RC Time Constant Calculator
Enter a resistance and a capacitance and get the circuit's time constant — the single number that sets how fast it charges, discharges, filters, and times.
Two values, one answer
Enter the resistance in ohms and the capacitance in farads and the calculator returns the time constant with the formula τ = R × C.
It is a speed
A bigger time constant means slower charging. After one τ the capacitor reaches ~63 %; after five it is essentially fully charged.
What is the RC time constant?
One number for how fast an RC circuit responds
When a resistor charges a capacitor, the voltage does not jump instantly — it rises along a smooth exponential curve. The time constant, written τ = R × C, is the single number that sets how fast that happens: resistance in ohms times capacitance in farads gives a time in seconds. After one time constant the capacitor has charged to about 63 % of the final voltage, and when discharging it falls to about 37 %. A bigger resistor or a bigger capacitor makes the circuit slower, because τ grows with both. This one figure is what timing in filters, blinkers, and debounce circuits is built on.
Enter the resistance in ohms and the capacitance in farads to get the time constant instantly.
One short formula, built from the resistance R and the capacitance C.
τ = R × CMultiply the resistance in ohms by the capacitance in farads and the answer comes out in seconds. The only thing to watch is the units: capacitor values are usually printed in microfarads (µF) or nanofarads (nF), so convert to farads first — 100 µF is 0.0001 F, and 1 µF is 0.000001 F.
Suppose you charge a 100 µF capacitor through a 10 kΩ resistor.
Convert
100 µF = 0.0001 F and 10 kΩ = 10000 Ω — put both into base units.
Multiply
10000 × 0.0001 = 1 — resistance times capacitance.
Read it off
τ = 1 s — after 1 s the capacitor is at ~63 %, and after ~5 s it is essentially fully charged.
The time constant is a speed, not a finishing time, and the first thing to read from it is the 63 % rule: after one τ the capacitor has charged to about 63 % of the final voltage, and on discharge it has dropped to about 37 %. Because the curve is exponential, each further time constant covers 63 % of whatever is left, so after 2τ you are near 86 %, after 3τ near 95 %, and by 5τ the capacitor is at roughly 99 % — close enough that engineers treat it as fully charged. That makes τ a quick way to size a circuit: a 1-second time constant settles in about 5 seconds. The relationship is a straight product, so the value moves in lockstep with the parts — double the resistor or double the capacitor and you double the time. To make a circuit react faster, reach for a smaller R or C; to slow it down, make either one bigger. This is exactly the lever you pull when setting the cutoff of a filter or the blink rate of a timer.
The formula τ = R × C is exact for a simple resistor-capacitor pair, but a few practical points are worth keeping in mind.
One R, one C, in base units
This calculator covers a single resistor charging a single capacitor. Keep the resistance in ohms and the capacitance in farads — converting microfarads or nanofarads first is the most common slip. The result assumes ideal components; real resistors and capacitors carry a tolerance, and capacitors have leakage and equivalent series resistance that shift the timing slightly. Circuits with several resistors or capacitors must first be reduced to one equivalent R and one equivalent C before this formula applies.