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.  
SARAH SAYS: Bakers often talk about the percent hydration of a dough or water Baker's %; that is the amount of water (liquid) in a formula in ratio to the amount of flour.  

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 easy to scale, or multiple 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.

QUESTION: What percentage of each ingredient is in my formula?
SARAH SAYS: Divide the Ingredient Weight by the Total Flour Weight then multiply that number by 100%.
  Ingredient Percentage=Ingredient Weight/Total Flour x 100%
For example, if a formula calls for 60 pounds of water and 100 pounds of flour, the baker’s percent would be 60% water. If there are two types of flour being used, the combination of the weight of both flours will be 100%. 

QUESTION: What is the required weight of each ingredient I need in my recipe?
SARAH SAYS: Take the Ingredient Percentage and multiply it by the Total Flour Weight.
  Ingredient Weight=Ingredient Percentage x Total Flour Weight

Check out more about how to calculate with this great tutorial.