TempoLife › Sunburn and vitamin D calculator › spf
How SPF works
SPF — sun protection factor — measures how well a sunscreen shields against the UVB that causes sunburn. Higher blocks more, but the numbers are not linear, and how you apply it matters as much as the figure on the bottle.
General sun-safety information, not medical advice. Burn times vary enormously with skin, the UV index, altitude, reflection off snow or water, and some medicines — when in doubt, cover up. Any new, changing or unusual mole or spot should be checked by a doctor.
The numbers are not what they seem
SPF 30 filters about 97% of UVB and SPF 50 about 98% — so the jump from 30 to 50 is small, not "nearly double". No sunscreen blocks everything, and SPF only rates UVB; look for "broad spectrum", which means it also covers the UVA that ages skin. The single biggest mistake is using too little: most people apply a fraction of the amount tested, which cuts the real protection sharply.
Apply enough, and again
Use it generously — roughly a teaspoon for the face and neck and a shot-glass-sized amount for the whole body — put it on before going out, and reapply every two hours and after swimming, sweating or towelling. Sunscreen is the last layer, not the only one: shade, clothing, a hat and timing around the midday peak do the heavy lifting, especially for children.
Related
The vitamin D window
Your skin only makes vitamin D when the sun is high enough — which, up north, it is not for months.
The UV index
A single number for how strong the sun is today, with the protection each level calls for.
Find your skin type
The Fitzpatrick scale, from always-burns to never-burns, with advice for each.
Source: WHO Global Solar UV Index and general dermatology and public-health guidance. General information, not medical advice. · checked 2026-09-04
Frequently asked questions
Is SPF 50 much better than SPF 30?
Only slightly — about 98% of UVB blocked versus 97%. Applying enough of either, and reapplying, matters far more than the extra number.
How often should I reapply sunscreen?
Every two hours in the sun, and straight after swimming, heavy sweating or towelling off, however "water resistant" the label claims to be.
Top up vitamin D from your plate
TempoLife's food database shows the vitamin D in foods, so winter does not leave you short.