To achieve a good sun protection, a layer thickness of 2 mg/sq cm is often recommended. Fifty individuals were asked to apply five different sunscreens ad libitum. Ten percent dihydroxyacetone was added to the sunscreens in order to make them fluoresce when irradiated with Wood's light. The layer thickness was calculated by dividing the amount applied by the area. The thicknesses of the sunscreen layers varied little between different parts of the body and different brands; in general it was close to 1 mg/sq cm. The corresponding protection factor was measured for two sunscreens on 20 persons. The results indicate that the sun protection factor under ad libitum conditions is only 50% of what would be achieved using a layer thickness of 2 mg/sq cm.