One way by which you can calculate the number of protons, neutrons, and electrons is to refer to your periodic table.
1. Obtain your periodic table and locate the element you want to find its proton, neutron, and electron.
2. There would be a number at the top left part of your element. This number is the atomic number of that element. However, the atomic number is equal to the number of protons and electrons. Just that proton is a positive charge, while the electron is a negative charge. For example, 16 is the atomic number of Oxygen. Therefore, its number of protons is 16, and the number of electrons is also 16.
3. Now, check the lower left part of your element, and you will see a number there. This number is referred to as the mass number of that element. Thus, to calculate the number of neutrons of that element, simply subtract the proton number from the mass number; this will give you the number of neutrons.