Wait 15 minutes before swimming with chemical sunscreen.

At least 15 minutes. That’s the standard wait time for chemical sunscreens to absorb into the skin and start working properly. If you jump in right away, you’re basically rinsing it off before it does anything useful.

Physical (mineral) sunscreens with zinc oxide or titanium dioxide work immediately — they sit on top of the skin and reflect UV. But they still wash off faster if you don’t let the lotion dry down for a few minutes. Either way, give it a few minutes.

Water-resistant labels tell you how long it lasts while swimming — 40 or 80 minutes. But that clock starts after you’ve waited, and it resets every time you towel off or sweat. Reapply immediately after getting out.

Don’t trust the bottle’s claims too much. Reapply after every swim.