Last updated: 2026-06-16

Formal charge is a bookkeeping tool that assigns a hypothetical charge to each atom in a Lewis structure, as if every bond shared its electrons perfectly evenly. It does not represent a real charge, but it is invaluable for deciding which of several possible structures is the most reasonable. This calculator computes the formal charge on an atom from three electron counts, the standard method taught alongside Lewis structures and resonance.

The formula

$\text{FC} = V - N - \frac{B}{2}$

Here $V$ is the number of valence electrons in the free, neutral atom (its main-group number), $N$ is the number of nonbonding electrons in lone pairs, and $B$ is the number of bonding electrons shared with other atoms. Because bonding electrons are shared, the atom is credited with only half of them.

Worked example

Consider the oxygen atom in a water molecule. It has two bonds and two lone pairs, so $V = 6,\ N = 4,\ B = 4$:

The zero formal charge reflects that oxygen in water is in its most comfortable bonding arrangement. By contrast, the nitrogen in the ammonium ion, with $V = 5,\ N = 0,\ B = 8$, gives $\text{FC} = +1$, accounting for the ion's overall charge.

Why formal charge guides structure

The formal charges of all atoms in a species sum to its overall charge, which is a quick consistency check on any Lewis structure. Among valid structures, the preferred one usually has formal charges closest to zero, the fewest charged atoms, and any negative charge placed on the most electronegative atom. These guidelines pick out the dominant resonance form.

Formal charge differs from oxidation state, which instead assigns both bonding electrons to the more electronegative atom; the two answer different questions and often disagree for the same atom. To count rings and multiple bonds in a structure before drawing it, use the Degree of Unsaturation Calculator, and to verify a molecular formula against composition data, the Percent Composition Calculator.