# How do you calculate the formal charge of CO_2?

Do you mean $C {O}_{2}$ or $C {O}_{3}^{2 -}$?
None of the atoms in our representation of the $O = C = O$ molecule bear a formal charge. For carbon and oxygen, $Z = 6$ and $Z = 8$ respectively, i.e. there are 6 and 8 formal positive charges in the carbon and oxygen nuclei.
For neutrality, the carbon and oxygen atoms must also be each associated with 6 and 8 formal negative charges. Around oxygen there are 2 lone pairs (i.e. 4 electrons), and the oxygen has a half share in the four electrons that constitute the $O = C$ bond. That's $4 + 2 + 2 \text{ (inner core)}$ electrons; 8 electrons in total, and thus $O$ is electrically neutral. Carbon shares 4 electrons from the double bonds, and with its 2 inner core electrons, has the required 6 electrons, 6 formal negative charges to balance the 6 positive nuclear charges.