How this instrument works
Most people apply far less sunscreen than dermatologists test it at, which is one reason SPF ratings often underperform in real-world use. The dermatological testing standard — and the American Academy of Dermatology's own guidance — calls for about 2 milligrams of sunscreen per square centimeter of skin for full protection, which works out to roughly 1 ounce (a shot-glass-full) to cover an average adult's exposed skin for one full-body application.
To scale that 2 mg/cm² rate to an individual person, this calculator first estimates body surface area (BSA) using the Mosteller formula — BSA (m²) = √(height in cm × weight in kg ÷ 3,600) — a simplified, widely-used clinical formula published in the New England Journal of Medicine in 1987. From BSA, the amount needed per full-body application follows directly: 2 mg/cm² converts to roughly 20 mL per square meter of body surface, at sunscreen's typical density of about 1 gram per milliliter.
Reapplication frequency is the other half of the equation, and it's governed by time in the sun, not just skin area: the AAD recommends reapplying sunscreen at least every 2 hours, and immediately after swimming or sweating. Multiplying application amount by applications-per-day and trip length gives a total volume, which this calculator then divides by your bottle size to tell you how many bottles to actually pack or buy.
- Enter your height and weight — used to estimate body surface area via the Mosteller formula.
- Enter expected sun exposure per day, in hours.
- Enter your reapplication interval — 2 hours is the American Academy of Dermatology's standard minimum guidance.
- Enter your trip length in days, and your sunscreen bottle size in mL.
- Read Bottles to buy — the total sunscreen needed for the whole trip, rounded up to a whole number of bottles.
Worked example — a 180 cm, 80 kg traveler on a week-long trip
A traveler standing 180 cm tall and weighing 80 kg has a Mosteller BSA of √(180 × 80 ÷ 3,600) = √4 = exactly 2.0 m². At 20 mL per square meter, that's 20 × 2.0 = 40 mL per full-body application — roughly a quarter of a standard 150 mL bottle, for one application.
With 6 hours of sun exposure a day and the standard 2-hour reapplication interval, that's ceil(6 ÷ 2) = 3 applications a day. Over a 7-day trip: 3 applications × 7 days × 40 mL = 840 mL total. Divided by a 150 mL bottle size and rounded up: ceil(840 ÷ 150) = 6 bottles to pack for the full week.
Questions
Why does this calculator need my height and weight for a sunscreen amount?
Because the amount of sunscreen needed for full coverage scales with how much skin you have to cover, and body surface area (BSA) — not weight alone — is the standard clinical way to estimate that from simple measurements. The Mosteller formula used here is a well-established, widely-cited method (New England Journal of Medicine, 1987) for estimating BSA from height and weight without needing a body scan.
Is 2 mg per square centimeter really how much sunscreen people should use?
Yes — it's the standard application rate used in SPF testing and endorsed in American Academy of Dermatology guidance, working out to roughly 1 ounce (a shot glass full) for an average adult's exposed skin in one application. Most people apply meaningfully less than this in practice, which is a well-documented reason real-world sun protection often falls short of a product's tested SPF rating.
Why reapply every 2 hours if my sunscreen is water-resistant?
Even water-resistant formulas degrade with UV exposure, sweat, friction from towels and clothing, and time, so the American Academy of Dermatology recommends reapplying at least every 2 hours regardless of water resistance claims, and immediately after swimming or heavy sweating. This calculator's reapply-interval input defaults to that 2-hour guidance but is fully adjustable if your product's label specifies something different.
Does this account for sunscreen that isn't a cream, like a spray or stick?
Not directly — the volume-to-mass conversion assumes roughly 1 gram per milliliter, which is a reasonable approximation for lotion and cream sunscreens but less exact for sprays (which apply less product per spray than users often assume, making it easy to under-apply) or sticks (which are typically used only for smaller areas like the face). For sprays and sticks, treat this calculator's mL figure as a useful minimum target rather than a precise match to how the product is packaged.