How do you calculate the atomic mass of an element?

its really simple. its the top number on the element on the peridic table. thats all
For example, the atomic mass of lithium is 7 and the atomic mass of oxygen is 16

You can add the number of protons and number of neutrons together for a rough estimate of the atomic mass (in AMU).  1 atomic mass unit (AMU) is about 1.66 × 10^-24 grams.  Electrons weigh about 1/2000 of an AMU and are inconsequential for a rough estimate of atomic weight (with consideration to significant digits).

Conversely, you can determine the number of neutrons in an average atom of an element by subtracting the number of protons (the atomic number) from the atomic weight (in AMU).

Check the link, as it may point you in the right direction.

The atomic mass is the number of protons and neutrons in an atom.  You can figure out how many protons in an atom by the position in the periodic chart.  It has to be neutral, so the number of electrons equals the number of protons. Now, on the periodic chart is the mass number, so subtract the number of protons from that, you you get the number of neutrons.  You round the mass number to the nearest whole number.

Atomic mass number, or simply mass number, is the number of neutrons and protons found in one atom of a specific element.

Its the number on top of the symbol of your periodic table...