RC Time Constant Calculator
How to calculate an RC time constant
- Enter the resistance and choose its unit.
- Enter the capacitance and choose its unit.
- The time constant appears with the charge time, the cutoff frequency and, if you enter a supply voltage, the stored energy.
Frequently asked questions
What is the time constant?
τ = R × C, the time a capacitor takes to charge to 63.2% of the supply through that resistor. 10 kΩ with 100 µF gives τ = 1 second.
How long until it is fully charged?
After 5τ it reaches 99.3%, which is treated as full in practice. Discharging follows the same curve downwards.
What is the cutoff frequency?
f = 1 ÷ (2πRC), the −3 dB point of a simple RC low-pass or high-pass filter.
Are my numbers sent anywhere?
No. Everything is worked out in your browser and nothing is stored or uploaded.
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