The dangers of overexposure to sunlight have been well publicized, but less attention has been given to an acknowledged benefit of exposure to UV radiation; that being the cutaneous synthesis of vitamin D₃. Here we define a standard vitamin D dose on the basis of recently recommended requirements for vitamin D that take account of its risk reduction role in a variety of diseases, and present a web-based tool that enables the reader to calculate associated exposure times for any time and place using either default values or user-selected conditions. Either it is not possible to synthesize vitamin D₃ at high latitudes in winter, or the exposure time required to reach a standard dose is sometimes impractical. Where solar UV is sufficient, a risk-benefit analysis of sunburn vs. vitamin D₃ synthesis shows that the best time for brief sun exposure is in the middle of the day. For low solar elevation angles common at high latitudes, a fine line exists between adequate UV exposure for vitamin D₃ synthesis and a risk of sun burn.