How to Choose & Use SPF Sunscreen
What SPF numbers really mean, the difference between SPF 30 and 50, how much to apply (the two-finger rule) and when to reapply — so your sunscreen actually works.
The quick answer
Choose a broad-spectrum SPF 30 or higher, apply it generously (most people use far too little), and reapply every two hours. The SPF number matters less than using enough and topping up.
What the SPF number means
SPF (Sun Protection Factor) is roughly how much longer you can stay in the sun before burning, and how much UVB is blocked. SPF 30 lets through about 1 part in 30 of burning UV; SPF 50 about 1 in 50.
| SPF | UVB blocked | UV let through |
|---|---|---|
| SPF 15 | ~93% | 1 in 15 |
| SPF 30 | ~97% | 1 in 30 |
| SPF 50 | ~98% | 1 in 50 |
| SPF 100 | ~99% | 1 in 100 |
See how this stretches your safe time in the SPF calculator.
How much to apply — the two-finger rule
- Whole body: about a shot glass (35 ml) of lotion.
- Face & neck: two full finger-lengths of product.
- Apply 15–30 minutes before sun so it binds to the skin.
Under-applying is the most common mistake — use half the amount and you get far less than half the protection.
When to reapply
- Every 2 hours in the sun.
- Straight after swimming, heavy sweating or towelling.
- Don’t trust “all day” claims — nothing survives a full day of real use.
Choosing a sunscreen
- Broad spectrum (UVA + UVB) — non-negotiable.
- SPF 30+ for everyday, SPF 50 for high UV, water or altitude.
- Water resistant for swimming or sport (still reapply).
- Mineral (zinc/titanium) filters suit sensitive skin and work immediately.
Then plan the rest of your day with the UV index and tanning time calculator.