Croissants finish proofing when the dough has expanded to two-and-a-half times its original volume.
This expansion is produced mainly by yeast fermentation. As yeast consumes sugars in the dough, it generates carbon dioxide, and the gas collects in the dough’s flexible gluten structure. The result is a larger, aerated piece of dough ready for the oven.
The volume benchmark is more useful than relying on time alone. Fermentation speed depends on yeast level, dough temperature, room conditions, and the size and shape of each croissant. A dough that has not expanded enough may bake up dense, while over-proofed dough can lose strength and collapse or produce weak layers.
Proofing also affects the laminated structure. The dough contains thin sheets separated by butter or another roll-in fat, and yeast-generated bubbles expand between those layers. During baking, steam and expanding gases provide additional lift. The two-and-a-half-times figure therefore marks the end of fermentation, not the final size of the baked pastry.