Work out a half-life — or what remains after it
Radioactive decay, medication clearance, capacitor discharge — the same equation solves for whichever of the four numbers you do not have.
Tools used in this guide
The job
Exponential decay follows Nt = N0·(1/2)^(t/T). Know any three of initial amount N0, remaining Nt, half-life T, elapsed t — and the fourth falls out. Homework problems and real-world 'how much is left' questions are the same shape.
Fill three, leave one
Enter the three numbers you have and leave the unknown empty. The solver figures which variable is missing — half-life from two amounts and a time, or elapsed time from a decayed sample.
Worked example
100 g of a substance with a 5-year half-life: after 15 years, 12.5 g remains — three halvings (100 → 50 → 25 → 12.5). Asked the other way — when does 100 g drop to 25 g? — the same tool returns 10 years.
Where it stops
One substance, one decay constant — no decay chains, no activity units (becquerels), no biological two-phase elimination. Everything positive and finite; values above 1e15 are rejected.