How Is the Mass Number Calculated for an Element?
To calculate the mass number of a specific element, find the number of protons and the number of neutrons that element has. Then, add them together. A periodic table of elements is needed to complete this task.

Find the number of protons
Using the periodic table of elements, find the atomic number of the element with mass numbers that need to be calculated. The atomic number of an element is equivalent to its number of protons. For example, the atomic number of sodium is 11. Thus, its number of protons is 11.

Find the number of neutrons
To find the number of neutrons, use the periodic table of elements again. This time, find the element’s atomic weight, and round it to the nearest whole number. Subtract the number of protons from the atomic weight. The result is the number of neutrons. Therefore, to find the number of neutrons of sodium, subtract sodium’s number of protons, which is 11, from its atomic weight, which is 23. Sodium’s number of neutrons is 12.

Add neutrons and protons together
Add the number of neutrons and protons together. The sum of these two values is the mass number. Thus, sodium’s mass number is 23.