RC time constant calculator
Calculate the ideal first-order RC time constant, τ = R × C, and the usual charging milestones.
Result: τ =
| Time | Elapsed time | Ideal charging |
|---|
Equations and interpretation
τ = R × C
V(t) = Vfinal × (1 − e−t/RC)
V(t) = Vinitial × e−t/RC)
The first equation describes charging; the second describes discharging. For an ideal first-order charging circuit, 1τ is about 63.2%, 2τ 86.5%, 3τ 95.0%, 4τ 98.2%, and 5τ 99.3% of the final voltage.
Example and common mistakes
1 kΩ with 1 µF gives τ = 1 ms. So does 10 kΩ with 100 nF. Check unit prefixes carefully: µF and nF differ by 1,000, and real circuits include tolerances, leakage, source impedance and parasitic capacitance.