About two hours—that's the reapplication interval dermatologists recommend. One morning coat won't carry you through a beach day. Sweat, water, and towel-drying strip product off faster than most people realize.

The two-hour rule

Set a timer. Reapply every two hours while you're outside, using the same generous amount as the first coat—about one ounce (a shot glass) for your body, half a teaspoon for your face. This holds whether you're wearing SPF 30 or SPF 50.

"Water-resistant" doesn't mean waterproof. Labels marked 40 or 80 minutes were tested in lab conditions; in real life, reapply as soon as you're out of the water and have dried off.

What shortens protection

Sweating and exercise can cut effective coverage to an hour or less. Toweling off removes a lot of product—reapply immediately after. Most people apply too little to start with, which means you're getting a fraction of the labeled SPF from minute one.

Chemical filters like avobenzone can break down under UV exposure. Mineral sunscreens (zinc oxide, titanium dioxide) stay stable longer on skin, but sweat and rubbing remove them just the same.

SPF doesn't extend duration

SPF tells you how much UVB is blocked, not how long you're protected. SPF 50 doesn't buy you more time than SPF 30—both need reapplication on the same schedule. The math where SPF 30 = 30× your burn time only works in a lab with perfect application and no sweating.

When to reapply sooner

• After swimming or getting wet
• After toweling off
• After heavy sweating or sports
• Every 1–1.5 hours during intense outdoor activity

If you're mostly indoors near a window, every three to four hours may be enough. On a hike or at the pool, stick to two hours or less.

Treat reapplication as part of the routine, not optional. A phone alarm at two-hour intervals beats discovering you burned because you forgot the second coat.