When writing a recipe (formula), Baker’s Percentage, Baker's Percent, Flour Weight, or Baker’s Math is a way to express the ratio of ingredients to one another by weight. Baker’s % is internationally used to express formulas for baked products such as bread, cookies, cakes, scones, and most any product where flour is the primary ingredient.
Each ingredient in a formula is expressed as a percentage of the largest ingredient, usually the flour weight, always expressed as 100%.
The advantage of this system is that it allows for the baker to easily convert their recipe into different weight indicators, such as pounds, ounces, kilograms, or grams. And once all of the weights of the ingredients in the recipe are calculated, it is easy to scale, multiply or divide the recipe. It also leads to greater consistency in recipes because it is always based on weight (pounds or kilograms). By looking at percentages it is easier to tell if one recipe is drier, sweeter, saltier, etc. than another recipe. It also makes predicting what the final product will look like easier. Baker’s % can be used to quickly and easily convert between batch sizes as well.
Source: Sarah Philips, CraftyBaking