What this calculator does
Radioactive decay & half-life works out amount of a substance remaining after a given elapsed time. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaN(t) = N₀ × (1/2)^(t / t₁ᐟ₂); λ = ln2 / t₁ᐟ₂
The inputs explained
| Field | What to enter |
|---|---|
| Initial amount (units) | A number, measured in units. Starts at 100. |
| Elapsed time (years) | A number, measured in years. Starts at 1000. |
| Half-life (years) | A number, measured in years. Starts at 5730. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with initial amount
Every other input is held at the calculator’s starting values while initial amount varies. Select any row to load that scenario into the calculator.
How the answer changes with initial amount