Half-life is defined as the time taken by a substance to lose half of its quantity. This term should not be confused with mean lifetime, which is the average time a nucleus remains intact.

What is the half life of radium?

The half-life of radium-218 is 25.2 x 10-6 seconds. On the other hand, one of the most common radium isotopes is radium-226, with a half-life of 1600 years!

What is the half life of carbon?

The half-life of carbon-14 is 5730 years. This means that after 5730 years have elapsed, half of an initial quantity of carbon-14 would have disintegrated.

What is the half life of uranium?

The half-life of uranium-238 is 4.5 billion years. It is one of the three natural occurring uranium isotopes, along with uranium-235 (700 million years), and uranium-234 (246,000 years).

Half-Life Calculator

Last updated: February 3, 2026922 people find this calculator helpful

Loading 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 stable and unstable nuclei. Stable nuclei don't change, but unstable nuclei undergo a type of radioactive decay, emitting alpha particles, beta particles, or gamma rays and eventually decaying into 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 a 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.

🙋 One of the applications of knowing half-life is radiocarbon dating. Learn more about that by checking out our Radiocarbon dating calculator.

Half-life formula

We can determine the number of unstable nuclei remaining after time $t$ t using this equation:

N (t) = N (0) \times 0. 5^{(t / T)},

N(t)=N(0)×0.5(t/T),

Where:

$N (t)$ N(t) – Remaining quantity of a substance after time $t$ t has elapsed;
$N (0)$ N(0) – Initial quantity of this substance; and
$T$ T – half-life.

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

N (t) = N (0) \times e^{(- t / τ)} N (t) = N (0) \times e^{(- λ t)},

N(t)=N(0)×e(−t/τ)N(t)=N(0)×e(−λt),

Where:

$τ$ τ – mean lifetime - the average amount of time a nucleus remains intact; and
$λ$ λ – decay constant (rate of decay).

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

T = \frac{ln ( 2 )}{λ} = ln (2) \times τ

T=λln(2)=ln(2)×τ

How to calculate the half-life

Determine the initial amount of a substance. For example, $N (0) =$ 2.5 kgN(0)=2.5 kg.
Determine the final amount of a substance - for instance, $N (t) =$ 2.1 kgN(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 $19.88 minutes$ 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.

Half-life is a similar concept to doubling time in biology. Check our generation time calculator to learn how exponential growth is both useful and a problem in laboratories! Also, we use a similar concept in pharmacology, and we call it the "drug half-life". Find out more about that in our drug half-life calculator.

FAQs

What is half life?: Half-life is defined as the time taken by a substance to lose half of its quantity. This term should not be confused with mean lifetime, which is the average time a nucleus remains intact.
How to calculate half life?: To find half-life: Find the substance's decay constant. Divide the natural logarithm of 2 or ln(2) by the decay constant of the substance. Alternatively, you can multiply ln(2) by the mean lifetime.
What is the half life of radium?: The half-life of radium-218 is 25.2 x 10-6 seconds. On the other hand, one of the most common radium isotopes is radium-226, with a half-life of 1600 years!
What is the half life of carbon?: The half-life of carbon-14 is 5730 years. This means that after 5730 years have elapsed, half of an initial quantity of carbon-14 would have disintegrated.
What is the half life of uranium?: The half-life of uranium-238 is 4.5 billion years. It is one of the three natural occurring uranium isotopes, along with uranium-235 (700 million years), and uranium-234 (246,000 years).

Half-Life Calculator

Half-life definition

Half-life formula

How to calculate the half-life

FAQs

Related calculators

Mole

Atom

Grams to moles

Molarity