How To Calculate Atoms?

First you need to know the molecular formula for the molecule. Lets use water, H₂O.
H has an atomic mass of 1, so H₂ = 2
O has atomic mass of 16
16+2=18, which is the relative atomic mass of 1 water molecule (relative to a carbon 12 atom).
We must also know the mass of water, lets say 50g.
We can find the number of moles of water using the equation:
  Number of moles = mass/mass of 1 mole
  number of moles = 50/18 = 2.77777777777moles

Each mole of any substance contains 6.022x10²³ molecules. So if we multiply 2.77777 by this number (avogadro's constant) then we get the number of molecules, =1.6727777x10²
Now, since there are 3 atoms present in each water (2 hydrogen, 1 oxygen), we can find the total number of atoms by multiplying this answer by 3, = 5.018333x10²⁴
If you mean the number of atoms in a single molecule, then you just need to know the molecular formula. I was assuming you meant for an amount of the molecule.

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First you need to know the molecular formula for the molecule. Lets use water, H₂O.
H has an atomic mass of 1, so H₂ = 2
O has atomic mass of 16
16+2=18, which is the relative atomic mass of 1 water molecule (relative to a carbon 12 atom).
We must also know the mass of water, lets say 50g.
We can find the number of moles of water using the equation:
  Number of moles = mass/mass of 1 mole
  number of moles = 50/18 = 2.77777777777moles

Each mole of any substance contains 6.022x10²³ molecules. So if we multiply 2.77777 by this number (avogadro's constant) then we get the number of molecules, =1.6727777x10²
Now, since there are 3 atoms present in each water (2 hydrogen, 1 oxygen), we can find the total number of atoms by multiplying this answer by 3, = 5.018333x10²⁴
If you mean the number of atoms in a single molecule, then you just need to know the molecular formula. I was assuming you meant for an amount of the molecule.

H2o