Radioactive Decay Calculator
For instructions, scroll to the middle of the page.

Time 1 >>>>>>>>>
Time 2 >>>>>>>>>
Day 0 Activity   >>

Choose Isotope >>

Half-Life >>>>>


The "Time 1" and "Time 2" input boxes are for inputting the start and end times of the interval you want to be calculated. Time 1 subtracted from Time 2 equals a difference that when divided by ten, yields the increments into which the interval will be divided.

(Time 2   –   Time 1)   ÷   10   =   Increment

For example, if you enter Time 1 as 50 and Time 2 as 70, your results will be in 2 day increments. (50, 52, 54, ... 70).

The "Day 0 Activity" is for inputting the radioactivity on the reference date and NOT for the activity at the start of the chosen interval. (Referring to the above example, this would NOT be the Day 20 activity).

As for the isotope "dropbox", we have chosen 70 of the most popular isotopes used in chemistry and medicine. They are listed in increasing mass number order. Choose the isotope, click the "Calculate" button and you have your calculations. Notice that the half-life of the isotope is displayed beneath the "Calculate" button.

Now, let's suppose you disagree with our half-life value OR you want calculations for an isotope we haven't listed. You can't use the boxes beneath the 'Calculate' button - these are output only. However, going to the drop-down menu, the last two choices are 'Other (Days)' and 'Other (Years)'. Make your selection from these two and then click "Calculate". An input 'prompt box' will appear and then you can input the half-life. Click the 'OK' on the prompt and your calculations will appear. DO NOT click the "Calculate" button again.

Radioactive Decay: Definition, Formula & Types

What is Radioactivity & Radioactive Decay?

Radioactivity is the phenomenon due to nuclear instability which is exhibited by the nuclei of an atom. It is a process by which the nucleus of an unstable atom loses energy and emits radiations. Henry Becquerel first discovered radioactivity. He wrapped a sample of a Uranium compound in a black paper and had kept it in a drawer that contained photographic plates. Later, while examining these plates, he found that they had already been exposed. This phenomenon was termed as Radioactive Decay. The element or isotope which emits radiation and undergoes the process of radioactivity is called Radioactive Element.

Radioactive decay occurs when the original nucleus or the parent nucleus of an unstable atom decomposes and forms a different nucleus or the daughter nucleus. The rate of radioactive decay is measured using half-life, which is the time required to decay half the amount of the parent nucleus.

The radioisotope of an element has unstable nuclei and so do not have sufficient binding energy to hold on to all the particles of an atom. For stabilisation of these isotopes, they require constant decaying. In the entire process of radioactive decay, they release a lot of energy in the form of radiations and often transform into a new element. The process of transformation of an isotope into an element with a stable nucleus is known as Transmutation. Transmutation can occur naturally or can be done artificially.

In a radioactive material, it is found that the radioactive decays per unit time are directly proportional to the total number of nuclei of radioactive compounds in the sample. Through this, we can mathematically quantify the rate of radioactive decay.

Radioactive Decay Formula

Radioactivity Decay Law

If the number of nuclei in a sample is N and the number of radioactive decays per unit time ?t is ?N then,

Article on Radioactive Decay - Definition, Formula & Types

Where ? is the constant of proportionality called the radioactive decay constant or disintegration constant. Also, thenumber of radioactive decays ?N is reducing the total number present in the sample. Convention tells us that this should be termed negative.

A rate of Decay:

Getting back to the expression, to see radioactive decays clearly, we should focus not on the number but on the rates. Rate here is the change per time. Calculating the rate of decay,

You can use our calculator for calculating radioactive decay rate.: Click here


Half-life is the time period in which both the number of nuclei, N and the rate of decay, R have been reduced to a half of the original value. If we start with 100 nuclei, after one half-life the number left will be 50. Second, 25. Third, 12.5. Fourth, 6.25 and so on.

Types of Radioactive Decay:

There are three major kinds of radioactive decays namely: Alpha decay, Beta decay, and Gamma decay.

Alpha decay:

The process of alpha decay involves the emission of a nucleus from an alpha particle. The alpha particle is usually a nucleus of helium which is very stable. It is a group of two neutrons and two protons. This process of transformation of one element into another element is known as transmutation. The example of alpha decay involves the uranium-238:

Beta Decay:

A beta particle is merely a high energy electron that is emitted from the nucleus. Nuclei do not contain electrons and yet during beta decay, an electron is emitted from a nucleus. At the same time that the electron is being ejected from the nucleus, a neutron is becoming a proton. Neutron breaks into two pieces with the pieces being a proton and an electron. The proton stays in the nucleus, increasing the atomic number of the atom by one. The electron is ejected from the nucleus and is the particle of radiation called beta. An electron is referred to as a beta particle, but it can also be a positron, which is a positively-charged particle. If the electrons are involved in the reaction, then the number of neutrons decreases in the nucleus one by one. Also, the proton number increases one by one. The example of the beta decay process involves Thorium-234:

Gamma Decay:

The electrons orbiting around the nucleus have energy levels, and that each time an electron moves from a high energy level to a low energy level, it emits a photon. The same thing happens in the nucleus: when it rearranges into a lower energy state, it shoots out a high-energy photon known as a gamma ray.

You can use this calculator for all-purpose mentioned below. If you find this calculator helpful then email us on .

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  • Calculator: My Corner Group
  • Content: Akriti Singh (My Corner Group Team Member)