RC Time Constant Calculator

The time constant tau is resistance times capacitance (tau = R x C), measured in seconds. It is the time for the capacitor to charge to about 63.2% of the supply voltage, or to fall to about 36.8% when discharging. Larger R or C means a slower circuit.

Roughly five time constants. After 1 tau a capacitor reaches about 63%, after 3 tau about 95%, and after 5 tau about 99%, which is usually treated as fully charged. The tool shows the 3 tau and 5 tau times directly.

The corner frequency is f = 1 / (2 pi R C), the point where a first-order RC filter attenuates the signal by 3 dB. For 1k ohm and 1 microfarad the calculator shows 159.154943 Hz, which rounds to about 159.15 Hz.

Convert to farads first, because the field expects farads. 1 microfarad (uF) is 0.000001, or 1e-6, and 1 nanofarad (nF) is 1e-9. You can type scientific notation like 1e-6 directly into the capacitance box.

Yes. Set the Solve for menu to Resistance or Capacitance, then enter the time constant plus the one component you know. The calculator rearranges tau = R x C, so R = tau / C and C = tau / R, to find the missing value.

The tool auto-scales the time unit for readability. Values of 1 second or more show in seconds, milliseconds appear as ms, microseconds as us, and smaller values as ns. So tau of 0.001 s is displayed as 1 ms rather than a string of leading zeros.

No. Tau depends only on R and C, not on the supply voltage. A higher supply charges the capacitor to a higher final voltage but follows the same exponential timing curve, reaching 63% at 1 tau regardless of the level.

Yes, the timing is symmetric. The same tau that charges a capacitor to 63.2% of the supply in one time constant also discharges it to 36.8% of its starting voltage in one time constant, and about five tau to settle either way.

No. The calculation runs entirely in your browser, so the component values you enter never leave your device and nothing is stored on a server.