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How often should you reapply sunscreen?
When to actually reapply sunscreen, from your exposure, activity, and how generously you applied — because a thin smear of SPF 50 is not SPF 50.
CostFree
TimeUnder 1 min
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DataKept on device
Where will you be?
Activity level
How much did you apply?
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The science
Where the “every 2 hours” rule comes from — and when it is wrong
01
The 2-hour rule exists because sunscreen degrades in real use: UV exposure breaks down some filters, and sweat, touch, and clothing physically remove the film. Two hours is the regulator’s safe average for direct-sun conditions — but your actual window depends on three things this calculator asks: exposure intensity, activity, and how much you applied.
02
Application amount is the one everyone gets wrong. SPF is tested at 2 mg/cm² — about a quarter teaspoon for the face — and studies consistently find people apply 25–50% of that. Protection does not scale linearly: applying half the tested dose can drop an SPF 50 to the effective protection of a single-digit SPF. A thin smear of SPF 50 is not SPF 50, which is why the calculator shortens your window when you tell it you applied lightly.
03
Water and sweat override everything: “water resistant” on a label means tested for 40 or 80 minutes of immersion, after which reapplication is due regardless of the clock. And reflection matters — water, sand, and especially snow bounce UV back up, which is why the beach and slopes get the shortest windows here.
Reapplication in real life: makeup, sweat, and the office
The office case: morning SPF holds for an indoor day away from windows; sit beside glass and a midday top-up earns its keep (UVA penetrates glass). Over makeup: SPF sticks, cushions, and setting sprays with SPF are imperfect but real re-application tools — imperfect coverage of a real SPF beats perfect coverage of nothing. Sweat and water: "water resistant 40/80 minutes" is a tested claim — reapply at that mark regardless of the clock. The beach case needs the full protocol: generous base coat 15 minutes before exposure, reapplication every clock-cycle AND after every swim or towel — toweling removes sunscreen mechanically, no formula survives it.
FAQ
Common questions
Do I need to reapply sunscreen indoors?+
Near windows, eventually — glass blocks most UVB but passes UVA, the aging-and-pigment wavelength. A morning application holds for a normal indoor day; reapply around midday if you sit beside a bright window.
How much sunscreen should I apply to my face?+
About a quarter teaspoon — the two-finger rule (two full finger-lengths of product) gets most people close. Applying half the amount can cut the effective SPF far more than proportionally.
Does makeup with SPF count?+
Rarely. You would need roughly ten times a normal foundation application to reach the tested dose. Treat SPF makeup as a bonus over a real sunscreen layer, not a replacement.
Do I need to reapply sunscreen if I stay inside?+
Away from windows, no — one morning application covers incidental exposure. The reapplication clock is driven by UV dose and film degradation; a windowless office delivers little of either.
How much sunscreen for face AND body?+
The face takes a quarter teaspoon; a full adult body in swimwear takes about a shot glass (30ml) per application. A family beach day legitimately finishes a bottle — budgeting less means underdosing someone.