Calculate how much of a substance remains after a given time, or how long decay to a target amount takes. Formula: N = N₀ × (1/2)^(t/t½).
A half-life is the time taken for half of a substance to decay or break down. After each half-life the amount halves again, which gives the formula N = N₀ × (1/2)^(t/t½), where N₀ is the starting amount, t is the elapsed time and t½ is the half-life. The first mode applies that formula directly. The second mode rearranges it with logarithms to find the time needed to reach a target amount. Amounts and times can use any units, as long as the half-life and elapsed time share the same time unit.
How many half-lives does it take to reach 25 percent? Two half-lives: after one half-life 50 percent remains, and after the second half-life half of that remains, which is 25 percent of the starting amount.
Can the target amount be larger than the starting amount? No. Decay only reduces the amount, so a target above the starting amount has no solution and the calculator asks for a target at or below the starting amount.