☢️ Half-Life Calculator
Radioactive decay remaining over time
About this calculator
Half-life is the time for half of a substance to decay, and this calculator tracks how much remains after any elapsed time. Enter the starting amount, the half-life, and how much time has passed — it works for radioactive isotopes, medications leaving the bloodstream, and anything else that decays exponentially.
The formula is: remaining equals initial amount times one-half raised to (elapsed time divided by half-life). After one half-life 50 percent remains, after two 25 percent, after ten less than a tenth of a percent — it never quite reaches zero mathematically.
Tip: For medications, five half-lives clears about 97 percent of a drug — pharmacists use this rule to estimate when a dose has effectively worn off.