The sun protection factor is the most visible number on the bottle and the least understood. It doesn’t describe time, and it only holds true if you apply the right amount.
What SPF actually measures
SPF shows how much UVB radiation the product filters out, not how many hours it lasts:
| SPF | UVB radiation filtered |
|---|---|
| 15 | 93% |
| 30 | 97% |
| 50 | 98% |
| 50+ | over 98% |
Between 30 and 50 there’s only a one-percentage-point difference. What really changes is how much radiation gets through: an SPF 50 lets through half as much as an SPF 30. For fair skin, children, recent scars or photosensitising treatments, that difference matters.
UVA, the other range
SPF says nothing about UVA rays, which drive skin ageing and play a significant role in melanoma. Look for the UVA-in-a-circle logo on the bottle: under EU rules, it guarantees the UVA protection is at least a third of the labelled SPF.
A sunscreen without that logo is only protecting you halfway.
The amount: the two-finger rule
SPF is tested in the lab by applying 2 mg of product per cm² of skin. In practice, most people apply about a third of that, and real protection drops far more than proportionally.
In everyday measurements:
- Face and neck: two fingers’ worth of cream, from knuckle to fingertip
- A whole adult body: around 30 ml, roughly six teaspoons
- A 200 ml bottle should only last around six full-body applications, not a whole holiday
If a bottle lasts you all summer, you’re under-applying.
How often to reapply
- Every two hours of sun exposure, always
- Straight after swimming, even if the product is water-resistant
- After towelling off or sweating heavily
“Water-resistant” means it keeps 50% of its protection after two 20-minute baths — not that it lasts all day.
Apply it 20–30 minutes before going out: chemical filters need that time to form their protective film on the skin.
The spots everyone forgets
Ears, the tops of the feet, the back of the neck, lips (use an SPF stick), the scalp along your hair parting, and the backs of the knees. These are classic beach sunburn sites.
Mineral vs chemical filters
Mineral filters (zinc oxide, titanium dioxide) reflect radiation, work instantly, and are the usual choice for reactive skin and babies over 6 months old. They leave more of a white cast.
Chemical filters absorb radiation, have a lighter texture, and need that 20–30-minute head start.
Neither is objectively better: the best sunscreen is the one you apply properly and in the right amount.
Babies under 6 months
Sunscreen isn’t recommended for babies under 6 months old. The strategy is physical: shade, long sleeves with UV protection, a hat, and avoiding sun exposure between 12:00 and 16:00.
When to see a doctor
See a doctor if a sunburn comes with extensive blistering, fever or vomiting, or if you notice a mole that changes size, shape, colour or border, bleeds or itches.
This information is general guidance only. It does not replace a consultation with your doctor or pharmacist. Sources: Regulation (EC) 1223/2009 on cosmetic products, European Commission Recommendation 2006/647/EC on the efficacy of sunscreen products.