SPF Reapplication Timer
Sunscreen doesn’t last all day. Tell the timer what you’re doing and what your bottle’s water-resistance rating is, and it’ll tell you when the film has likely worn off and it’s time to put more on.
When to reapply
Why reapplication matters more than the number
The single biggest reason sunscreen fails people isn’t the SPF they chose — it’s that the coat they put on in the morning is long gone by the afternoon. Sweat, swimming, rubbing against a towel or a chair, and the sheer passage of time all strip and break down the film. A bottle labelled SPF 50 protects like SPF 50 only while a full, fresh layer is actually on your skin.
The habit that works is boring and effective: apply generously fifteen minutes before you go out, reapply every two hours, and reapply immediately after you get out of the water or towel off — within the limits of your product’s water-resistance rating. This timer just turns that rule into a concrete next time so you don’t have to guess.
Frequently Asked Questions
Does a higher SPF mean I can reapply less often?
No. SPF describes how much UV the film blocks, not how long it lasts on your skin. A very high SPF still needs reapplying, because water, sweat, sand, and towelling physically remove it and it degrades in sunlight. The general rule — reapply every two hours, and immediately after swimming or heavy sweating — applies at every SPF.
What does 'water resistant 40 or 80 minutes' actually mean?
It's the only water-resistance claim allowed on a label, and it means the product kept its stated SPF for that many minutes of swimming or sweating in testing. After that window you should reapply. If a product isn't water resistant at all, treat any real contact with water or heavy sweat as a reason to reapply right away.
Do I need to reapply on a cloudy day?
Often yes. Up to 80% of UV can pass through light cloud, and UVA is fairly constant through the day. If the UV index is 3 or higher and you're outdoors for a while, keep to the same reapplication habit as on a sunny day.
How much should I use when I reapply?
The same generous amount as the first coat — roughly a shot glass (about 30 mL) for a whole adult body, or a teaspoon for the face and neck. Under-applying is the most common reason real-world protection falls short of the SPF on the bottle.
General guidance only, not medical or dermatological advice. Always follow the directions on your sunscreen’s label, and see a professional about your own skin.