Half-life calculator guide

How to use the Half-Life Calculator

The Half-Life Calculator helps you solve exponential decay questions. Use it to find how much remains after time passes, how much time a decay process took, or what the half-life is when you know the starting amount, ending amount, and elapsed time.

Open the half-life calculator

Quick start

Choose Remaining amount, Elapsed time, or Half-life.
Enter the values the selected mode asks for.
Use the same time unit for elapsed time and half-life.
Press Calculate half-life.
Review the answer, percent remaining, half-lives passed, and formula steps.

The half-life formula

The remaining amount formula is remaining amount = initial amount x (1/2)^(elapsed time / half-life). The exponent tells how many half-lives have passed.

For example, if 18 hours pass and the half-life is 6 hours, then 18 / 6 = 3 half-lives. After 3 half-lives, 100 mg becomes 12.5 mg.

Choose the right mode

Remaining amount Use this when you know the initial amount, half-life, and elapsed time.

Elapsed time Use this when you know the initial amount, final amount, and half-life.

Half-life Use this when you know the initial amount, final amount, and elapsed time.

Unit tips

Keep time units consistent. If the half-life is in hours, enter elapsed time in hours. If the half-life is in years, enter elapsed time in years.

The amount unit is a label for readability. You can use mg, g, kg, %, atoms, or another unit as long as the initial and final amounts use the same unit.

Examples from the half-life calculator

Remaining amount 100 mg, half-life 6 hours, time 18 hours

12.5 mg remaining

Find elapsed time 80 g to 10 g, half-life 12 hours

36 hours

Find half-life 100 g to 25 g in 10 days

5 days

Common mistakes

Do not mix time units, such as entering half-life in hours and elapsed time in days. Convert one value first so the calculator compares the same kind of time.

For elapsed-time and half-life modes, the final amount must be greater than zero. To solve the half-life itself, the final amount must also be less than the initial amount.

Safety and privacy

This calculator is for general math, science study, and planning examples. It is not medical advice, dosing advice, or radiation safety guidance.

Recent half-life answers stay only in the current browser tab while you work. They are not sent to a server.