Chess FEN Validator & Parser
Verify the syntax and structural integrity of your Fen strings. Ensure compatibility with Stockfish, ChessBase, and Lichess.
Developer API
Need to integrate FEN validation into your app? Our validator logic follows the UCI protocol exactly as specified by the Stockfish team.
Visual breakdown of the 6 fields that make up a valid chess position string.
What is a Chess FEN Validator?
A FEN (Forsyth-Edwards Notation) string is the standard method for representing a specific board position. However, hand-editing FENs or buggy engine exports can lead to invalid strings that crash chess software.
Our FEN Validator performs a deep structural analysis of your position. It checks for the correct number of ranks, ensures each rank has exactly 8 squares, validates castling rights, and verifies that side-to-move and move counters are formatted correctly.
Anatomy of a FEN
- Piece Placement: 8 ranks separated by '/', describing pieces from the 8th to the 1st rank.
- Active Color: 'w' or 'b' indicating whose turn it is to move.
- Castling Rights: KQkq symbols or '-' if no castling is allowed.
- Counters: Halfmove clock (for the 50-move rule) and Fullmove number.
Frequently Asked Questions
Why is my FEN showing 'Invalid Rank Count'?
A valid chess FEN must have exactly 8 sections separated by forward slashes (/). Each section represent one rank on the 8x8 board.
Does this validator support Chess960?
Yes. Chess960 FENs (X-FEN) are fully supported as long as they follow the standard formatting rules for piece placement and castling.
Can this tool fix my FEN automatically?
The validator points out errors. To fix a FEN, we recommend using our Interactive Board tool to set up the position visually and generate a new string.