What this calculator does
Sunscreen is tested at 2 mg per square centimetre of skin, and that is the rate the SPF on the bottle assumes. Working out the amount needs a body surface area, which the Mosteller formula estimates from height and weight.
For someone 170 cm and 70 kg that is a surface area of 1.82 m² and about 36 g of sunscreen for full-body cover, roughly seven teaspoons. Most people apply between a quarter and half of that, which is why real-world protection routinely falls well short of the SPF on the label.
The formula
Body surface area uses the Mosteller formula: the square root of height in centimetres multiplied by weight in kilograms, divided by 3,600. That area is multiplied by 2 mg per square centimetre, the rate used in SPF testing, to give the mass of sunscreen for a full-body application.
| Term | Meaning |
|---|---|
| 2 mg/cm² | The application rate used in SPF laboratory testing, which the label figure assumes. |
| Mosteller formula | A widely used body surface area estimate from height and weight. |
| Teaspoon rule | A common simplification: about a teaspoon per body area, roughly seven for a full body. |
| Reapplication | Every two hours and after swimming or towelling, regardless of the SPF. |
The inputs explained
| Field | What to enter |
|---|---|
| Height (cm) | Height in centimetres. |
| Weight (kg) | Weight in kilograms. |
When to use it
Working out how much to actually apply
The gap between what people apply and what the label assumes is large, and seeing the figure in teaspoons makes it concrete.
Estimating how long a bottle lasts
A 200 ml bottle gives about five full-body applications at the tested rate, which is considerably fewer than most people expect.
Packing for a trip
Multiplying the per-application figure by applications per day and days away gives a realistic quantity to take.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much sunscreen does a full application need?
Three body sizes with the sunscreen quantity for full-body cover.
| Height (weight varies with it) | Sunscreen per full-body application | In teaspoons | Body surface area (Mosteller) |
|---|---|---|---|
| 150 cm | 34.2 g (≈34.2 ml) | 6.83 tsp | 1.71 m² |
| 170 cm | 36.4 g (≈36.4 ml) | 7.27 tsp | 1.82 m² |
| 185 cm | 37.9 g (≈37.9 ml) | 7.59 tsp | 1.90 m² |
Questions
How much sunscreen should I use?
About 35 to 40 g for a full-body application on an average adult, which is roughly seven teaspoons or a shot glass. For face and neck alone, about half a teaspoon. Most people apply far less, which is the single biggest reason sunscreen underperforms its label.
Why does applying less matter so much?
Because SPF is measured at 2 mg per square centimetre. Applying half that does not give half the SPF; the relationship is worse than linear, so an SPF 50 applied at half the rate can behave closer to SPF 15. Applying enough matters more than choosing a higher number.
How often should I reapply?
Every two hours during sun exposure, and immediately after swimming, sweating heavily or towelling, regardless of what the bottle claims about water resistance. Water resistance is tested for a limited time and is not a substitute for reapplying.
Is sunscreen enough on its own?
No. Public health guidance consistently puts shade, clothing and timing ahead of sunscreen, with sunscreen covering what those cannot. Anyone with specific concerns about skin cancer risk or a changing mole should see a doctor rather than rely on any product.
For body measurements in another context, see the belt size calculator. For blanket weight from body weight, see the weighted blanket calculator.