The chemical formula of rust is:

Rust is hydrated ferric oxide and its chemical formula is Fe₂O₃.xH₂O . The generic name of a very common compound iron oxide is rust. Iron oxide, the Fe₂O₃  is abundant because iron is so easily mixed with oxygen so readily, in fact, that pure iron is rarely present in nature. So, rusted iron is always present in  Fe₂O₃.xH₂O form.

The chemical equation for the formation of rust is given as:

$4\mathrm{Fe}+3{\mathrm{O}}_2+{\mathrm{xH}}_2\mathrm{O}\to 2{\mathrm{Fe}}_2{\mathrm{O}}_{3.}{\mathrm{xH}}_2\mathrm{O}$

Hence, we can say that the chemical formula of rust is Fe₂O₃.xH₂O.