Sunscreen Amount Calculator
Work out sunscreen for a whole day from your height, weight and what you are wearing.
Uses Du Bois body surface area at the 2 mg/cm² lab standard.
Most people apply only 25 to 50% of the recommended amount of sunscreen, which quietly throws away most of the SPF they paid for. This page works out how much a whole day actually takes, sized to your own body rather than to an average one.
The core formula
Total per application = exposed skin area × 2 mg/cm²
The 2 mg/cm² figure is the application density used in every SPF laboratory test, set by the FDA and matched by the international standard. It is not a recommendation on top of the label. It is the assumption baked into the number on the bottle.
Skin area comes from the Du Bois formula, the same one used for chemotherapy dosing:
BSA (m²) = 0.007184 × height(cm)^0.725 × weight(kg)^0.425
A 175 cm, 75 kg adult works out to 1.90 m², which is 19,000 cm². Cover all of it at 2 mg/cm² and you need 38 g, or close to 38 mL since sunscreen is roughly the density of water.
Which fraction of you is exposed
Regional shares come from the Wallace rule of nines, the burn-assessment chart every emergency department uses:
| What you are wearing | Share of skin exposed |
|---|---|
| Face, neck and hands only | 9% |
| Face, neck and bare arms | 27% |
| Shorts, legs bare | 36% |
| Short sleeves and shorts | 40% |
| Swimwear | 85% |
Each leg is 18% of you and each arm is 9%, which is where those add up from. It is also why a swimsuit needs nine times the product of a walk to the shops, not twice.
The teaspoon rule, for when you have no scales
Seven regions, one 5 mL teaspoon each: head and neck, each arm, each leg, front of torso, back of torso. Total 35 mL.
That is not a separate folk rule. Take the same 175 cm, 75 kg adult, expose the 85% a swimsuit leaves bare, and the arithmetic gives 32 mL. The teaspoon rule rounds up to 35. If this calculator and the teaspoons ever disagree by much, check what you typed.
Why under-application costs so much
Protection does not fall off in proportion to how much you skipped. Faurschou and Wulf measured it in 2007 and found effective SPF = label SPF raised to the power (applied dose ÷ 2 mg/cm²):
| Applied | SPF 50 bottle really gives |
|---|---|
| 2.0 mg/cm² | 50.0 |
| 1.5 mg/cm² | 18.8 |
| 1.0 mg/cm² | 7.1 |
| 0.5 mg/cm² | 2.7 |
Half the product, square root of the number. This is the single most useful thing to know about sunscreen, and it is on none of the packaging.
Reapplication
Every 2 hours of sun exposure, and immediately after swimming or heavy sweating whatever the clock says. “Water-resistant” is a tested claim of either 40 or 80 minutes of immersion, and the label has to state which. Beyond 80 minutes no product may claim it at all.
Worked example
A 175 cm, 75 kg adult at the beach from 8 AM to 5 PM in swimwear, reapplying every 2 hours.
BSA 1.90 m², 85% exposed, so 16,200 cm² at 2 mg/cm² = 32 mL per application. Nine hours means 5 applications, at 8, 10, 12, 2 and 4. Total 160 mL.
An 88 mL travel tube does not cover one person for one beach day. Buy the 200 mL bottle and stop rationing it.
Tip: sprays are the worst offenders for under-application. Spray until the skin visibly glistens, then rub in. A quick pass at arm’s length lays down a fraction of a proper coat, and the wind takes some of that.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.