Sunscreen Timer

How much sunscreen to use: the real amount

The single biggest gap between sunscreen science and real-world results: under-application. A bottle that promises SPF 50 only delivers it if you apply the lab-tested thickness. Most people apply roughly half that — which can drop effective SPF by 75%.

Want a personalized reapply schedule based on your location and skin? Use the smart sunscreen timer →

The shot glass rule

For the whole body in a swimsuit, the recommended amount is about one ounce (a shot glass) per application. For the face alone, two finger-lengths — squeeze sunscreen along the entire length of your index and middle fingers and use that for your face and neck combined. If your bottle is lasting an entire summer for one adult, you're under-applying.

Why under-application drops SPF fast

SPF doesn't scale linearly with thickness. Applying half the recommended amount doesn't give you half the SPF — research suggests it gives you roughly the square root of the rated SPF. So SPF 50 applied at half thickness behaves like SPF 7. That's why higher-SPF products give a buffer against your own habits.

Common missed spots

The ears (especially the tops). The back of the neck. The tops of the feet. The hairline. The lips (use a lip balm with SPF — regular sunscreen tastes terrible and gets eaten). Hands, especially for tattoos. Most people who get an asymmetric burn missed one of these by reflex.

Reapply the same amount, every time

When the smart sunscreen timer alerts, don't just dab a token amount. Reapply the full shot-glass quantity (or two-finger amount for face). The alert is calibrated to a full reapplication, not a touch-up. Touch-ups are extra, not a replacement.

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