RC time constant calculator
When a capacitor charges or discharges through a resistor it follows an exponential set by the time constant τ = R × C. One τ gets you 63% of the way; five τ is close enough to call it settled.
Results
- Time constant τ
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- Time to 63.2% (1τ)
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- Time to 95% (3τ)
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- Time to 99.3% (5τ)
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Results update as you type.
Formula
τ = R × C
Vc(t) = Vs · (1 − e^(−t/τ))
t(63.2%) = 1τ · t(99.3%) = 5τ
How it works
With R in kΩ and C in µF the time constant comes out directly in milliseconds — a handy shortcut worth remembering (10 kΩ × 10 µF = 100 ms).
After 1τ the capacitor reaches 63.2% of its final voltage, after 3τ about 95%, and after 5τ roughly 99.3% — engineers usually treat five time constants as "fully charged".
The RC charging example plots this exact curve on the oscilloscope so you can watch the exponential build in real time.