Iron oxide can refer to a few different compounds, each with its own formula. Here are the two most common:

* Iron(II) oxide: This is also known as ferrous oxide. Its formula is FeO. Iron has a +2 charge (Fe²⁺) and oxygen has a -2 charge (O²⁻), so they combine in a 1:1 ratio.

* Iron(III) oxide: This is also known as ferric oxide or rust. Its formula is Fe₂O₃. Iron has a +3 charge (Fe³⁺) and oxygen has a -2 charge (O²⁻), so they combine in a 2:3 ratio.

There are other iron oxides, but these are the most common and important.