Count the number of atoms bound to the central atom. For example, H20, or water, has two hydrogen atoms bound to the central oxygen atom.
Count the number of lone pairs of electrons of the central atom. These pairs of electrons orbit the central atom but are bonded to each other and are illustrated in atomic representations. For the example, the oxygen atom has two lone electron pairs.
Add the number of bonds and electron pairs together. For the example, an H2O molecule has two bonds and two pairs, resulting in four. The steric number of a molecule of water is 4.