The empirical formula of Calgon is

The formula of calgon is ${\mathrm{Na}}_2\left[{\mathrm{Na}}_4\right({\mathrm{PO}}_3{)}_6].$ Calgon is a trade name of a complex salt, sodium hexametaphosphate $({\mathrm{NaPO}}_3{)}_6$. It is used for softening hard water.

The empirical formula of a chemical compound is the simplest whole number ratio of atoms present in a compound.

So, empirical formula of calgon will be ${\mathrm{NaPO}}_3$.