RC charge and discharge time calculator

Timing

For charge, this is the voltage reached from 0 V.

Not aiming for a specific voltage? How long until a capacitor is basically fully charged? — the 1τ–10τ settling table and why it never reaches 100%.

Voltage after a delay

The inverse question: you already have R, C, and a clock edge. What voltage is on the capacitor then?

Design a delay

The same threshold conditions apply. The result includes the exact value and the nearest preferred part.

Common questions

Why does τ = RC matter, and what is a “time constant”?

τ = RC is the time it takes an RC circuit to close about 63.2% of the gap to its final voltage — that 63.2% figure is 1 − 1/e, and it is where the “time constant” name comes from. This page's own defaults show it directly: R = 10 kΩ and C = 100 nF give τ = 1 ms, and the default threshold of 3.16 V is exactly 63.2% of the 5 V supply — charging to it takes 999.7 µs, essentially one full time constant.

What does “voltage after a delay” answer that the timing section doesn't?

It's the inverse question. Timing asks “how long until I reach this voltage?”; this asks “what voltage is on the capacitor after this much time?” Using this page's own defaults, after exactly 1 ms of charging (one time constant) the capacitor sits at 3.1606 V, 63.21% of the 5 V supply — the same 63.2% figure the first question describes, reached from the other direction.

Why does the design tool pick a slightly different part than the exact math?

Because resistors and capacitors are only sold in a fixed set of standard values (E24 for resistors, E12 for capacitors), not any number the algebra produces. Asking this page's own default 1 ms target time to solve for resistance (with the default 100 nF capacitor) calls for an exact 10003.28 Ω — but the nearest standard value happens to be 10 kΩ, which is itself an E24 value, so the snapped part still lands the delay at 999.7 µs, only 0.03% off target. A less convenient target time can snap by several percent instead.