We get it—when you're staring at a bunch of sunscreens, it's tempting to reach for the one with the highest number. SPF 50 has to be twice as good as SPF 25, right? Not exactly.

The truth is, SPF math isn’t as simple as doubling the number. Let's break it down.

What Does SPF Actually Mean?

SPF stands for Sun Protection Factor, and it measures how well a sunscreen protects your skin from UVB rays—the ones that cause sunburn and contribute to skin cancer.

The number tells you how much longer it would take your skin to burn with sunscreen on, compared to unprotected skin. So SPF 25 means it would take 25 times longer to burn than if you weren’t wearing anything. SPF 50? Fifty times longer.

But here's the catch: SPF doesn’t increase protection in a straight line.

The Real Difference Between SPF 25 and SPF 50

SPF 25 blocks about 96% of UVB rays
SPF 50 blocks about 98% of UVB rays

That’s only a 2% difference in protection—not double the effectiveness.

So while SPF 50 does offer slightly more UVB protection than SPF 25, it's not a huge leap. And no sunscreen blocks 100% of UV rays.

What Actually Matters More Than the Number?

Here’s the not-so-secret secret: the way you apply your sunscreen matters way more than whether it's SPF 25 or 50.

✅ Apply enough. Most people only use 25–50% of the recommended amount. You need about a nickel-sized dollop for your face, and a shot glass full for your body.

✅ Reapply often. SPF doesn’t last all day. Reapply every 2 hours—especially after swimming, sweating, or toweling off.

✅ Use broad-spectrum formulas. These protect against both UVA (aging) and UVB (burning) rays.

So Which SPF Should You Use?

For most everyday activities—walking the dog, running errands, going to work—SPF 25 to 30 is plenty if you apply it properly and reapply as needed.

If you’re heading to the beach, spending time in direct sun, or have very fair or sensitive skin? Go for SPF 50. Just remember: it’s not a free pass to skip reapplying.

Bottom line: Don’t let SPF numbers lull you into a false sense of security. Apply generously, reapply often, and choose a sunscreen you like enough to use every day. That’s the real magic.

Sources: Australian Academy of Science, American Oncology Network, EWG