Sunscreen Calculator (how much to apply and when to reapply)

Understanding the sunscreen calculator — concept and principle

This calculator works out how much sunscreen you need to cover the uncovered parts of your body at the amount used in SPF testing, how many applications you need during your time outside and how many bottles that takes. Most people apply far less than the tested amount and get less protection than the label suggests.

How it is calculated

Body surface area (m²) = √(height cm × weight kg ÷ 3,600)

Uncovered share (%) from the adult Lund–Browder chart: head and neck 9, forearms with hands 11, whole arms 19, lower legs with feet 21, whole legs 40, front of trunk 13, back 13

Amount per application (mL) = area (cm²) × 2 mg/cm² ÷ 1,000 (1 g ≈ 1 mL)

Applications = time outdoors ÷ reapplication interval, rounded up

Total = amount × applications; bottles = total ÷ bottle size, rounded up

The principle

The SPF on a label is measured at 2 mg of sunscreen per cm² of skin. Using half that amount gives much less than half the protection. Multiplying the uncovered skin area by 2 mg/cm² gives the amount needed to get the labelled protection. Sunscreen wears off with time, sweat, towelling and water, so it must be reapplied.

Worked example

Default inputs: 170 cm, 65 kg; face and neck, forearms and hands, lower legs and feet uncovered; 5 hours outside; reapply every 2 hours; 50 mL bottle.

1. BSA = √(170 × 65 ÷ 3,600) = 1.75 m².
2. Uncovered = 9 + 11 + 21 = 41% → 0.718 m² = 0.72 m².
3. Per application = 7,183 cm² × 2 ÷ 1,000 = 14.4 mL ≈ 2.9 teaspoons.
4. Applications = 300 ÷ 120 = 2.5 → 3; total = 14.37 × 3 = 43 mL → 1 bottle.

How to read the result

The amount may look large, but it is what the SPF test assumes. A common guide for the whole adult body in swimwear is about 30–35 mL, a shot-glass amount. Apply 15–30 minutes before going out and reapply after swimming or towelling.

Common mistakes

• Applying a thin layer and assuming full SPF protection.
• Forgetting ears, neck, feet and the backs of hands.
• Not reapplying after swimming, even with water-resistant products.

Frequently asked questions

How much sunscreen for the face?

By this method about 3 mL, a little over half a teaspoon, for face, ears and neck (9% of an average adult's skin).

Does SPF 50 last longer than SPF 30?

No. Both need reapplying at least every 2 hours.

Does water-resistant mean waterproof?

No. It keeps its stated protection for 40 or 80 minutes in water, then needs reapplying.

Sources: ISO 24444:2019, Cosmetics — Sun protection test methods — In vivo determination of the sun protection factor (SPF) · Mosteller RD, "Simplified calculation of body-surface area", N Engl J Med 1987;317:1098