It's the time it takes for half of a radioactive (or otherwise decaying) substance to decay or transform into something else.

How is remaining quantity calculated after a given time?

Remaining amount = initial amount × (1/2)^(elapsed time ÷ half-life).

Is half-life used outside of radioactivity?

Yes — the same exponential decay math applies to drug elimination from the body (pharmacokinetics), certain chemical reactions, and other decay processes.

Half-Life Calculator - CalculatorLabs.net

Half-Life Calculator

Half-life is the time it takes for half of a radioactive (or otherwise exponentially decaying) substance to break down.

Initial Amount

Half-Life (any time unit)

Elapsed Time (same unit)

Instant Result

Remaining Amount -

What Is a Radioactive Half-Life Calculator?

Half-life is the time it takes for half of a radioactive (or otherwise exponentially decaying) substance to break down. Whatever the starting amount, after one half-life only half remains, after two half-lives a quarter remains, and so on. This calculator finds how much is left after any elapsed time.

How to Use This Radioactive Half-Life Calculator

Enter the initial amount of the substance (in whatever units you're using — grams, count, activity, etc.).
Enter the half-life of the substance.
Enter the elapsed time, using the same time unit as the half-life.
Click Calculate to find the remaining amount.

Formula

Remaining Amount = Initial Amount × 0.5^(t / half-life)
N = N₀ × (1/2)^(t / t½)

Worked Example

100 g of a substance with a half-life of 5 years, after 15 years. Remaining = 100 × 0.5^(15/5) = 100 × 0.5³ = 100 × 0.125 = 12.5 g.