About the RC Time Constant Calculator
When a capacitor charges or discharges through a resistor, the voltage changes along a curve whose speed is set by the product of the resistance and the capacitance. That product is the time constant, written τ. This calculator gives it, and also the times to reach 63 percent, 86.5 percent and about 99 percent of the final voltage, and the cut-off frequency if the same pair is used as a filter. Type the resistance and the capacitance with their units and the RC time constant appears at once, with a few related figures underneath, and you can copy the result with one tap. The sums run in your browser, so nothing you enter is uploaded.
Updated October 2026
How to use the RC Time Constant Calculator
- 1Enter the resistance and the capacitance with their units.
- 2Pick the units or options from the menus if the RC time constant tool offers them.
- 3Read the RC time constant in the large result box.
- 4Look at the extra rows for related figures.
- 5Press Copy result to keep the answer.
When you need the RC time constant
Hobbyists use the time constant to set delays with a 555 timer, to debounce switches and to smooth signals. Designers use it to choose filter values for audio, sensors and power supplies.
The tool answers questions such as how long an LED will glow after power is removed from a capacitor, or how fast a sensor reading will settle. Use the 5τ figure as the practical time to a full charge.
How the RC time constant is worked out
τ = R × C. After one τ a charging capacitor reaches 63.2 percent of the supply, after 2τ 86.5 percent, and after 5τ about 99.3 percent. The cut-off frequency of an RC filter is 1 ÷ (2π × R × C). With 10 kΩ and 100 µF, τ = 10,000 × 0.0001 = 1 s. The capacitor is about fully charged after 5 s, and the same pair used as a low-pass filter has a cut-off of 0.159 Hz.
Every answer on this page comes from that one rule, so you can check any result by hand with a calculator. The figures shown underneath, such as other units, are conversions of the same answer.
- Resistance in Ω, kΩ, MΩ
- Capacitance in pF to F
- Shows cut-off frequency
Mistakes to avoid with RC time constant
A capacitor never charges completely in theory; it only gets closer. Component tolerances, often 10 to 20 percent for capacitors, mean the real time varies. Make sure that the units are right: kΩ with µF gives milliseconds, not seconds.
On phone and computer
On a phone, type into the boxes one after another and read the RC time constant underneath; Copy result puts it on the clipboard for a message or a note. On a computer the inputs sit on the left and the answer on the right, so you can change one figure and watch the RC time constant move. The rc time constant calculator works the same on both. Choose the parts first. Capacitor Calculator and Resistor Calculator handle each value, Ohm's Law Calculator covers the current at the start, and Frequency Converter turns a cutoff into hertz or kilohertz.
Features
- 5τ is about full
- Mind kΩ and µF units
- Tolerances change the real time
- Clear messages when the RC time constant inputs make no sense
- Copy the result with one tap
- Private: nothing is uploaded
Frequently asked questions
How do I calculate RC time constant?
τ = R × C. After one τ a charging capacitor reaches 63.2 percent of the supply, after 2τ 86.5 percent, and after 5τ about 99.3 percent. The cut-off frequency of an RC filter is 1 ÷ (2π × R × C). With 10 kΩ and 100 µF, τ = 10,000 × 0.0001 = 1 s. The capacitor is about fully charged after 5 s, and the same pair used as a low-pass filter has a cut-off of 0.159 Hz.
What is the formula for RC time constant?
τ = R × C. After one τ a charging capacitor reaches 63.2 percent of the supply, after 2τ 86.5 percent, and after 5τ about 99.3 percent. The cut-off frequency of an RC filter is 1 ÷ (2π × R × C). The rc time constant calculator applies it exactly and rounds only the displayed answer to six significant figures.
What does 63 percent mean?
In one time constant a charging capacitor reaches 63.2 percent of its final voltage; a discharging one falls to 36.8 percent.
What is the cut-off frequency?
The frequency at which an RC filter passes half the power, where the output is 70.7 percent of the input. It is 1 ÷ (2πRC).
How long until fully charged?
About five time constants, when the capacitor reaches 99.3 percent. Some designs use ten to be sure.
Is this rc time constant calculator free and private?
Yes. It is free, needs no account and runs in your browser, so the numbers you type are not uploaded or stored.