How to calculate ingredient percentage in formulations (%w/w)
Question
How is the percentage of an ingredient (%w/w) calculated in a formulation, and how do I determine the required weight of an ingredient for a specific batch size?
For instance, how much Alpha Arbutin is needed for a 100-gram cream formulation if the formula requires 2% (%w/w)?
Answer
Calculating ingredient percentages in formulations is usually done as percentage by weight per volume (%w/v) or more commonly, percentage by weight per weight (%w/w). This refers to the number of grams of that ingredient in 100 grams of the total formulation.
Simple Calculation Method:
If you need X percent (%w/w) of ingredient A in a total formulation of Y grams:
- Weight of Ingredient A needed (grams) = (X / 100) * Y
Example:
If you want to prepare a 100-gram cream formulation and need to include 2% (%w/w) Alpha Arbutin:
- Weight of Alpha Arbutin needed = (2 / 100) * 100 grams = 2 grams
- Other ingredients in the formulation = 100 grams (total formulation) - 2 grams (Alpha Arbutin) = 98 grams
So, in a 100-gram formulation, you would use 2 grams of Alpha Arbutin and a total of 98 grams of other ingredients.
Important Notes:
- The sum of the percentages of all ingredients in the formulation must always equal 100%.
- This calculation is a basic foundation. Actual formulation preparation might involve additional details, such as the density of certain ingredients or calculating as %w/v in some cases. However, generally for solid ingredients in liquid or semi-solid formulations, %w/w is commonly used.
Hope this explanation helps make the calculation of ingredient percentages easier to understand.