The amount used to test SPF
The SPF on a sunscreen label is measured in the laboratory with a thick layer of 2 milligrams of sunscreen per square centimeter of skin. Most people apply a quarter to a half of that, and since protection falls quickly with a thinner layer, the real SPF can end up a fraction of the number on the bottle.
The calculator estimates your skin area with the Mosteller formula, then applies 2 mg per cm² to the part you leave uncovered:
- Body surface area (m²) = √(height in cm × weight in kg ÷ 3,600)
- Sunscreen per application (g ≈ mL) = body surface area × 10,000 × share of skin exposed × 2 ÷ 1,000
Worked example
An adult 5 ft 9 in tall and 180 lb, wearing a swimsuit, spends 4 hours a day at the beach for 7 days.
- Body surface area: about 2.0 m².
- Skin exposed: 90%, about 18,000 cm².
- Sunscreen per application: about 36 mL, or 1.2 fl oz, about 7 teaspoons.
- Applications: one at the start and one 2 hours later, 2 a day.
- For the week: about 17 fl oz, or 3 bottles of 8 oz.
That matches the familiar advice of “a shot glass for the whole body.”
The teaspoon guide
| Body area | Amount |
|---|---|
| Face, neck and ears | 1 teaspoon |
| Each arm | 1 teaspoon |
| Each leg | 2 teaspoons |
| Chest and abdomen | 2 teaspoons |
| Back | 2 teaspoons |
Applying it right
- Apply 15 minutes before going out, on dry skin, and do not forget ears, feet, the back of the neck and the scalp along a part.
- Reapply every 2 hours, and right after swimming, sweating or towel drying. “Water resistant” labels last 40 or 80 minutes in water.
- Use a lip balm with SPF 30 or higher.
- Use enough spray: spray until the skin glistens, then rub in. Never spray directly on the face.
Choosing a sunscreen
The American Academy of Dermatology recommends a broad-spectrum, water-resistant product with SPF 30 or higher. Broad spectrum means it also blocks UVA rays, which age the skin and contribute to skin cancer. Mineral sunscreens with zinc oxide or titanium dioxide suit sensitive skin and young children. Keep babies under 6 months out of direct sun instead of using sunscreen.
Sunscreen is one part of sun protection: seek shade between 10 a.m. and 4 p.m., wear a wide-brimmed hat, sunglasses and tightly woven or UPF clothing.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.