Here’s the rumor: the higher the SPF number, the longer you can stay in the sun.
Not actually so. You could use an SPF 15 which only shields you from 93% of UV rays, but if it’s applied every half hour, that will give you more coverage than applying an SPF 30 once!
The SPF number indicates how well a sunscreen will protect you from UVB rays. This is how it works: If you typically get a sunburn in 10 minutes without protection, an SPF 15 sunscreen protects you from burns 15 times longer than those 10 minutes (or 150 minutes); an SPF 20 protects you for 20 times longer (or 200 minutes). [source]
SPF 30 shields out 97% of UV light. Anything higher doesn’t heighten its efficacy. So, solid protection is based on how often you re-apply it. Your best bet is to use an SPF 30 labeled “broad spectrum,” meaning it’s blocking out UVA/UVB rays and re-apply every two hours. And of course, always opt for ones with zinc or titanium as active ingredients!