Aqua regia does not have a specific chemical formula because it is a mixture, not a pure compound.

However, it's typically prepared by mixing concentrated nitric acid (HNO₃) and hydrochloric acid (HCl) in a molar ratio of 1:3.

The most common formula used to represent the mixture is:

HNO₃ : HCl (1:3)

This indicates the relative proportions of the two acids in the mixture.