By SPF Rating
A good suggestion is choosing a broad-spectrum sunscreen [4] with at least SPF 30. Bailey says “with less than that, in an SPF 15, you are blocking 93 percent of the UVB,” “at SPF 30, you are blocking up to 97 percent, and at SPF 50, you are blocking 98 percent.
The popular question asked is that can you double the amount of SPF [5] 15 and call it an SPF 30. Ilyas says “we can’t really judge it like that,” “you might not apply one as evenly. You can’t guarantee the coverage” she suggests that your best bet is to stick with at least an SPF 30 and apply the .04 ounces to your face using whatever tool or visualization works best for you.