RC Time Constant Calculator
Find the time constant of a resistor-capacitor circuit.
Results update as you type.
How it works
The RC time constant sets how quickly a capacitor charges or discharges through a resistor, and this calculator finds it from the resistance and capacitance. You enter the two values and it returns the time constant, written with the Greek letter tau, along with the roughly five-time-constant interval after which the capacitor is essentially fully charged or discharged. The time constant equals resistance times capacitance, so larger resistors or capacitors slow the process down. Its physical meaning is precise: after one time constant a charging capacitor reaches about 63 percent of the final voltage, after two about 86 percent, and after five over 99 percent, at which point it is treated as complete. This exponential behaviour governs a huge range of electronics, from the timing in oscillators, blinkers, and 555-timer circuits to the smoothing of power supplies, the debouncing of switches, and the cutoff of filters that shape audio and radio signals. Engineers and hobbyists use the time constant to design delays and frequency responses, and students meet it in every introductory circuits course. Be careful with units: resistance in ohms and capacitance in farads give the time constant in seconds. Enter the resistance and capacitance to compute it.
τ = R · C; full charge ≈ 5τ
The time constant of a resistor-capacitor circuit equals the resistance multiplied by the capacitance. After one time constant, a charging capacitor reaches about 63 percent of the supply voltage, and a discharging one falls to about 37 percent. The capacitor is considered essentially fully charged or discharged after five time constants, when it has reached over 99 percent of its final value.
Source:Electronics (RC circuit)
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Worked example
How to use this calculator
- 1
Enter your values
Enter resistance (r) and capacitance (c).
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Time constant (τ) updates instantly as you type — no button to press.
Frequently asked questions
What is the RC time constant?
The RC time constant is the time it takes a capacitor to charge to about 63 percent of the supply voltage, or discharge to about 37 percent, through a resistor. It equals the resistance multiplied by the capacitance and is denoted by the Greek letter tau. It characterises how fast an RC circuit responds to changes in voltage.
How long until a capacitor is fully charged?
A capacitor is considered essentially fully charged after five time constants, when it has reached more than 99 percent of the supply voltage. After one time constant it is at 63 percent, after three at about 95 percent, and after five at over 99 percent. In practice, engineers use the five-time-constant rule to judge when charging is complete.
What affects the time constant?
Only the resistance and the capacitance affect the time constant, and it is simply their product. Increasing either the resistance or the capacitance lengthens the time constant and slows charging and discharging. The supply voltage does not change the time constant; it only sets the final voltage the capacitor approaches. This makes RC timing easy to tune by choosing component values.
Where are RC time constants used?
RC time constants appear throughout electronics: in timing circuits like blinkers and the classic 555 timer, in filters that pass or block certain frequencies, in smoothing capacitors that steady power supplies, and in switch debouncing. Any circuit where a signal needs a controlled delay or a frequency-dependent response relies on choosing the right resistance and capacitance.
What units should I use?
Use ohms for resistance and farads for capacitance to get the time constant in seconds. Since real capacitors are usually rated in microfarads or nanofarads, convert them to farads first: one microfarad is 0.000001 farad. Mixing units, such as ohms with microfarads, will give a result that is off by a large factor, so convert carefully before entering values.