2 mg/cm² · the dose used in SPF tests

Sunscreen amount calculator

The SPF on a bottle is measured with a generous, even layer. Find out how much that layer is for your body and the skin you will leave bare, in teaspoons, millilitres and grams.

At the SPF test dose (2 mg/cm²) 6¼teaspoons

About 31 mL (31 g) of lotion, or 1 US fl oz.

Skin covered: 85%
Area: 15363 cm²
Body surface: 1.82 m²

What people usually put on: studies of sunbathers have found they apply only about a quarter of the test dose. Here that would be about 1½ teaspoons, and protection falls much faster than the amount does.

Test dose

Typical real-life amount

Sunscreen by body area
Area	Share	Grams	Teaspoons
Face and ears	3.5%	1.3 g	¼
Neck	2%	0.7 g	¼
Upper arms	8%	2.9 g	½
Forearms	6%	2.2 g	½
Backs of hands	2.5%	0.9 g	¼
Chest and stomach	13%	4.7 g	1
Back	13%	4.7 g	1
Thighs	19%	6.9 g	1½
Lower legs	14%	5.1 g	1
Tops of feet	3.5%	1.3 g	¼
Total	85%	31 g	6¼

How much to pack

Hours outside a day

Extra coats a day

Days

Bottle size

One coat per two hours outside, plus extra coats after swims or towelling.

4 applications a day × 31 mL = 123 mL a day.

For 7 days: 860 mL, which is 5 bottles of 200 mL. One bottle lasts about 1.6 days.

How the calculator works

Every SPF is measured on skin covered with 2 mg of sunscreen per square centimetre, the dose set in the FDA's sunscreen rule, ISO 24444 and the EU recommendation. To match the number on the label, you need that much on every bit of bare skin.

Enter height and weight in metric or US units. The calculator works out your body's surface area with the Mosteller formula: the square root of height in cm × weight in kg ÷ 3,600.
Choose an age group. The share of skin each part takes up comes from the Lund and Browder chart, which gives young children a larger head and shorter legs.
Tick the skin you will leave bare, or pick an outfit. Palms, soles and skin under swimwear are left out.
Read the amount in teaspoons (5 mL), millilitres, grams and US fluid ounces. Volumes assume a lotion of about 1 g per mL; sprays and gels vary.

Worked example: an adult of 170 cm and 70 kg has a surface area of √(170 × 70 ÷ 3,600) ≈ 1.82 m². A swimsuit leaves about 84.5% of it bare, roughly 15,400 cm². At 2 mg/cm² that is about 31 g: some 6 teaspoons, or just over 1 US fluid ounce. That matches the American Academy of Dermatology's “about 1 ounce, a shot glass” for an adult body.

The trip planner multiplies one coat by the coats you need: one per started two hours outside, plus any extra after swimming or towel drying.

Most of us use far less

The EU's recommendation on sunscreen claims puts the test dose at about 36 g, around six teaspoons of lotion, for an average adult body, and notes that this is more than people usually apply. Measurements of sunbathers have found layers of about 0.5 mg/cm², a quarter of the test dose.

Protection does not fall in step with the amount; it falls faster. In an in-vivo study by Faurschou and Wulf, half the dose gave roughly the square root of the SPF and a quarter gave roughly its fourth root. The EU text puts it more cautiously: halve the quantity and protection “may fall by as much as two-thirds”.

Other rules of thumb

Whole body: about 1 ounce, a shot glass (AAD); 6 to 8 teaspoons (NHS); about 36 g or six teaspoons (EU).
Face: the AAD suggests at least 1 teaspoon, about two finger-lengths. That is more generous than the area-based figure here, which covers the test dose only.
Two coats: the British Association of Dermatologists suggests applying 15 to 30 minutes before going out and again 15 to 30 minutes after, to catch missed spots.

Questions about sunscreen amounts

Why 2 mg per square centimetre?

It is the amount laboratories use when they measure SPF, under the FDA's rule (21 CFR 201.327), ISO 24444 and the EU recommendation. The SPF on the label only describes skin covered that thickly, so it is the amount to aim for if you want the protection the label states.

Is a shot glass of sunscreen enough?

For an average adult in swimwear, roughly yes: the AAD's “about 1 ounce” is close to the calculator's figure. Taller or heavier people need more, children less, and someone in a T-shirt and shorts needs about a third as much.

How much sunscreen does my face need?

Tick only “Face and ears” and “Neck” to see the test-dose amount for your size, usually just under half a teaspoon for an adult. The AAD's simpler advice is at least 1 teaspoon for the face, which leaves room for uneven spreading.

Can I use less if my sunscreen has a higher SPF?

A thin layer of a high SPF can still beat a thin layer of a lower one, but neither reaches its label. Studies show protection drops faster than the amount, so the EU recommendation warns that half the quantity may cut protection by as much as two-thirds.

Does this work for children?

Yes, from age 1: choose the age group so the head and legs get the right share, and enter the child's own height and weight. For babies under 6 months the FDA label tells parents to ask a doctor, and the BAD advises shade and clothing rather than sunscreen.

What about sprays, sticks and gels?

The grams are the same, but the volumes here assume a lotion of about 1 g per mL. Sprays and sticks are hard to measure, so it is easy to put on too little: follow the label, cover every exposed area evenly, and rub sprays in.

Sources

US FDA, 21 CFR 201.327, labeling and testing of OTC sunscreen products (2 mg/cm² test dose; directions to apply liberally, reapply and ask a doctor for children under 6 months).
ISO 24444:2019, Cosmetics — Sun protection test methods — In vivo determination of the sun protection factor (SPF).
European Commission, Recommendation 2006/647/EC on the efficacy of sunscreen products and the claims made relating thereto (about 36 g for an average adult; half the amount may cut protection by up to two-thirds).
Mosteller RD. Simplified calculation of body-surface area. N Engl J Med 1987;317:1098. doi:10.1056/NEJM198710223171717.
Lund CC, Browder NC. The estimation of areas of burns. Surg Gynecol Obstet 1944;79:352–358 (share of body surface by region and age).
Faurschou A, Wulf HC. The relation between sun protection factor and amount of sunscreen applied in vivo. Br J Dermatol 2007;156:716–719. doi:10.1111/j.1365-2133.2006.07684.x.
American Academy of Dermatology, How to apply sunscreen (1 ounce for the body, 1 teaspoon for the face).
NHS, Sunscreen and sun safety (6 to 8 teaspoons for the whole body).
British Association of Dermatologists, The sunscreen fact sheet (two coats; babies under 6 months).