# Half Life Calculator

The half life calculator is a tool that helps you understand the principles of radioactive decay. You can use it to not only learn how to calculate half life, but also as a way of finding the initial and final quantity of a substance or its decay constant. This article will also present you with the half life definition and the most common half life formula.

## Half life definition

Each radioactive material contains a stable and an unstable nuclei. Stable nuclei don't change, but unstable nuclei undergo radioactive decay, emitting alpha particles, beta particles or gamma rays and eventually decaying into a stable nuclei. Half life is defined as the time required for half of the unstable nuclei to undergo their decay process.

Each substance has a different half life. For example, carbon-10 has a half life of only 19 seconds, making it impossible for this isotope to be encountered in nature. Uranium-233, on the other hand, has the half life of about 160 000 years.

This term can also be used more generally to describe any kind of exponential decay - for example, the biological half-life of metabolites.

Half life is a probabilistic measure - it doesn't mean that exactly half of the substance will have decayed after the time of the half life has elapsed. Nevertheless, it is an approximation that gets very accurate when a sufficient number of nuclei are present.

## Half life formula

The number of unstable nuclei remaining after time **t** can be determined according to this equation:

`N(t) = N(0) * 0.5^(t/T)`

where:

**N(t)**is the remaining quantity of a substance after time**t**has elapsed.**N(0)**is the initial quantity of this substance.**T**is the half-life.

It is also possible to determine the remaining quantity of a substance using a few other parameters:

`N(t) = N(0) * e^(-t/τ)`

`N(t) = N(0) * e^(-λt)`

**τ**is the mean lifetime - the average amount of time a nucleus remains intact.**λ**is the decay constant (rate of decay).

All three of the parameters characterizing a substance's radioactivity are related in the following way:

`T = ln(2)/λ = ln(2)*τ`

## How to calculate the half life

- Determine the initial amount of a substance. For example,
`N(0) = 2.5 kg`

. - Determine the final amount of a substance - for instance,
`N(t) = 2.1 kg`

. - Measure how long it took for that amount of material to decay. In our experiment, we observed that it took 5 minutes.
- Input these values into our half life calculator. It will compute a result for you instantaneously - in this case, the half life is equal to 19.88 minutes.
- If you are not certain that our calculator returned the correct result, you can always check it using the half life formula.

Confused by exponential formulas? Try our exponent calculator.