Sunscreen Calculator
Work out how much sunscreen you need per application and for your trip, and how many bottles to buy.
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Disclaimer: this calculation is for informational purposes only. For important decisions, consult a qualified professional.
How much sunscreen you really need
The SPF on the bottle is measured by applying 2 mg of sunscreen per cm² of skin. That is much more than most people use: for an adult in swimwear it comes to about 30 ml, a full shot glass, on each application. This calculator estimates the amount from your body surface area and exposed skin, and tells you how much sunscreen and how many bottles you need for a holiday.
How the calculation works
- Body surface area: the Mosteller formula, the square root of height in cm times weight in kg divided by 3,600.
- Amount per application: exposed area in cm² × 2 mg; sunscreen weighs about 1 g per ml, so grams equal millilitres.
- Period: amount × applications per day × days; bottles are rounded up.
Amount by body size
With 85% of skin exposed, as in swimwear:
| Person | Body surface area | Per application | Per day (3 applications) |
|---|---|---|---|
| Child, 110 cm and 19 kg | 0.76 m² | 13 ml | 39 ml |
| Teen, 150 cm and 45 kg | 1.37 m² | 23 ml | 70 ml |
| Adult, 165 cm and 60 kg | 1.66 m² | 28 ml | 85 ml |
| Adult, 170 cm and 70 kg | 1.82 m² | 31 ml | 93 ml |
| Adult, 180 cm and 80 kg | 2.00 m² | 34 ml | 102 ml |
| Adult, 190 cm and 95 kg | 2.24 m² | 38 ml | 114 ml |
Amount by body area
Split according to the rule of nines, used in medicine to estimate the surface of body parts:
| Body area | Share of surface | Sunscreen (adult 170 cm, 70 kg) |
|---|---|---|
| Head and neck | 9% | 3.3 ml |
| One arm | 9% | 3.3 ml |
| Front of torso | 18% | 6.5 ml |
| Back | 18% | 6.5 ml |
| One leg | 18% | 6.5 ml |
A teaspoon holds about 5 ml: an adult needs a little under a teaspoon for the head and neck and for each arm, and more than a teaspoon for each leg, the front of the torso and the back.
How to read the result
- Sunscreen per application: the amount that gives the protection stated on the bottle.
- Bottles needed: how many whole bottles to buy; if a lot is left over, the calculator says so.
- Body surface area: the starting point of the calculation, also useful to compare adults and children.
Tips and limits
Reapply roughly every two hours and after swimming, sweating or towelling, even with water-resistant sunscreen. Sunscreen does not replace shade, clothing and a hat, especially around midday. For young children and sensitive skin, ask your paediatrician or dermatologist. The calculation is an estimate: the areas covered by swimwear and the amount left on your hands change the result slightly.
Frequently asked questions
How much sunscreen do I need for my whole body?
For an adult of 170 cm and 70 kg in swimwear, about 31 ml per application, a little more than a shot glass.
Why 2 mg per cm²?
It is the amount used to measure SPF: with less sunscreen, real protection is lower than stated.
How often should I reapply?
Roughly every two hours and after swimming, sweating or towelling.
How many bottles for a week at the beach?
With 3 applications a day for 7 days an adult uses about 650 ml: four 200 ml bottles.
How much for the face?
Head and neck are about 9% of the surface: for an adult about 3 ml, just over half a teaspoon.
Does water-resistant sunscreen need reapplying?
Yes: it resists water for a limited time and must be reapplied after swimming and towelling.
Does it work for children?
Yes, the calculation uses the child's height and weight; for young children follow your paediatrician's advice.
Is last year's sunscreen still OK?
Check the expiry date or the open-jar symbol, which shows how many months to use it after opening.
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