SPF 50 means the sunscreen filters about 98% of UVB rays, based on the FDA’s standardized sunburn test.
The number comes from a standardized test that measures how much UV exposure it takes to redden protected skin compared to unprotected skin.
How the SPF Number Is Measured
The SPF value on a sunscreen label comes from a lab test where researchers apply sunscreen to skin and shine UV light on it. The amount of UV needed to cause sunburn on protected skin, divided by the amount needed to cause sunburn on unprotected skin, gives the SPF rating. An SPF 50 product therefore filters about 98% of UVB rays, while SPF 30 filters about 97%.
The FDA explains that SPF values primarily indicate UVB protection because they’re based on sunburn testing, which UVB rays cause. The label doesn’t tell you everything about UVA protection, even though “broad spectrum” sunscreens do offer some.
What SPF 50 Does NOT Mean
SPF 50 doesn’t mean you can stay in the sun 50 times longer without burning, and it certainly doesn’t mean you’re fully protected. The rating assumes you applied a full two milligrams of sunscreen per square centimeter of skin — roughly a shot glass full for your whole body. Most people apply about half that, which effectively drops an SPF 50 down to around SPF 22.
SPF is not a percentage of all UV rays blocked, either. The FDA explicitly notes that SPF values only indicate a sunscreen’s UVB protection, so don’t treat the number as a total solar shield.
Why Sunscreen Must Be Reapplied
The FDA frames sunscreen as one part of a broader sun-protection routine, not a one-time shield. Sunscreen chemicals and minerals break down, rub off, and wash away over time, so reapplication matters more than chasing a higher number.
Apply sunscreen 15 minutes before going outside to every exposed area, and reapply at least every two hours, or immediately after swimming or heavy sweating. The FDA also recommends adding shade, hats, and protective clothing to your routine, since SPF alone doesn’t fully protect you.
For a practical guide to tested formulas, see our roundup of the best 50 SPF mineral sunscreens currently worth your money.
Why Labels Cap Out at SPF 50
You’ll notice most U.S. sunscreens top out at SPF 50 or SPF 50+, and that’s deliberate. The FDA has proposed limiting maximum SPF labeling to “50+” because the evidence isn’t strong enough to show that products above SPF 50 offer users meaningfully more protection than SPF 50 products.
The difference between SPF 50 and SPF 100 is about one percentage point of UVB filtering, which the FDA’s testing standards don’t treat as a real benefit.
Here’s the practical takeaway:
- SPF 30 blocks about 97% of UVB rays
- SPF 50 blocks about 98% of UVB rays
- SPF 50+ is the highest tested and labeled tier in the U.S.
The real difference between SPF 30 and SPF 50 matters most in the afternoons or for people who burn easily. What makes a bigger difference is whether you actually reapply.
FAQs
Is SPF 50 enough for a full beach day?
SPF 50 provides strong UVB protection, but it only works if you reapply it every two hours and after swimming or sweating. The FDA recommends pairing sunscreen with shade, hats, and clothing for full coverage. No SPF rating lasts an entire day without reapplication.
Does SPF 50 protect against UVA rays?
SPF values primarily measure UVB protection because they’re based on sunburn testing. For UVA protection, look for a “broad spectrum” label on the bottle. A broad spectrum SPF 50 sunscreen protects against both UVA and UVB rays, making it a solid everyday choice.
Is SPF 100 better than SPF 50?
You get nearly identical sunburn protection from both, so the higher price of SPF 100 usually isn’t worth it.
References & Sources
- U.S. Food and Drug Administration. “Sunscreen: How to Help Protect Your Skin from the Sun.” Official consumer guidance on SPF testing and sun-safety practices.
- U.S. Food and Drug Administration. “Questions and Answers: FDA Announces New Requirements for Over-the-Counter Sunscreen Products Marketed in the U.S.” Explains proposed SPF 50+ labeling cap and testing standards.
- McGill University Office for Science and Society. “What Does SPF in Sunscreen Really Mean?” Accessible explanation of SPF measurement and UVB filtering percentages.