What 2,476 analyzed games reveal about chess blunders

Where blunders actually happen — measured, not guessed. Every position below was scored by a real chess engine.

Based on 2,476games analyzed on Chess2EZ, and growing. We measure win-chance drops with a chess engine; there are no rating buckets here — the sample isn't rated.

Where blunders happen: it's not the opening

Beginners obsess over opening theory, but that is rarely where games are lost. Splitting every logged blunder by move number tells a different story. The middlegame is where the most damage is done — 49% of all blunders land there.

How bad are the mistakes?

Not every error is game-ending. We grade each move by how much win chance it throws away: an inaccuracy is a small slip, a mistake is a real error, and a blunder is a potential game-loser. Here is the split across every graded move.

What one blunder actually costs

A single serious mistake — a blunder or a mistake — costs an average of 20.8 percentage points of win chance in our data. That is often the difference between a win and a loss: hang one piece, and a comfortable position becomes a fight for survival. Multiply that by 3.9 serious mistakes per game and it is clear why cutting blunders is the single fastest way to climb.