RC Time Constant Calculator
An RC circuit is a resistor and a capacitor wired in series. The product of the two sets how quickly the capacitor charges. This page works out that product, the time constant, and how long the capacitor takes to reach any share of its full charge.
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- Works offline after your first visit
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How it works
The time constant is resistance times capacitance. Here resistance is in kilohms and capacitance in microfarads, so their product divided by 1,000 gives seconds.
After one time constant, a capacitor has charged about 63 percent of the way. The time to reach any other share is the time constant times the negative natural log of one minus that share.
Worked example
A 10 kilohm resistor with a 100 microfarad capacitor takes 1 s to reach 63.2% of full charge. One time constant is 1 s.
Questions
Why does charging slow down near the end?
The current falls as the capacitor fills, so each further share of charge takes longer. Each time constant covers about 63 percent of what is left.
How many time constants count as full?
Five time constants reach about 99 percent of full charge, which many people treat as full in practice. The right share depends on the circuit.
Does the supply voltage change the time constant?
Not the time constant itself. The supply voltage sets the level the capacitor finally reaches, not how fast it gets there.
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