RC Time Constant Calculator
Find how fast a capacitor charges through a resistor.
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Time constant τ (s)
Time constant τ (s)
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Time to 90% (s)
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Time to 99% (s)
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Capacitor voltage at t (V)
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Charge level at t
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RC Time Constant Calculator by TheCalcTools
How the RC Time Constant Calculator works
Enter the resistance, capacitance, supply voltage and a time since the supply was connected.
The time constant τ is R × C.
A charging capacitor reaches 1 − e^(−t/τ) of the supply voltage.
The formula
τ = R × CV(t) = V × (1 − e^(−t/τ))
Worked example
Using the default values shown in the calculator:
| Resistance (kΩ) | 10 |
|---|---|
| Capacitance (µF) | 100 |
| Supply voltage (V) | 5 |
| Time elapsed (s) | 1 |
- τ = 10,000 × 0.0001 = 1.00 s.
- At 1 s the capacitor is at 63.2% of 5 V, which is 3.161 V.
- It reaches 99% after 4.61 s.
Result:
| Time constant τ (s) | 1.0000 |
|---|---|
| Time to 90% (s) | 2.3026 |
| Time to 99% (s) | 4.6052 |
| Capacitor voltage at t (V) | 3.1606 |
| Charge level at t | 63.2% |
Frequently asked questions
What does τ mean in practice?
After one time constant the capacitor reaches about 63.2% of the supply voltage. After 5τ it is over 99%.
How does discharge differ?
On discharge the voltage falls as V × e^(−t/τ), reaching 36.8% after one time constant.
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.