⏱️ RC Time Constant Calculator
Time constant (τ)1 s
Charging %63.21%
Remaining after discharge36.79%
Enter resistance and capacitance to calculate an RC circuit’s time constant (τ=RC), and find what percentage the capacitor has charged or discharged after a given elapsed time, using the standard exponential formulas.
How to use
- Enter the resistance (Ω).
- Enter the capacitance value and select its unit.
- Enter the elapsed time (seconds).
- The time constant and the charge/discharge percentage at that time are calculated automatically.
FAQ
What is the time constant (τ)?
It’s the time for a capacitor’s voltage in an RC circuit to reach about 63.2% of its final value, calculated as τ = R × C (resistance × capacitance).
Why is the charge percentage 63.2% at t=τ?
Substituting t=τ into the charging equation V(t)=V0(1-e^(-t/τ)) gives 1-e^(-1) ≈ 0.6321 — a universal constant that follows from the exponential function.
Why is 5τ often considered "fully" charged or discharged?
At t=5τ, the charging percentage is about 99.3% and the remaining discharge is about 0.7%, which is close enough to complete for most practical purposes.