Half Life Calculator

What this calculator helps you do

The half-life calculator estimates how much of a substance remains after repeated half-life periods. It is useful in physics, chemistry, pharmacology, and any context where a quantity decreases by a fixed proportion over time.

How half-life works

After one half-life, 50 percent remains. After two half-lives, 25 percent remains. After three half-lives, 12.5 percent remains. The quantity keeps shrinking by the same fraction, not by the same absolute amount.

Worked example

If you start with 80 units and the half-life is 5 hours, then 40 units remain after 5 hours, 20 units after 10 hours, and 10 units after 15 hours.

How to use the result

Use the answer to understand decay patterns, compare scenarios, or check whether your manual calculation is reasonable. If the topic is medical dosing, follow qualified clinical guidance rather than relying on a generic web tool alone.

Bottom line

This page is meant to make exponential decay easier to estimate and explain.