Sunscreen Amount Calculator (How Much for Your Body?)
The amount of sunscreen to cover the skin you leave exposed, using the 2 mg per cm² of the SPF test, plus how many applications a bottle gives.
Results
What this tool does
SPF is measured by applying 2 mg of product per square centimetre of skin, and the label only holds if you use about that much: for a whole adult body it comes to roughly 30–40 g, well over a shot glass. Studies of real use find that people usually apply a quarter to a half of it, so the protection they get is far below the number on the bottle. An SPF 30 blocks about 97 % of the UVB that causes burning when applied properly and SPF 50 about 98 %, a smaller gain than the numbers suggest. Reapply after about two hours, and after swimming or sweating, whatever the SPF.
Formula
area = body surface × share exposed · sunscreen = 2 mg per cm² × area · UVB blocked = 1 − 1 ÷ SPF
Variables
| Symbol | Meaning | Unit |
|---|---|---|
ht | Height | cm |
kg | Body weight | kg |
ex | Share of the body not covered by clothes | % |
sp | SPF of the sunscreen | — |
bt | Size of your bottle | mL |
GR | Sunscreen for one application | g |
TS | That is about this many teaspoons | — |
AR | Skin area to cover | cm² |
UV | Share of UVB the SPF blocks when applied properly | % |
AP | Applications per bottle | — |
Worked example
- Height175 cm
- Body weight70 kg
- Share of the body not covered by clothes60 %
- SPF of the sunscreen30
- Size of your bottle200 mL
- Sunscreen for one application22 g
- That is about this many teaspoons4.4
- Skin area to cover11,068 cm²
- Share of UVB the SPF blocks when applied properly96.7 %
- Applications per bottle9.0
Limitations
- This tool is informational only. It does not diagnose, treat or replace a qualified health professional. Speak to a doctor before making decisions about your health.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.