Cosmetics Formulation Development

Hello, I have a question and would like to ask. Let's say that the serum says that there are ingredients. Niacinamide 5% followed by a moisturizer with Niacinamide 5% followed, equating to having Niacinamide is on our page 5% or 10%, thank you

Hello, % is a unit of concentration, meaning there is such a substance. at a concentration of ...% in the formula, then when applied to the skin It cannot be counted as concentration. Because it's on the skin, not just the aforementioned formula. But it's sweat, moisture, fat on the skin. If you can count it, it must be how many grams (or milligrams) are on the surface.