Formal Charge Calculator
Formal charge is a bookkeeping tool for Lewis structures: it is the charge an atom would have if every bond split its electrons evenly. Comparing formal charges helps you pick the most plausible Lewis structure, since stable structures keep formal charges small and place negatives on the more electronegative atoms. To compute it you take the atom's valence electrons, subtract its lone-pair electrons, and subtract half its bonding electrons (one per bond). This calculator does that from the valence electron count, the number of lone-pair electrons, and the number of bonds.
Formal charge formula
bonding electrons = 2 * number of bonds
formal charge = valence electrons - lone-pair electrons - (bonding electrons / 2)
equivalently = valence - lone-pair - bonds
Valence electrons, lone-pair electrons, and bonds must each be zero or greater. The most favorable Lewis structures have formal charges as close to zero as possible.
Formal charge context
- Formal charge assumes bonding electrons are shared equally between atoms.
- For main-group atoms, valence electrons equal the main-group number.
- Each bond, single or part of a multiple bond, contributes two bonding electrons.
- The formal charges of all atoms sum to the molecule or ion's overall charge.
- The best Lewis structure minimizes formal charges and places negatives on electronegative atoms.
Formal charge: frequently asked questions
What is formal charge?
Formal charge is the hypothetical charge an atom would carry in a molecule if all bonding electrons were shared equally between the bonded atoms. It helps compare and rank Lewis structures: the most stable structure usually has formal charges closest to zero.
How do I calculate formal charge?
Subtract the non-bonding (lone-pair) electrons and half the bonding electrons from the atom's number of valence electrons: formal charge = valence electrons - lone-pair electrons - (bonding electrons / 2). Bonding electrons equal twice the number of bonds.
How many valence electrons does an atom have?
For main-group elements the number of valence electrons equals the main-group number: hydrogen 1, carbon 4, nitrogen 5, oxygen 6, fluorine 7, and so on. Read the group from the periodic table and enter that count.
What is the difference between formal charge and oxidation state?
Formal charge assumes electrons in a bond are shared equally, while oxidation state assigns both bonding electrons to the more electronegative atom. They answer different questions and usually give different numbers for the same atom.
Why should formal charges sum to the overall charge?
The sum of formal charges over all atoms equals the overall charge of the molecule or ion, because formal charge just redistributes the same total electron count. Checking this sum is a quick way to verify a Lewis structure is consistent.
Official sources
- IUPAC Gold Book: formal charge and Lewis structure terminology.
- NIST: periodic table and group numbers for valence electrons.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.