To convert shortening amounts to butter amounts in a recipe, multiply the weight of the shortening by 1.25 to obtain the weight of the butter required. Multiply the butter weight by 0.15 and subtract a corresponding amount of liquid from the recipe.
Shortening contains 100 percent fat, but butter typically contains approximately 80 percent fat, 15 percent water and 5 percent milk solids. Multiplying the shortening weight by 1.25 ensures that the amount of butter used in the recipe yields an equivalent amount of fat. Similarly, reducing the liquid by the butter weight multiplied by 0.15 ensures that the butter doesn't increase the liquid content and alter the results.