Quick Tips: SPF Numbers

Chances are, when you see the acronym "SPF," you immediately know it has something to do with sunscreen. But do you know what it really means? And, more importantly, do you know how to determine whether one type of SPF is better than another?

SPF, which stands for sun protection factor, is a decades-old rating system designed to give consumers guidelines for determining the amount of ultraviolet (UV) light protection a particular sunscreen offers. But despite the widespread use of SPF on product labels, there is still a great deal of confusion among sunscreen users over what the ranking really indicates.



If you count yourself as someone who's in the dark about the levels of sun protection, here are some quick tips that will not only make you a more informed consumer, but also help you better protect your skin against dangerous UV rays:

Determine What SPF Means

To ascertain a product's exact SPF, developers test a group of sun-sensitive people to see how many UV rays it takes them to burn with and without the sunscreen. They divide the "with sunscreen" total by the "without sunscreen" total, and that number is then rounded down to the nearest five to get the SPF.

So how will this knowledge help you select the right SPF? Think of your own history of sun exposure. How much time in the sun does it usually take you to burn? Multiply that number by the SPF number on your sunscreen, and that will tell you how long you can reasonably expect to wear that product before getting burned. So, theoretically, if it takes you 10 minutes to burn without sunscreen, and you're wearing an SPF of 15, you could go two and a half hours without burning.

Understand How Much Protection You'll Really Receive

Despite your calculations, you should be aware that SPF can give you a false sense of protection. You have to take several things into consideration before heading confidently out into the sun. For starters, the sunscreen can only meet its full SPF potential if it's used as directed, which means you need to wear a thick coating of it. You should also give your skin a chance to fully absorb the sunscreen (about 15 to 30 minutes) before you go outside. And, finally, you need to ensure that every inch of exposed skin -- even commonly missed areas, like the earlobes -- are covered.

Don't Be Tricked by High SPFs

These days, SPFs range from 2 to 100+. It might be easy to think that a really high SPF provides you with complete and unlimited coverage, but -- unfortunately -- that's not true. To better understand why, consider the absorption rates of different SPFs. Absorption doesn't increase exponentially in correspondence with SPF. Rather, it creeps up the scale by only a few percentage points. Take SPF 15 and SPF 30, for examples. An SPF 15 sunscreen absorbs 93.3 percent of UVB rays, while an SPF 30 product absorbs 96.7 percent. That's a difference of only 3.4 percent -- not exactly what you might expect for an SPF that is essentially doubled.

Know Which Kind of UV Coverage You'll Get

Many consumers probably don't realize that, traditionally, SPF has only applied to protection from UVB rays. However, there are two categories of UV light that can lead to skin damage: UVA and UVB. UVA is typically responsible for long-term problems, such as premature aging and skin cancer; whereas UVB is mostly associated with immediate harm like sunburn. Fortunately, many sunscreen manufacturers are now incorporating UVA protection into their formulas as well. The way you can determine whether an SPF indicates defense against both types of UV rays or just UVB is to see if it's labeled as a "broad-spectrum" SPF. Broad-spectrum formulas absorb both UVA and UVB rays.

Find out if You're Protected in All Conditions

Your sunscreen's SPF value decreases dramatically when it exposed to water or sweat. Some products are designed to hold up better under such conditions but cannot offer ideal coverage. New labeling rules by the FDA require the manufacturers of such products to indicate on their labels how long SPF protection can be maintained under more challenging scenarios.

Find out more about sun protection on the next page.

Related Articles


  • Bowers, Jan. "A New Day for Sunscreen." American Academy of Dermatology. June 1, 2012. (December 4, 2012)
  • Emedicine: Sunburn.
  • Mayo Clinic: Sunburn.