Sunscreen only gives the SPF on the label when applied at 2 mg per square centimetre – about a shot glass (30 mL) for an adult’s whole body. Most people apply a quarter to half of that. Reapply at least every two hours and after swimming or sweating.
How to use the Sunscreen Calculator
- Height
- Weight
- Skin exposed
- Hours in the sun per day
- Days
- Bottle size
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter your height and weight, the share of skin exposed, the hours you spend in the sun each day, the number of days and your bottle size.
Tips
- Use the amounts printed on the product (ml, sheets, applications) where available.
- Count everyone in the household who shares the product.
- Buy a little extra so you never run out unexpectedly.
- Small daily habits add up – check the yearly figures for the real impact.
Common mistakes to avoid
- Forgetting that hot water costs energy as well as water.
- Underestimating how much sunscreen a full application needs.
- Comparing pack prices without checking the size of each pack.
Inputs explained
| Field | What to enter |
|---|---|
| Height | Units: cm, in, ft |
| Weight | Units: kg, lb, g |
| Skin exposed | Choose one: Swimwear (≈ 85%), Shorts and T-shirt (≈ 50%), Face, neck, arms (≈ 20%), Face only (≈ 5%) |
| Hours in the sun per day | A number |
| Days | A number |
| Bottle size | Choose one: 50 mL / 1.7 fl oz, 100 mL / 3.4 fl oz, 150 mL / 5 fl oz, 200 mL / 6.8 fl oz … |
What each result means
- Body surface area = sqrt([Height]×100×[Weight]÷3600)
- One application = [Body surface area]×10000×[Skin exposed]×2÷1000÷1000×1e-3
- Applications per day = max(ceil([Hours in the sun per day]÷2 – 0.000001), 1)
- Sunscreen for the trip = [One application]×[Applications per day]×[Days]
- Bottles to pack = ceil([Sunscreen for the trip]÷[Bottle size] – 0.000001)
Example values
How the main result changes when Height changes and everything else stays at the example values above:
| Height (cm) | Sunscreen for the trip (mL) |
|---|---|
| 85 | 459 |
| 127.5 | 562 |
| 170 | 649 |
| 212.5 | 726 |
| 255 | 795 |
| 340 | 918 |
Formula
Body surface area (Mosteller) = √(height cm × weight kg ÷ 3,600)One application = 2 mg/cm² × exposed skin area (≈ 30 mL for an adult in swimwear)Applications per day = ceil(hours in the sun ÷ 2)Worked example
Inputs — Height: 170 cm; Weight: 70 kg; Skin exposed: Swimwear (≈ 85%); Hours in the sun per day: 6; Days: 7; Bottle size: 100 mL / 3.4 fl oz.
Results — Body surface area = 1.82 m²; One application = 30.9 mL; Applications per day = 3; Sunscreen for the trip = 649 mL; Bottles to pack = 7.
Frequently asked questions
How much sunscreen should I use?
About 30 mL (a shot glass) for an adult's whole body, or half a teaspoon for the face and neck.
How often should I reapply?
At least every two hours, and immediately after swimming, sweating or towelling.
Does SPF 50 mean I can apply less?
No – using half the amount gives far less protection than the SPF number suggests.
What formula does the Sunscreen Calculator use?
Body surface area (Mosteller) = √(height cm × weight kg ÷ 3,600).