Half-Life Calculator

What is the Half-Life Calculator?

Half-life is the time it takes for half of a substance to decay or break down. It's used in nuclear physics, radiocarbon dating, pharmacology (how fast a drug clears your system), and even finance. This calculator works for any exponential decay situation where you know the half-life. CalciHub's version lets you calculate the remaining quantity after a given time, or figure out how long it'll take to reach a certain level.

How Does It Work?

Every half-life period, the amount of substance drops by half. This follows an exponential decay curve, not a straight line. The formula accounts for how many half-life cycles fit into the time period you're looking at.

N(t) = N₀ × (1/2)^(t / t½)

• N(t) = Remaining quantity at time t

• N₀ = Initial quantity

• t = Time elapsed

• t½ = Half-life (in the same time unit as t)

How to Use CalciHub's Half-Life Calculator

1. Enter the initial quantity of the substance.

2. Enter the half-life value and its time unit (seconds, minutes, hours, years, etc.).

3. Enter the total elapsed time in the same unit.

4. Click Calculate to get the remaining amount.

Tip: Make sure your elapsed time and half-life are in the same units. Mixing hours and days without converting will give wrong results.

A Quick Example

A medical imaging technician is working with Technetium-99m, a radioactive tracer with a half-life of 6 hours. They start with 400 mg and need to know how much remains after 24 hours.

• N₀ = 400 mg

• t½ = 6 hours

• t = 24 hours

• Number of half-lives = 24 / 6 = 4

• N(t) = 400 × (1/2)^4 = 400 × 0.0625 = 25 mg

After 24 hours, only 25 mg of the original 400 mg remains — that's four halvings, which explains why short-half-life isotopes are preferred in medical imaging: they decay quickly and limit the patient's radiation exposure.

Frequently Asked Questions