Free Half-Life Calculator — Decay Formula & Chart

How to use the half-life calculator

Pick what to solve for. Remaining quantity, elapsed time, or the half-life itself — the labels update to match.
Enter the three known values. Initial quantity, and two of: remaining, half-life, elapsed time.
Press Calculate. You get the answer, the formula with your values substituted in, and a decay chart with half-life markers.
Read the chart. The dashed lines show where each half-life falls — the gold dot marks your exact point on the curve.

What is the half-life formula?

N = N₀·(1/2)^(t/T): after each half-life T, half the quantity remains. After t/T half-lives, the fraction left is (1/2) raised to that count.

How do you find elapsed time from decay?

Solve the decay law for t: t = T·log₂(N₀/N). It counts how many half-lives it takes to fall from N₀ to N, then multiplies by T.

What does one half-life do?

It halves the quantity — whatever you have, one half-life later you have exactly half. Two half-lives leave a quarter, three leave an eighth.

Can the remaining quantity exceed the initial quantity?

No — that would be growth, not decay. If your remaining amount is larger, either the initial value is wrong or you need a growth model instead.

Does the quantity ever reach zero?

Mathematically, never: each half-life halves what is left, so the curve approaches zero but only reaches it after infinite time.

You’re here

Decay math plus a decay chart.

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