Sunscreen amount.

Worked example

An adult 170 cm tall and 70 kg. Mosteller BSA = √(170 × 70 / 3600) = 1.82 m² = 18,181 cm². Face, head and neck is 9%: 1,636 cm² × 2 mg/cm² = 3,273 mg = 3.3 g, about 3.3 ml (density 1 g/ml), 0.65 teaspoon, or 2.6 two-finger strips at 1.25 ml. All five areas together (99% of the body) come to about 36 g.

How it works

amount = 2 mg/cm² × area. The 2 mg/cm² figure is the dose the US FDA's SPF test applies to skin (21 CFR 201.327), so it is the amount behind the SPF number on a label. Area is the rule-of-nines share of your body surface area: head and neck 9%, each arm 9% (both: 18%), front trunk 18%, back trunk 18%, each leg 18% (both: 36%). The remaining 1% (genitalia) is not offered, so all five areas add to 99%. Body surface area comes from the Mosteller formula, BSA (m²) = √(height cm × weight kg / 3600).

Assumptions that are not from a standard. Sunscreen density is taken as 1 g/ml (real products vary), a teaspoon is 5 ml, and the two-finger strip is a rule of thumb you can edit. The rule-of-nines head share includes the scalp and does not split out the face alone, so a face-only area is smaller. The percentages are an adult approximation; children's proportions differ. Trip bottles = amount × applications per day × days ÷ bottle size.

Not a guarantee. This is the labelled-SPF test dose, worked out as arithmetic. Real-world protection is lower with sweat, water, rubbing off and uneven coverage. Follow the directions and reapplication times on your product's label.

Sources (as of 2026-10-02): US FDA, 21 CFR 201.327 (SPF test, 2 mg/cm²); Mosteller RD, Simplified calculation of body-surface area, N Engl J Med 1987;317:1098; Wallace AB, The exposure treatment of burns, Lancet 1951;257:501-504 (rule of nines).

FAQ

How much sunscreen should I use on my face?

Using the labelled-SPF test dose of 2 mg/cm² and the rule-of-nines head-and-neck share of 9% of body surface, the calculator gives about 3.3 g (3.3 ml) for an adult of 170 cm and 70 kg. That covers the whole head and neck, so a face-only area is smaller. Everyday rules of thumb vary and are often lower than the test dose.

How many fingers of sunscreen is that?

The calculator converts ml into two-finger strips using a value you can edit (default 1.25 ml per strip). That figure is a common rule of thumb, not a standard: finger size and bottle nozzle change it. Use grams or ml when you can.

Why 2 mg/cm²?

It is the amount the US FDA's SPF test applies to skin (21 CFR 201.327). A label SPF is measured at that dose, so using less usually gives less protection than the number on the bottle.

How is the area worked out?

Body surface area comes from the Mosteller formula, the square root of height in cm times weight in kg divided by 3600. Each body area is then its rule-of-nines share of that total: head and neck 9%, each arm 9%, front trunk 18%, back trunk 18%, each leg 18%.

How many bottles do I need for a trip?

Enter days, applications per day and your bottle size. The tool multiplies the amount for your chosen areas by applications and days, then divides by bottle size. Reapplication timing comes from your product label.

Is this a guarantee of protection?

No. It is arithmetic on a test dose, not a guarantee. Sweat, water, rubbing and uneven application all reduce protection, and sunscreen is one part of sun safety alongside shade and clothing.

Staying hydrated outdoors? Try the water intake calculator.