RC time constant calculator
Calculate the time constant τ = R × C of an RC circuit, the capacitor voltage during charge and discharge at any moment, the time to reach a voltage and the filter cutoff frequency, with a chart of the curve.
Capacitor charge and discharge (RC)
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RC time constant calculator: what it is for
Calculate the time constant τ = R × C of a resistor-capacitor circuit, the capacitor voltage at any moment while charging, V = Vs × (1 − e^(−t/τ)), and discharging, the time to reach a target voltage and the cutoff frequency of the RC filter, fc = 1 ÷ (2πRC). A chart shows both curves, and a table lists the values from 1 τ to 5 τ.
The calculations run in your browser, without sending anything to a server. It accepts values with prefixes, such as 4k7, 100n and 10u.
How to use
- Enter the resistor, the capacitor and the source voltage.
- Choose the time and the target voltage.
- See the voltages, the times and the cutoff frequency.
When to use it
- Designing delays and reset circuits.
- Sizing low-pass and high-pass filters.
- Physics and electronics exercises.
Frequently asked questions
What is the RC time constant?
It is the product of resistance and capacitance, τ = R × C, measured in seconds. After 1 τ, the capacitor charges to about 63.2% of the source voltage; after 5 τ, to 99.3%, and in practice it is considered fully charged.
What is the formula for charge and discharge?
When charging, the capacitor voltage is V(t) = Vs × (1 − e^(−t/τ)). When discharging from V0, it is V(t) = V0 × e^(−t/τ). The time to reach a voltage V while charging is t = −τ × ln(1 − V/Vs).
What is the cutoff frequency?
In an RC low-pass or high-pass filter, the cutoff frequency is fc = 1 ÷ (2π × R × C). At that frequency the signal is attenuated by about 3 dB, that is, it drops to about 70.7% of its amplitude.
Is it really free?
Yes, 100% free. No sign-up, no subscription and no usage limits. All Tools123 tools are free and run right in your browser.