Just how long can you stay out on the beach? That’s a tricky question—but an important one for your health. The Sunbathing Calculator tries to answer it. Introduced by the web-based Omni Calculator Project, the online tool lets you plug in basic information about your location, skin type and sunscreen SPF and then comes up with an idea of how long you can safely stay in the sun before getting a burn. For example, if you’ve got a sunscreen SPF of 50, you should be safe for an afternoon in the Southwest Florida sun. If you’re going out sans cream, you better cover up: You’ll be good for only about 15 minutes. See more at omnicalculator.com/other/sunscreen.