Sunscreen SPF Explained: What the Numbers on the Bottle Really Mean

Sunscreen labels look simple. There’s a big number, a few claims, and a picture of a beach. But most people misunderstand at least one part of what’s printed there, and the most common mistake has nothing to do with which bottle you pick. It’s how much you put on.

Here’s what each part of the label means and how to use it well.

What SPF measures

SPF stands for sun protection factor. It measures how well a sunscreen protects against UVB rays, the type of ultraviolet light mainly responsible for sunburn. It’s determined in a lab by comparing how much UV exposure it takes to produce redness on protected skin versus unprotected skin.

The scale isn’t linear, and this is where people get tripped up. According to the Skin Cancer Foundation, SPF 15 filters out roughly 93% of UVB rays, SPF 30 about 97%, and SPF 50 about 98%. Going from 30 to 50 doesn’t make you nearly twice as protected. The gains get smaller as the number climbs, and no sunscreen blocks 100%.

That doesn’t make high SPF pointless. For people who burn easily, are spending long hours outdoors, or are being treated for skin conditions, the extra margin can matter. But a very high number is not a reason to stay out longer or reapply less often. The American Academy of Dermatology recommends a broad-spectrum sunscreen of SPF 30 or higher for most people.

Broad spectrum covers the other half

SPF only tells you about UVB. The sun also gives off UVA rays, which penetrate deeper into the skin, contribute to skin aging, and also play a role in skin cancer. UVA passes through clouds and window glass, which is part of why dermatologists recommend sunscreen even on overcast days.

In the U.S., the words “broad spectrum” on a sunscreen label mean the product has passed an FDA-required test showing it provides UVA protection in proportion to its UVB protection. A sunscreen without that label may protect you from burning while doing less against UVA. Look for both: broad spectrum and an SPF of at least 30.

Under FDA rules, sunscreens that are not broad spectrum, or that are broad spectrum but under SPF 15, must carry a warning that they only help prevent sunburn and have not been shown to reduce the risk of skin cancer or early skin aging.

Water resistant, not waterproof

You won’t see “waterproof” or “sweatproof” on a sunscreen sold in the U.S. anymore. The FDA doesn’t allow those claims because no sunscreen stays put indefinitely in water or sweat.

What you’ll see instead is “water resistant (40 minutes)” or “water resistant (80 minutes).” That means the SPF on the label held up in testing after that much time in water. After that window, or after toweling off, you need to reapply. Toweling is easy to forget. It rubs sunscreen off even if you’ve only been in the water a few minutes.

How much to use

This is the part that matters most. The SPF on the bottle is measured using a specific, fairly generous amount of sunscreen, 2 milligrams per square centimeter of skin. Most people apply far less than that in real life, which means they’re getting less protection than the number suggests.

The AAD’s rule of thumb is that most adults need about one ounce of sunscreen, roughly enough to fill a shot glass, to cover the body when wearing a swimsuit. For the face alone, a common guideline is about a nickel-sized amount for the face, or the “two finger” method of squeezing a line of sunscreen along the length of your index and middle fingers. Don’t forget the ears, the back of the neck, the tops of the feet, and your scalp along any part line.

Apply it about 15 minutes before you go outside so it has time to bind to your skin. Then reapply at least every two hours, and right after swimming or heavy sweating, according to the label.

Mineral or chemical filters

Sunscreen filters generally fall into two groups.

Mineral sunscreens, sometimes called physical sunscreens, use zinc oxide, titanium dioxide, or both. They sit on the skin and protect it by absorbing UV, and to a lesser degree reflecting and scattering it. Many people with sensitive skin tolerate them well. The tradeoff has traditionally been a white cast, particularly noticeable on darker skin tones, though many newer formulas are better on this front, and tinted versions help.

Chemical sunscreens, also called organic filters, use ingredients like avobenzone, homosalate, octisalate, and octocrylene. They absorb UV and convert it into a small amount of heat. They tend to rub in clear and feel lighter, which makes some people more willing to wear them every day.

In 2019 the FDA proposed rules on sunscreen ingredients that recognized zinc oxide and titanium dioxide as generally safe and effective and asked manufacturers for more data on several chemical filters. That was a request for more information, not a finding that those filters are harmful. The AAD continues to recommend using sunscreen, and notes that the best sunscreen is the one you’ll actually wear consistently.

If you’re shopping for a beach destination, check local rules. Hawaii, for instance, has banned the sale (since 2021) of sunscreens containing oxybenzone and octinoxate because of concerns about coral reefs.

Reading the Drug Facts box

In the U.S., sunscreen is regulated as an over-the-counter drug, so every bottle has a Drug Facts panel. It lists each active ingredient and its percentage, the uses, warnings, and directions, including how often to reapply. Inactive ingredients are listed separately. If you’re avoiding a particular filter or have sensitive skin, this panel is where you’ll find out exactly what’s in it.

Check the expiration date too. The FDA requires an expiration date unless the maker’s testing shows the product stays stable for at least three years, so if a bottle has no date, the FDA says to treat it as expired three years after purchase. If a bottle is past its date, or has changed color or consistency, replace it. Heat can break sunscreen down faster, so a bottle that’s been in a hot car all summer is worth replacing.

Sunscreen is one part of the plan

Even used well, sunscreen works best as part of a broader approach. Shade, clothing, wide-brimmed hats, and UV-blocking sunglasses all help, especially in the middle of the day when UV is typically strongest. Many weather apps now show the daily UV index, which is a simple way to know when to take extra care.

And if you notice a new spot or a mole that’s changing in size, shape, or color, get it checked by a dermatologist. Sunscreen lowers risk. It doesn’t replace looking at your own skin.