Science, Tech, Math ? Science How Sunscreen SPF Is Measured Share Flipboard Email Print RuslanDashinsky/Getty Images Science Chemistry Chemistry In Everyday Life Basics Chemical Laws Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated March 24, 2017 SPF (Sun Protection Factor) is a multiplication factor you can use to determine how long you can stay out in the sun before getting a sunburn. If you normally can stay out 10 minutes before burning, a sunscreen with an SPF of 2 will let you stay out twice as long, or 20 minutes, before feeling the burn. An SPF of 70 will let you stay out 70 times longer than if you had no protection (or 700 minutes in this example, which would be over 11 hours or a full day). How Is SPF?Determined? Think SPF is a?calculated value or an experimental lab value, based on how much ultraviolet light penetrates a coating of sunscreen? Nope! SPF is determined using human experimentation. The test involves fair-skinned volunteers (people who burn most quickly). They apply the product and bake in the sun until they start to fry.? What about water resistant? For a sunscreen to be marketed as 'water resistant', the time required to burn must be the same before and after two consecutive 20 minute soaks in a Jacuzzi. The SPF factors are calculated by rounding down the time required to burn; however, you may get a false sense of protection from an SPF because the amount of sunscreen used in the tests is a lot more product than the average person uses. The tests use 2 milligrams of formula per square centimeter of skin. That's like using a quarter of an 8-oz bottle of sunscreen for a single application. Still... a high SPF confers more protection than lower SPF.?How Sunless Tanning Works | How Sunscreen Works?