RC Circuit Time Constant Calculator (with Charge/Discharge Graph)
Calculate the time constant τ = R × C of an RC circuit (a resistor R and capacitor C in series). See the charging and discharging curves on a graph, plus the time to reach 63.2% charge and the practical "fully charged" time (5τ).
Tips
- Resistance can be entered in Ω/kΩ/MΩ and capacitance in F/mF/µF/nF/pF, so you can type in the numbers straight off a component's printed value (e.g. 10 kΩ, 100 µF).
- The time constant τ represents how long it takes the voltage to change by 63.2%. It depends only on the R × C product, not on the supply voltage V₀.
- By convention, a capacitor is considered "practically fully charged" after 5τ (about 99.3%). Use this as a rule of thumb when estimating wait times in a circuit design.
- The charging and discharging curves in the graph are two sides of the same phenomenon sharing the same τ — discharge right after power-off follows the same time constant.
- This calculation is also handy for estimating switching delays in digital circuits, such as a pull-up resistor paired with a decoupling capacitor.
Frequently Asked Questions
Side Note — How Far the Idea of a Time Constant Reaches
The time constant of an RC circuit is one of the first concepts taught in electrical engineering textbooks, but the underlying idea — a quantity approaching a new value at a constant proportional rate — shows up far beyond electronics. The rate at which something cools down, the rate a drug is metabolized in the body, and the rate radioactive material decays can all be described with the same exponential decay-and-approach model.
In real circuits, the charge/discharge behavior of a capacitor has long been used to build timers that switch on or off after a set delay. The famous 555 timer IC internally relies on an RC time constant to set its oscillation frequency. Because you can freely adjust the oscillation period just by changing the resistor and capacitor values, this technique is widely used in LED blinker circuits and simple clock generation.
That said, real-world circuits involve factors the ideal formula doesn't account for, such as a capacitor's leakage current, its ESR (equivalent series resistance), and capacitance drift with ambient temperature. For applications that demand precise timing, a crystal or ceramic oscillator is generally preferred over an RC oscillator. Treat the RC time constant as a useful approximation, and reach for another approach when tight precision is required.