Chess App is a free web tool that analyzes chess games using the Stockfish engine. Upload a PGN file and get move-by-move centipawn evaluation, with each move classified as a blunder, mistake, inaccuracy, or good move.
Chess App accepts chess games in PGN (Portable Game Notation) format. It passes each position to Stockfish, records the engine's evaluation before and after each move, and computes the centipawn loss for that move. The results are displayed on an interactive board with a centipawn evaluation bar, best-move arrows, and color-coded move classifications.
Each move is scored by comparing the Stockfish evaluation before the move to the evaluation after the move. The difference (centipawn loss) determines the classification:
One centipawn equals one hundredth of a pawn in value. A loss of 200 centipawns means the position deteriorated by roughly two pawns' worth of material or positional value compared to the engine's preferred move.
When you submit a PGN, the server runs a Python script that drives Stockfish using the python-chess library. Stockfish evaluates each position at the configured depth (higher depth is more accurate but slower). The script outputs JSON with per-move evaluations, which the app hands back to your browser and saves there for the replay screen. The server does not keep a copy once your browser has it.
Chess App is built by Nuthatch, LLC. The goal was a no-login, no-subscription tool for club players who want to review their games and understand their mistakes without paying for a commercial analysis service. The tool is free to use.
Can I analyze my chess game online for free?
Yes. Chess App is free to use. Upload a PGN file on the home page and the analysis runs immediately with no account required.
How do I check my chess game for blunders?
Export your game as a PGN from Chess.com, Lichess, or your chess software. Upload the PGN here. The tool highlights every blunder (200+ centipawn loss) in red so you can review them one by one.
Does the tool use Stockfish?
Yes. Stockfish is the engine performing all move evaluation. It is one of the strongest open-source chess engines available, rated above 3500 ELO.
What is a centipawn?
A centipawn is a unit of chess evaluation equal to one hundredth of a pawn. Stockfish expresses the advantage of one side over the other in centipawns. A score of +100 means the position is roughly one pawn better for White.
What is the difference between a blunder and a mistake?
Both are errors, but a blunder is more severe. A blunder causes a centipawn loss of 200 or more (roughly two pawns of value). A mistake causes a loss of 100 to 199 centipawns. An inaccuracy causes a loss of 50 to 99 centipawns.
Do I need to create an account?
No. There are no accounts, logins, or passwords. Submit a PGN and get the analysis right away.
Last updated: June 2026