SPF is a measure of how much solar energy (UV radiation) is required to produce sunburn on protected skin (i.e., in the presence of sunscreen) relative to the amount of solar energy required to produce sunburn on unprotected skin. As the SPF value increases, sunburn protection increases. There is a popular misconception that SPF relates to the time of solar exposure. For example, many consumers believe that, if they normally get sunburn in one hour, then an SPF 15 sunscreen allows them to stay in the sun 15 hours (i.e., 15 times longer) without getting sunburn. This is not true because SPF is not directly related to the time of solar exposure but to the amount of solar exposure. Although the amount of solar energy is related to solar exposure time, there are other factors that impact the amount of solar energy. For example, the intensity of the solar energy impacts the amount.

Source: U.S. Food & Drug Administration