SPF Reapplication Timer
Tell the timer what you’re doing and what your bottle’s water-resistance label says, and it gives the latest time to put more on, on the FDA’s rule.
When to reapply
- FDA: "Reapply sunscreen at least every two hours, and more often if you're sweating or swimming."
- American Academy of Dermatology: "To remain protected when outdoors, reapply sunscreen every two hours, and immediately after swimming or sweating."
- FDA: "This is not true because SPF is not directly related to time of solar exposure but to amount of solar exposure."
A deadline, not a wait
The FDA’s rule is: “Reapply sunscreen at least every two hours, and more often if you're sweating or swimming.” So the timer never tells you to wait: it gives the latest moment, and anything that takes sunscreen off (a swim, sweat, a towel) brings that moment forward to now.
Each coat should be as generous as the first, and the SPF number does not stretch the time between them. WHO’s advice puts shade and clothing ahead of sunscreen; the timer is for the skin they leave uncovered.
Frequently Asked Questions
What is the rule?
The FDA: "Reapply sunscreen at least every two hours, and more often if you're sweating or swimming." The American Academy of Dermatology: "To remain protected when outdoors, reapply sunscreen every two hours, and immediately after swimming or sweating." This timer shows the latest moment to reapply, never a wait.
Does a higher SPF mean I can reapply less often?
No. The FDA: "This is not true because SPF is not directly related to time of solar exposure but to amount of solar exposure." The two-hour rule is the same at every SPF.
What does 'water resistant (40 minutes)' or '(80 minutes)' mean?
The FDA: "Products labeled as water resistant are also required to state on their labels whether the sunscreen remains effective for 40 minutes or 80 minutes when swimming or sweating." and "All sunscreens eventually wash off." In the water, reapply as soon as you are out, and no later than the label's minutes.
Do I need to reapply on a cloudy day?
Keep to the same rule. WHO's UV Index guide: "Up to 80% of solar UV radiation can penetrate light cloud cover."
How much should I use when I reapply?
The same as the first coat. The FDA: "An average-sized adult or child needs at least one ounce of sunscreen (about the amount it takes to fill a shot glass) to evenly cover the body from head to toe." The Sunscreen Amount Calculator works out the amount for the areas you cover.
General information, not medical advice. Follow the directions on your sunscreen’s label. The words in quotation marks are the FDA’s, the AAD’s and WHO’s, checked on their sites on October 6, 2026.