Calculating SPF from Ingredient Percentage

Hello. The SPF value of a sunscreen product is not calculated directly from the percentage of sunscreen ingredients. Determining the SPF value requires testing the final product (either In-vivo on human skin or In-vitro in the laboratory). This is because the SPF value depends on many factors in the overall formulation, not just the concentration of each UV filter, such as the dispersion of the UV filters in the product base, other ingredients in the formula, and how the product forms a film on the skin when applied. Therefore, to know the SPF value of a formulation containing 1% Zinc Oxide and 1% Titanium Dioxide, the finished formula must be tested. It cannot be calculated directly from the percentages.