Very good question

In order to calculate the number of neutrons for any element in the Periodic Table of the Elements, you have to locate the element first. Potassium has the symbol “K” and it is located in the column at the very right of the table here . Once there, you proceed to locate the “atomic number” of potassium which is the number in the upper left-hand corner of the square. For Potassium this is 19.

To continue with your calculation of number of neutrons, you also need to find the atomic mass of the element, which is written underneath the symbol of the element. For Potassium this number is 39. Finally in order to calculate the number of neutrons of that element, you have to do some simple math:

Number of neutrons = atomic mass number - atomic number

However, potassium has 3 isotopes. An isotope is an atom of an element with the same number of protons, but differing numbers of neutrons, which means that isotopes have different atomic weights.

Because potassium has 3 isotopes, then we have to calculate for the different atomic weights of the isotopes:

Potassium-39 - It has 19 protons and atomic mass is 39. Hence, number of neutrons is 39 - 19 which is 20.
Potassium-40 - It has 19 protons and atomic mass is 40. Hence, number of neutrons is 40 - 19 which is 21.
Potassium-41 - It has 19 protons and atomic mass is 41. Hence, number of neutrons is 41 - 19 which is 22.

Hence, the Potassium number of neutrons are 20, 21 and 22.

I recommend you to read this link, where it is easy and fun to learn about the way to calculate the number of neutrons of any element on the periodic table of elements.