RC Circuit Time Constant Calculator

The engine computes both curves in parallel from the same τ and time value: discharge follows V(t) = V₀·e^(−t/τ) (capacitor emptying through the resistor) and charging follows V(t) = V₀(1 − e^(−t/τ)) (capacitor filling toward the supply voltage). You only need to look at whichever output matches your actual scenario — the other is computed for reference in case you want to compare both directions at once.

Because the exponential charge and discharge curves only approach their final value asymptotically and mathematically never fully reach it, there's no single finite 'done' time. The calculator instead reports the conventional milestones: one time constant (τ) reaches about 63% charged / 37% discharged, roughly 2.3τ reaches 90%, and roughly 4.6τ (τ·ln(100)) reaches 99% — industry-standard benchmarks for 'close enough to fully charged or discharged.'

It uses the standard capacitor energy formula U = ½CV², but evaluated using the discharge voltage at your specified time — not the charging voltage — so it represents the energy still stored in the capacitor as it discharges, not the energy delivered during a charging cycle. If you're modeling a charging scenario, treat this figure as informational rather than the energy stored at that point in your intended process.

Because τ = R × C is a direct product, an order-of-magnitude error in capacitance produces the same order-of-magnitude error in the time constant and in every downstream value (voltage, current, and all three milestone times) computed from it. Since farads are enormous for real components, always double-check that you've converted microfarads (×10⁻⁶) or nanofarads (×10⁻⁹) to their equivalent decimal-farad value before entering it.