = 99.326% of the supply — a common convention, but a convention, not a law. On this page's own defaults that is 4.9663 V out of a 5 V supply, 3.37 mV short. Whether that is close enough depends entirely on what the circuit does next: 0.67% off is nothing for an indicator LED and can be a real error for a precision ADC reference.
How do I find the time for a different threshold, like 99.9%?
Use the target-percent tool above rather than reading the fixed table: it inverts the same equation, t = −RC × ln(1 − P/100), for any P you choose. On this page's defaults, 99.9% takes 6.9078 time constants — 6.9078 ms — noticeably longer than the 5τ convention for one more '9' of precision, which is the general shape of an exponential tail: each added 9 costs roughly one more time constant (ln 10 ≈ 2.303τ per decade).
Does this apply to discharging too, not just charging?
Yes, symmetrically. A discharging capacitor follows V(t) = V0 × e^−t/RC, closing 63.2% of the remaining voltage every time constant on the way down to zero instead of up to the supply — same τ = RC, same never-quite-zero asymptote, same 5τ/99.33% convention, just approaching from the other direction. The parent RC calculator's own discharge mode uses the identical formula this page's table is built from.