FEN Position Builder
Set up any chess position and get its FEN, or paste a FEN and see the board. Checks for positions that are well-formed but impossible.
Click a square to place the selected piece · click it again to clear it
What a syntax check will not catch
- A FEN can be perfectly well-formed and still describe a position that cannot exist. Syntax checking is not enough on its own.
- The commonest fault in a hand-written FEN is castling rights the pieces do not support — "KQkq" with a king that has already moved off e1.
- The second commonest is the side NOT to move being in check. That means the previous move left a king attacked, which is illegal, and most engines will refuse the position outright.
- The halfmove clock counts moves since the last capture or pawn move, for the fifty-move rule. The move number counts full moves and starts at 1.
- The en passant square is given whenever a pawn has just moved two squares, whether or not anything can actually capture it.
How to use
- Pick a piece and click squares to place it.
- Or paste a FEN into the box and the board follows.
- Read the checks — they catch positions that parse but cannot exist.
Frequently asked questions
Why does it say my position is impossible when the FEN looks fine?
Because being well-formed is not the same as being possible. The two commonest faults are castling rights the pieces do not support — KQkq with a king that is not on e1 — and the side NOT to move being in check, which means the previous move left a king attacked. Both parse perfectly and both are rejected by real engines.
Why can a pawn not be on the first or last rank?
Because it would have promoted. A pawn reaching the eighth rank must become another piece immediately, and a pawn can never move backwards to the first, so neither square can hold one.
What are the last two numbers?
The halfmove clock and the move number. The clock counts moves since the last capture or pawn move and drives the fifty-move rule; the move number counts full moves and starts at 1, going up after Black moves.
When is the en passant square set?
Whenever a pawn has just moved two squares, whether or not anything can actually capture it. The square recorded is the one the pawn skipped over, not the one it landed on.
Does it check the position is reachable from the start?
No, and nothing practical does — that question is genuinely hard. It checks the rules a position must obey to be legal at all: piece counts, kings, pawn placement, castling rights, en passant, and whether the wrong side is in check.
Is being in check a problem?
Not if it is your move — that is an ordinary position and it is only noted. It is an error when the side NOT to move is in check, because that could only arise from an illegal previous move.
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