Sunscreen Amount Calculator
Work out how much sunscreen to apply for your coverage and how much a day outdoors needs with reapplication.
How it works
amount = exposed area (cm²) × 0.002 g/cm² · daily = amount × (1 + hours ÷ 2 reapplications)
Sunscreen SPF is tested at 2 mg per square centimetre — far more than most people actually use, which is why real-world protection often falls short of the label. Converting typical exposed-skin areas at that dose gives the "teaspoon rule" totals dermatologists teach: about a teaspoon for the face and neck, and roughly a shot glass (an ounce) for the whole body. Reapplying every two hours, and after swimming or heavy sweating, keeps protection up through the day.
Worked example
Whole body (~16,000 cm²) at 2 mg/cm² is about 32 g per application. Over 4 hours outdoors you apply 1 + (4 ÷ 2) = 3 times, roughly 96 g total.
Sources: FDA — Sunscreen and your skin
Frequently asked questions
How much sunscreen is "enough"?
The tested dose is 2 mg/cm². In practice that is about a teaspoon for the face and neck and a shot glass for the whole body — a visible layer, not a thin smear.
How often should I reapply?
Every two hours during sun exposure, and immediately after swimming, toweling off or heavy sweating, even with "water-resistant" products.
Does a higher SPF mean I can use less?
No. SPF ratings assume the full 2 mg/cm² dose. Applying a thin layer of high-SPF sunscreen gives much less protection than the number suggests.
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This is a general educational estimate, not medical advice. Individual responses vary — for decisions about exercise, sun exposure, heat, cold or health conditions, talk to a qualified professional.