SPF Calculator
How long does your sunscreen actually protect you, and when should you put more on? Enter your skin type, the UV index and your SPF for a personal protection time and reapply schedule.
What SPF really blocks
| 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 |
The jump from SPF 30 to 50 is small. What matters far more is applying enough and reapplying — see our guide to choosing and using SPF.
The two-finger rule
- Whole body: about a shot glass (35 ml) of lotion.
- Face & neck: two full finger-lengths of product.
- Reapply every 2 hours, and right after swimming or towelling.
Apply 15–30 minutes before you go out so it can bind to the skin. Then check how long you can stay out with the Tanning Time Calculator.
Frequently asked questions
How long does SPF 30 last?
SPF is about how much UV is blocked, not a fixed clock. SPF 30 lets through roughly 1/30th of burning UV, so it multiplies your natural burn time by about 30 — but only with a thick, even layer. Regardless of the number, reapply every 2 hours and after swimming or sweating.
Is SPF 50 twice as strong as SPF 30?
No. SPF 30 blocks about 97% of UVB and SPF 50 about 98% — a small difference. The bigger factor is applying enough and reapplying. Going above SPF 50 brings very little extra benefit.
How much sunscreen should I apply?
About 2 mg per cm² of skin — roughly a shot glass (35 ml) for the whole body and a two-finger length for the face and neck. Most people apply only a quarter to a half of this, which sharply lowers the real SPF.
Do I need to reapply if it says “once a day”?
Yes. No sunscreen reliably lasts a full day in real use — it rubs, sweats and washes off. Reapply every two hours of sun exposure and straight after towelling dry.
Your tan, planned by the hour.
Everything on this site, plus live UV by the hour, a burn-timer that counts down for your skin, SPF reapply reminders and push alerts the moment your safe window opens.