The coming holiday weekend is expected to see temperatures in the 90s, and it’s traditionally a time when people spend their time outdoors. Mayo Clinic experts say sunscreen is a must – but knowing what is the best sun protection factor (SPF) is tough when people have no idea what SPF actually means.

Mayo Clinic Dermatologist Dr. Dawn Davis said there’s a specific formula for calculating SPF and understanding it can be the key to avoiding sunburns. “It stands for sun protection factor — it’s simply a ratio of the number of minutes that you can stay outside with the product on before getting minimal redness to the skin,” said Dr. Davis. She added preventing that painful redness is one of the biggest factors in preventing skin cancer.

So, how do the makers of sunscreen calculate an SPF?

“So if you stand outside in a particular location and you’re testing the sunscreen, and it takes you 10 minutes to develop redness to the skin without the product on, but then you apply the product on a different area of skin, and it takes 50 minutes for the skin to show redness, then that is an SPF factor of 50 over 10, which equals five,” Dr. Davis said.

She recommends a minimum of SPF 30, which theoretically means people could stay protected from ultraviolet rays 30 times longer than without sunscreen.