Half-Life Calculator

This half-life calculator works out how much of a substance is left after a period of decay, along with the fraction remaining and the number of half-lives that have passed. A half-life is the time it takes for half of a quantity to decay away, and it is the natural way to describe any process that shrinks by a constant proportion over equal time steps. After one half-life, half of the original is left; after two, a quarter; after three, an eighth, and the amount keeps halving without ever quite reaching zero. You enter the starting amount, the half-life of the substance and the time that has elapsed, making sure the half-life and the elapsed time use the same unit, and the calculator returns the amount remaining, the percentage left and how many half-lives have gone by. While half-lives are most famous in radioactivity, where they range from fractions of a second to billions of years and underpin carbon dating, the same exponential decay describes the clearance of a medicine from the bloodstream, the discharge of a capacitor in electronics, and many other settings. The formula is the starting amount times one half to the power of the elapsed time over the half-life. Enter your three values to see how much remains straight away.

Use the same time unit for the half-life and the elapsed time. Estimate only.