How this calculator works
An initially uncharged capacitor approaches the supply voltage exponentially. Its time constant is resistance times capacitance. One time constant reaches about 63.2% of final charge; five reach about 99.3%. A table shows these milestones. The final 100% is approached asymptotically, so no finite charging time reaches it in the ideal model.
Formula
tau = R C; p = 1 - exp(-t/tau); t = -R C ln(1-p); t = k tau for k time constants.
Worked example
With 1000 ohms and 0.001 F, tau is 1 s. Charging to 50% takes 0.693147 s; five time constants take 5 s and reach 99.3262%.
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Frequently asked questions
Is five time constants fully charged?
It is about 99.3262% of the final value, often close enough for engineering estimates. Exact 100% requires infinite time in this model.
Does this cover discharge?
For discharge the remaining fraction is exp(-t/RC). This page reports the fraction gained during charging from zero.
What changes the effective resistance?
Source resistance, wiring resistance and the capacitor's series resistance add to a simple series charging path. Leakage and nonlinear supplies require more analysis.