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About this tool
Build a clock-time sunscreen reapplication schedule sized to your activity, sweat and water exposure, plus how much to use each time.
The Sunscreen Reapply Timer turns standard sunscreen label directions into clock times for a specific outing: apply 15 minutes before you go outdoors, reapply at least every 2 hours when dry, and switch to your bottle's water-resistance rating — 40 or 80 minutes, the only two claims regulators allow — once you are swimming or sweating. It also sizes each application using the teaspoon rule, about 5 ml per body region and roughly 35 ml for a whole adult body. Useful for beach days, long rides, outdoor shifts and match days where a vague 'reapply often' is easy to forget.
Open Sunscreen Reapply Timer on AltFTool — it loads instantly in your browser.
Set Time you go outdoors and Time outdoors (minutes), then pick "What you will be doing" — dry, sweating heavily, or in and out of water — and the Water resistance on the bottle: no claim, 40 minutes or 80 minutes.
Tick the areas you are covering under "Areas you are covering": Face, ears, head and neck at 1 tsp, Both arms and hands at 2 tsp, Front of torso, Back of torso and Both legs and feet.
"Reapply every" gives the interval and the reason for it, with First application 15 min before you step out, Applications needed, Amount each time in ml and tsp and Total for the session, then Today's schedule lists each clock time; press Copy result.
Dry exposure uses the 2-hour rule; wet or sweaty exposure uses your bottle's 40 or 80 minute rating.
Under-applying is the most common failure — the teaspoon rule shows the millilitres each round actually needs.
A sunscreen with no water-resistance claim must be reapplied straight after swimming, and the tool says so.
At least every two hours during dry sun exposure, which is the standard label direction, and immediately after swimming, towel drying or heavy sweating. If the bottle says water resistant (40 minutes) or water resistant (80 minutes), that number is how long the labelled SPF survives in water, so reapply on that shorter cycle while you are wet.
About 35 ml — roughly seven teaspoons or a shot glass — for a whole adult body in swimwear, which works out to about one teaspoon each for the face, head and neck, each arm, each leg, the front of the torso and the back. Most people apply between a quarter and a half of that, which is why real-world protection falls well short of the SPF on the bottle.
No. Regulators do not permit the words waterproof or sweatproof on sunscreen, only water resistant with a 40 or 80 minute rating, and that rating comes from a standard immersion test. After that window the film has thinned enough that the tested SPF no longer holds.
The label direction exists so the product can spread and dry into an even film before UV exposure begins; organic (chemical) filters in particular need that time to settle. Applying at the door leaves gaps and lets some of the product transfer onto clothing or a towel before it has set.