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The factor that greatly affects a beer's final gravity is the amount of fermentable sugars present in the wort. Final gravity refers to the specific gravity of the beer once fermentation has completed. The initial gravity is influenced by the concentration of sugars extracted from the grains during mashing. These sugars are what the yeast consumes during fermentation to produce alcohol and carbon dioxide.

If there are higher amounts of fermentable sugars in the wort, the potential alcohol content will be higher, leading to a higher final gravity once fermentation occurs. Conversely, if there are fewer sugars, the final gravity will naturally be lower after fermentation finishes, resulting in a less alcoholic beer.

While temperature of fermentation, type of yeast, and length of the fermentation process can influence the fermentation's efficiency and the beer's overall characteristics, the primary determining factor of the final gravity remains the initial concentration of fermentable sugars. Thus, it is the initial sugar content that serves as the foundation for how much the yeast can ferment, ultimately affecting the beer's final gravity.