Every bottle on the shelf shouts a number at you. Here is the unglamorous truth: the gap between SPF 30 and SPF 50 is about one percentage point of UVB, and almost nobody applies enough sunscreen to get the number on the label anyway. How much you use matters more than which bottle you buy.
What the number actually measures
SPF stands for Sun Protection Factor, and it describes one thing only: how well a product filters UVB — the shorter wavelength that burns you. It is measured in a lab by applying a standard, generous layer of product and comparing how long skin takes to redden with and without it.
The useful way to read the number is as the fraction of UVB that still gets through:
| SPF | UVB transmitted | UVB filtered |
|---|---|---|
| SPF 15 | 1 in 15 — about 6.7% | ≈ 93.3% |
| SPF 30 | 1 in 30 — about 3.3% | ≈ 96.7% |
| SPF 50 | 1 in 50 — about 2% | ≈ 98% |
| SPF 100 | 1 in 100 — about 1% | ≈ 99% |
Doubling the number does not double the protection. Going from SPF 50 to SPF 100 doubles the price and the thickness on your skin to buy back a single percentage point. This is exactly why the US FDA has proposed capping sunscreen labels at "SPF 60+" — beyond that, the extra digits sell better than they protect.
The mistake that undoes all of it
Laboratories test sunscreen at 2 mg per square centimetre of skin. For an adult body that is roughly 30 ml — a full shot glass. For your face and neck alone, about half a teaspoon.
Almost nobody does this. Studies of real-world use consistently find people applying between a quarter and a half of the tested amount — often around one third.
Protection does not fall off politely in proportion. A 2018 study in Acta Dermato-Venereologica applied a very high SPF sunscreen at 2.0, 1.3 and 0.75 mg/cm² and measured epidermal DNA damage directly. At 2.0 and 1.3 the sunscreen significantly reduced that damage; at 0.75 — roughly what people actually apply — the reduction was largely not statistically significant. Application amount matters as much as the number on the label — an SPF 50 applied too thinly can give substantially less protection than an SPF 30 applied properly.
So which one should you buy?
The World Health Organization’s baseline is broad-spectrum SPF 15 or higher, reapplied after swimming, sweating, or anything that wipes it off. In Bangkok, where the UV index sits in the high-to-extreme band for most of the year, we would treat that as the floor and not the target.
- SPF 15 — errands, a walk to lunch, an overcast commute. Genuinely fine for short, incidental exposure.
- SPF 30 — a normal Thai day outdoors. The sensible everyday default.
- SPF 50 · PA+++ — the beach, the boat, the motorbike, midday, or any day the UV index is 8 or above.
- Above 50 — you are buying a rounding error. Spend the difference on applying more of the SPF 50.
A note on our tanning oils
We should be straight with you, because the numbers on our own shelf can mislead. Our tanning oils are SPF 2 and our tanning lotions are SPF 4. Those are not sun protection. They are conditioning oils that carry a token filter; at SPF 2 roughly half the UVB still reaches your skin.
If you want colour and you want to keep your skin, the honest routine is a real broad-spectrum SPF on the exposed skin you care about most — face, shoulders, chest, the tops of your ears — and oil where you are deliberately tanning, with a hard limit on the clock. And it is worth saying plainly: dermatologists are united that a tan is the skin’s response to DNA damage. There is no such thing as a safe tan, only a slower one. We cover how to do it with the least harm in the field guide.