For photolabile sunscreens, the agreement between in vitro and in vivo measurements of SPF has been relatively low.To improve agreement between in vitro and in vivo SPF measurements, UV doses must be administered before or during the measurements.We have previously reported an in vitro method that involves irradiating a sunscreen film with a solar simulator and measuring the cumulative applied and transmitted UV doses at intervals during irradiationA least squares curve fit of the applied dose (x) vs. the transmitted dose (y) yields a useful math. function that accurately models the relationship: y = αxβ.When β is approx. 1, the sunscreen is photostable and the slope, α, is the reciprocal of the SPF.When β is significantly greater than 1, the sunscreen is not considered photostable.The SPF is the value of x when y reaches 1 and may be computed as: SPF = (1/α)1/β.An equally important consideration for accurate in vitro measurements of sunscreen SPF is the amount of sunscreen applied to the substrate.We present an approach to evaluating the volume of sunscreen on the substrate, that offers potential for significantly improving agreement between in vitro and in vivo measurements of SPF.