The short answer: SPF by skin type and situation
Two things decide your factor: your Fitzpatrick skin type and how strong the sun will be. Here's the everyday-versus-holiday matrix — and if you sit between two options, take the higher one.
| Skin type | Everyday city (UV ≤ 5) |
Beach, holiday & high UV (6+) |
|---|---|---|
| Type I | SPF 50+ | SPF 50+ |
| Type II | SPF 30–50 | SPF 50+ |
| Type III | SPF 30 | SPF 50 |
| Type IV | SPF 30 | SPF 30–50 |
| Type V | SPF 15–30 | SPF 30 |
| Type VI | SPF 15–30 | SPF 30 |
Broad-spectrum in every case. Not sure of your type? UV Buddy's six-question quiz sorts you into I–VI — ties resolved toward the more sensitive type, which also means the higher factor.
What the SPF number actually means
SPF is a lab-measured multiplier on how long skin takes to redden. The more useful translation is the fraction of UVB blocked: SPF 15 blocks about 93%, SPF 30 about 97%, SPF 50 about 98%. Notice the shape of that curve — the jump from 15 to 30 buys you real protection, while the jump from 30 to 50 buys one percentage point.
That's not an argument that factor 30 and factor 50 are the same. It's an argument that once you're at 30+, application quality beats the label number. A generous, even layer of SPF 30 outperforms a stingy mist of SPF 50 every single time.
The 2 mg/cm² problem — and why you should buy 50 anyway
Every SPF rating assumes a test dose of 2 mg of sunscreen per square centimeter of skin — a visibly thick, slightly ridiculous-feeling layer. Real people apply about 25–50% of that. Protection falls off steeply with thickness, so the SPF 50 on the bottle performs more like SPF 15–25 on an actual human at an actual beach.
Two conclusions follow directly:
- Buy the higher factor. Under-application already halves (or worse) whatever you bought, so SPF 50 is how you end up wearing something like 25. It's headroom, not vanity.
- Reapply on schedule. A second coat two hours later is the cheapest SPF upgrade that exists — it fixes both the thin layer and the worn-off film at once.
Torn between two bottles? Take the higher factor. Real-world application is thin enough that SPF 50 often performs like a 20 — buy your headroom.
Read the rest of the label
The big number is only a UVB rating. Three more things on the bottle matter just as much:
- Broad spectrum — UVB burns; UVA penetrates deeper and drives photoaging. In the US, look for the words "broad spectrum"; on European and British sun cream, look for the letters UVA in a circle or a UVA star rating. No broad-spectrum claim, no purchase.
- Water resistance — always stated as 40 or 80 minutes. That's how long the SPF held up in water during testing, and it becomes your reapply interval on swim days.
- Texture you'll actually wear — the best factor 50 in the world protects nobody from the back of a cabinet. Pick one you'll happily use in real quantities.
When SPF is optional — and what about tanning?
Sunscreen need scales with the UV index. Below UV 3, unprotected time is a minor concern for most skin types on an ordinary errand-running day — which is why UV Buddy literally flips between "SPF optional" and "Lotion up!" the moment your local UV crosses 3. From UV 3 up, the table above applies; at 6+, the holiday column does.
And no, SPF doesn't cancel your tan. You still tan with factor 30 on — more slowly, with less damage, and the color tends to last longer because the skin beneath it isn't burning and peeling. That's the whole logic of UV Buddy's guided Glow sessions, which build SPF on as a step rather than treating it as the enemy.
Your SPF lives on your profile
Set each person's factor once and UV Buddy carries it everywhere — "SPF 45 · Type III · reapply 3:20 PM" — with a different SPF per family member. Reapply timers and the live burn math both account for the factor you're actually wearing, not a generic assumption.
Mini-FAQ
Is SPF 100 worth it?
Marginal at best. SPF 100 blocks about 99% of UVB versus 98% for SPF 50 — a whisper of extra protection, often wrapped in a heavier texture that people apply more thinly and less happily. Compliance is protection; put the effort into reapplying your 50 on time instead.
Chemical or mineral — does it change the SPF answer?
No. Both are tested and rated the same way, so an SPF 50 is an SPF 50 either way. The differences are in feel, wait time and reef rules — we've decoded them in the zinc & reef-safe guide.
Can I layer two SPF 30s to get SPF 60?
No — SPF doesn't stack. Two layers behave roughly like the strongest one, applied a bit better. That second coat still helps, but by fixing thin patches and missed spots, not by adding the numbers together.
The right factor, at the right minute
Store everyone's SPF, let the live burn math size your safe window, and get a "Lotion up!" nudge the moment UV crosses 3.
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