To calculate the formal charge of boron in BF4, we must first determine the number of valence electrons in a neutral boron atom and the number of electrons that boron contributes to the molecule.

Boron has an atomic number of 5, so a neutral boron atom has five valence electrons. In BF4, boron forms four single bonds with four fluorine atoms. Each bond represents two electrons shared between boron and fluorine, so boron contributes a total of 4 electrons to the molecule.

Using the formal charge equation (formal charge = valence electrons - non-bonding electrons - 1/2 bonding electrons), we can calculate the formal charge on boron in BF4:

Formal charge on boron = 5 - 0 - 4/2

= 5 - 0 - 2

= +3

Therefore, the formal charge on boron in BF4 is +3.