SPF stands for Sun Protection Factor. It’s a number that tells you how much UVB radiation (the kind that causes sunburn) your sunscreen can filter before your skin starts to burn. The higher the number, the more protection you get, though the differences shrink as the numbers climb.
How SPF Is Measured
SPF isn’t a measure of time. It’s a ratio of energy. Specifically, it compares the amount of UV radiation needed to cause sunburn on sunscreen-protected skin versus unprotected skin. If it takes 30 times more UV energy to burn your skin while wearing sunscreen than it would bare, that sunscreen has an SPF of 30.
To earn its SPF rating, sunscreen is tested at an application thickness of 2 milligrams per square centimeter of skin. That’s a very specific, generous layer, and it’s almost certainly more than most people apply on their own (more on that below).
What the Numbers Actually Filter
The jump between SPF values is smaller than it looks on the label. Here’s what each level blocks in terms of UVB rays:
- SPF 15: blocks 93% of UVB rays
- SPF 30: blocks 97% of UVB rays
- SPF 50: blocks 98% of UVB rays
Going from SPF 15 to SPF 30 cuts the remaining UVB exposure roughly in half, from 7% to 3%. But going from SPF 30 to SPF 50 only shaves off another percentage point. This is why dermatologists often recommend SPF 30 as a practical sweet spot: you’re getting very close to the maximum UVB filtration without chasing diminishing returns. That said, a higher SPF does provide a slightly larger margin of error if you don’t apply enough or miss reapplication windows.
SPF Only Covers Half the Story
The sun emits two types of ultraviolet radiation that reach your skin: UVA and UVB. SPF measures protection against UVB only. UVB rays are the primary cause of sunburn and play a major role in skin cancer, but UVA rays penetrate deeper into the skin, accelerate aging, and also contribute to cancer risk.
This is where “broad spectrum” comes in. To carry a broad spectrum label in the U.S., a sunscreen must pass a separate test showing it absorbs UVA radiation up to a critical wavelength of at least 370 nanometers. In plain terms, the product has to prove it covers a wide enough range of UV light, not just the burning rays. If you grab a bottle that only lists an SPF number without “broad spectrum” on the label, you’re getting incomplete protection.
Why You Probably Apply Too Little
SPF testing uses 2 mg of sunscreen per square centimeter of skin. For a full adult body, that works out to roughly one ounce, about the amount that would fill a shot glass. Most people apply somewhere between a quarter and half of that amount, which dramatically lowers the real-world protection they’re getting. If you apply half the tested thickness, your effective SPF drops far below what’s on the label.
For your face alone, a nickel-sized dollop is a reasonable target. If the sunscreen feels like too much at first, that’s probably closer to the right amount than what you’re used to.
When and How Often to Reapply
Sunscreen breaks down with UV exposure, and it physically rubs or washes off over time. The FDA recommends reapplying at least every two hours, regardless of the SPF number. A higher SPF doesn’t buy you more time between applications.
If you’re swimming or sweating, the clock moves faster. Sunscreens labeled “water resistant” are tested to maintain their SPF for either 40 or 80 minutes in water, and the label is required to state which. Once that window closes, you need to reapply immediately. No sunscreen is waterproof, and the FDA doesn’t allow that word on labels.
SPF and Skin Tone
Everyone’s baseline tolerance to UV radiation is different. Lighter skin burns faster, meaning the “unprotected” side of the SPF ratio starts at a lower threshold. Darker skin contains more melanin, which provides some natural UV filtration and a longer time-to-burn. The Fitzpatrick scale classifies skin into six types based on burning and tanning tendencies, and it was originally developed to calculate UV therapy doses.
But SPF works the same way regardless of your skin type. It multiplies your individual baseline tolerance by the same factor. If your unprotected skin would burn in 10 minutes, SPF 30 means it would theoretically take 30 times more UV energy to burn. If your skin wouldn’t burn for 30 minutes, that same SPF 30 extends protection proportionally. The key point: darker skin still absorbs UV damage that contributes to aging and cancer risk, even without a visible burn. SPF matters across all skin tones.
What’s on the Label
U.S. sunscreen labels are regulated by the FDA. Current rules require the SPF number, broad spectrum status (if applicable), and water resistance duration. The FDA has proposed raising the maximum labeled SPF from 50+ to 60+, a change aimed at giving consumers slightly more information about high-protection products without encouraging the false sense of invincibility that comes with triple-digit SPF claims.
When you’re choosing a sunscreen, the three things that matter most are: broad spectrum coverage, an SPF of at least 30, and a texture you’ll actually wear consistently. The best sunscreen is the one you use correctly and reapply on schedule. An SPF 50 applied once in the morning and forgotten protects far less than an SPF 30 reapplied every two hours throughout the day.