The relationship between skin cancer and exposure to the sun is now clearly established. It is therefore necessary to ensure that consumers have effective sun protection. The effectiveness of the anti-solar products is quantified using a universal indicator, the SPF (sun protection factor). This value can be given in two different ways: by in vivo (standard ISO 24444:2010) and in vitro methods. The in vitro method was adopted for this study, for ethical reasons. The protective effect regarding non-melanoma cancers given by sun products has been proven. It is nevertheless a fact that consumers need to be made aware. Indeed, the quantity of sun protection product applied in reality by the consumer is clearly lower than the recommended amount. Under these conditions, the following question can be asked: What is the level of protection attained if half or even a quarter of the recommended dose of product is applied? In order to answer this question, 20 oils available on the market were tested in vitro at five different doses (5, 7.5, 10, 12.5, 15.0 mg over a surface of 25 cm(2)). We showed that a ratio of two polynomial functions exists between the SPF and the final mass of the product. The factors reducing the efficacy when the dose is divided by 2 are very variable, ranging from 2 to 4 according to which product is studied.