Each person depending upon skin type is affected differently by the sun. Something  to consider when looking for sunscreen is how quickly your skin reddens in the sun.

Here’s a calculation from Web MD:

If you’d normally burn in 10 minutes, SPF 15 multiplies that by a factor of 15, meaning you could go 150 minutes before burning.  SPF stands for sun protection factor it tells you how much longer you can stay in the sun with out burning.  There is not a single sunscreen that blocks 100% of the suns uv rays but apply properly and SPF 15 filters 93% and then SPF 30 filters 97%. You want to look for a sunscreen that covers both UVA and UVB rays. FDA has approved titanium the oxide, zinc oxide and avobenzone. So keep an eye out for some screens that have one or more of these ingredients, don’t just grab for the high SPF .

Here’s a few more Great tips from Web MD:

Apply the sunscreen 15 to 30 minutes before you go out in the sun. For woman, sunscreen can be applied under makeup. Use about 1 ounce (or 2 tablespoons) to cover your whole body. Don’t skimp. A number of studies show that people simply don’t use enough — and only get 10% to 25% of the benefit.

Don’t forget the easy-to-miss spots, like the tips of your ears, your feet, the back of your legs, and, if you have one, your bald spot. Since your lips can also get sunburned, use a UV-protective lip balm and reapply it regularly, Fairbrother says.

No matter how long-lasting it’s supposed to be, reapply sunscreen at least every 2 hours, and more often if you’re sweating or getting wet.

The FDA defines water resistant sunscreen as meaning that the SPF level stays effective after 40 minutes in the water. Very water resistant means it holds after 80 minutes of swimming. These sunscreens are in no way water-proof, so you’ll need to reapply them regularly if you’re taking a dip.

Pay attention to the expiration date on the bottle. Sunscreen loses its effectiveness over time.

Wear sunscreen whenever you’re out during the day — and not only when it’s hot and sunny. On a grey, overcast day, up to 80% of the dangerous UV rays still make it through the clouds. And during the winter, exposure to the sun’s rays still can have damaging effects on your skin.