How this calculator works
The standard application rate for sunscreen testing and labelling is 2 milligrams per square centimetre, which is 20 grams per square metre of skin. For the whole body, the calculator estimates skin area with the Mosteller formula, which uses height and weight: the square root of height in centimetres times weight in kilograms divided by 3,600. For the face, it uses the entered length and width as a simple rectangle. It then assumes reapplication every two hours of sun exposure, with at least one application on any day, and multiplies by the days of the trip to get a total in grams, which is treated as millilitres because sunscreen is close to the density of water.
Formula
Body surface area = square root of (height in centimetres x weight in kilograms / 3600) square metres. Face area = length x width. Sunscreen per application = area x 20 grams per square metre. Applications per day = hours in the sun / 2, rounded up, and at least 1. Total = per application x applications x days. Bottles = total / bottle size, rounded up.
Worked example
An adult 170 cm tall and 70 kg has an estimated body surface area of the square root of 170 x 70 / 3600, which is 1.82 square metres. At 20 grams per square metre that is 36.4 grams per full body application. Four hours in the sun means two applications a day, so seven days need 509.1 grams, or three 200 mL bottles.
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Frequently asked questions
Why 2 milligrams per square centimetre?
That is the amount used when sunscreens are tested for their SPF, so it is the rate at which the labelled protection is defined. Most people apply far less, often a quarter to a half of this, which gives much lower protection than the label suggests.
How often should I reapply?
The calculator assumes every two hours of sun exposure, which is the common guidance, and also after swimming, heavy sweating or towelling off. No sunscreen is fully waterproof, and reapplication matters more than the label number for a long day outside.
Is the body surface area estimate accurate?
The Mosteller formula is a standard clinical estimate and is close for most adults, but it is less accurate at the extremes of height and weight. The result is a planning figure for buying sunscreen, not a clinical measurement.
Does the calculator include the face and neck when I choose body?
The body surface area covers the whole skin surface, so the body figure already includes the face. In practice people apply a separate face product, but the total amount needed is the same. Use the face mode when you only want to size a face application.