Sensor Selection Guide Calculator

63.2% is exactly 1 − e⁻¹, the fraction of the supply voltage a capacitor reaches after exactly one time constant (τ = R × C) in the standard exponential charging equation V(t) = Vs(1 − e^(−t/RC)). It's the defining reference point for RC timing — every other milestone in electronics (like 3τ for ~95% and 5τ for ~99%) is measured relative to it — which is why it's the calculator's default rather than an arbitrary round number.

Mathematically no — the exponential charging curve V(t) = Vs(1 − e^(−t/RC)) only approaches Vs asymptotically and never reaches it in finite time. In practice, engineers treat 5τ (about 99.3% of the supply voltage) as 'fully charged' because the remaining gap becomes negligible for almost any real application, which is why this calculator reports the 5τ milestone rather than a true 100% time.

The identical resistor-capacitor pair that sets your charging time constant also forms a first-order low-pass filter, with a −3dB cutoff frequency of f = 1/(2πRC) — the frequency where the filter's output amplitude has dropped to about 70.7% of the input. Signals below that frequency pass through largely unattenuated, while signals above it are increasingly rolled off, which is why the same R and C values used for switch debouncing or PWM smoothing are reported here as both a charge time and a cutoff frequency — they're two views of the same physical time constant.

The formulas here assume the capacitor starts fully discharged and charges through a single resistor to a perfectly fixed supply voltage; a real circuit with meaningful source impedance, a parallel discharge path, or multiple RC stages in series will settle differently than this ideal single-stage model. Component tolerance matters too — ceramic capacitors commonly vary ±20% from their marked value — so expect your breadboarded circuit's actual timing to drift from this calculation by a comparable margin.