Are you wearing enough sunscreen?
Almost certainly not — studies find most people apply a quarter to half of the dose SPF ratings are tested at, which is how SPF 50 ends up performing like SPF 15. This calculator gives you the real numbers: your face dose, your whole-body dose from your height and weight, and how many bottles your next vacation honestly needs.
Face & neck — the standard dose
The face needs about ¼ teaspoon (~1.25 g) of sunscreen per application — a full ribbon squeezed along the length of your index and middle fingers. The neck and ears take another ¼ teaspoon. No inputs needed; this one is the same for almost every adult.
If it feels like too much, apply it in two thinner coats a minute apart — the dose is what protects you, not the comfort of a thin layer.
Whole body — your personal dose
We estimate your body surface area with the Mosteller formula — √(height × weight ÷ 3600) — then apply the 2 mg/cm² test dose across it. The defaults show an average adult; type your own numbers for your personal figure.
The classic "one shot glass" (30–35 g / ~1 oz) rule is this same math pre-computed for an average adult.
| Region | Share of skin | Sunscreen |
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Shares come from the clinical rule of nines (the remaining ~1% is the groin). Only the skin that's actually exposed needs covering — which is what the trip planner below accounts for.
Trip planner — bottles for the vacation
Two is the honest minimum for a sun day; the standard advice is to reapply every two hours in the sun and after every swim.
Sunscreen is close to water-dense, so we count 1 ml ≈ 1 g.
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Where the math comes from
Every SPF number on every bottle was earned in a lab at exactly 2 mg of sunscreen per square centimetre of skin — a far thicker layer than most people ever apply. This calculator starts from that test dose and scales it to you: your body surface area is estimated with the Mosteller formula, √(height in cm × weight in kg ÷ 3600), which for a 170 cm, 70 kg adult gives about 1.8 m² of skin — and 2 mg/cm² across 1.8 m² is roughly 36 g per full-body application, the famous "shot glass". The per-region split uses the clinical rule of nines, the same body-share map used to assess burns.
The trip planner multiplies your personal dose by how much skin is actually out (a beach day is most of your body; a clothed day is your face and arms), by applications per day, and by the length of the trip — then divides by your bottle size at 1 g ≈ 1 ml to get bottles and, if you pick your sunscreen from the Dermington database, an estimated cost. It pairs naturally with our sunscreen finder (which bottle to buy), the sun safety tool (your skin type and UV index), and the fingertip unit calculator (dosing for medicated creams, where the same under-application problem applies).
Common questions
How much sunscreen should I use on my face?
About a quarter teaspoon — roughly 1.25 g — for the face alone, with another quarter teaspoon for the neck and ears. The two-finger rule gets you to the same place: squeeze a ribbon of sunscreen along the full length of your index and middle fingers, and that is one facial dose. Most people apply half that or less, which quietly cuts their real protection to a fraction of the number on the bottle.
How much sunscreen do I need to cover my whole body?
For an average adult, roughly 30–40 g per application — the classic "shot glass" (about one ounce) rule. The precise figure depends on your body surface area: at the tested dose of 2 mg per square centimetre, a 170 cm, 70 kg adult works out to about 36 g, or around seven teaspoons. The calculator above computes your personal number from your height and weight.
How many bottles of sunscreen do I need for a week at the beach?
More than you think. Full-body coverage for an average adult runs about 33 g per application; reapplied twice a day for seven days, that is roughly 450–460 g — about ten standard 50 ml tubes. A single 50 ml bottle covers less than one full beach day at the proper dose, which is exactly why the trip planner above does this arithmetic before you pack.
Why did I burn even though I wore SPF 50?
Almost always under-application. SPF is measured in the lab at 2 mg per square centimetre, and real-world studies consistently find people apply only 25–50% of that. Because protection falls off steeply with dose, SPF 50 at a typical real-world layer can behave more like SPF 15 — and missed patches, skipped reapplication, sweat, and water all take a further cut.
Do I really have to reapply sunscreen every two hours?
When you are actually out in the sun, yes — the film thins as it rubs off on clothes, towels, and sweat, and some UV filters degrade with exposure, so the two-hour rule (plus straight after swimming or heavy sweating) is standard dermatology guidance. For an office day with a short walk at lunch, a morning application and a midday top-up on exposed skin is a reasonable compromise.