Sunscreen Calculator

Estimate how much sunscreen to apply per use, based on your body surface area and the dermatology-standard 2 mg/cm² application rate — plus how many uses a bottle will last.

Quick Facts

Application rate
2 mg of sunscreen per cm² of skin
The FDA's SPF-testing dose; most people apply well under half this much.
Body surface area
Mosteller formula: √(height × weight ÷ 3600)
The same equation clinicians use to estimate BSA from height and weight.

Your Results

Calculated
Per application
-
At 2 mg/cm² over the chosen area
In teaspoons
-
US teaspoons, ~4.93 mL each
Total per day
-
Per application × reapplications
Bottle life
-
Uses from a 100 mL (3.4 fl oz) bottle

Ready

Enter height, weight, coverage area, and daily reapplications, then press Calculate.

Understanding the Sunscreen Calculator

This tool estimates how much sunscreen you actually need for a given application, based on the same dosing standard the FDA uses to test SPF ratings: 2 milligrams of product per square centimeter of skin. Most people apply only a quarter to a half of this amount, which is one of the main reasons real-world sun protection falls well short of the SPF number printed on the bottle.

The formulas

The calculation runs in three steps:

  • Body surface area (BSA): estimated with the Mosteller formula, BSA (m²) = √(height in cm × weight in kg ÷ 3600). This is the same equation used clinically (for example, to dose medications by body surface area).
  • Target dose: BSA is converted to cm² (×10,000) and multiplied by 2 mg/cm², then scaled by the percentage of the body you're covering, to get the total amount in milligrams.
  • Unit conversion: milligrams are converted to milliliters assuming a lotion density of roughly 1 g/mL (a standard approximation for water-based sunscreen lotion), then to US teaspoons using 1 tsp ≈ 4.93 mL.

For an average adult (170 cm, 70 kg), BSA works out to about 1.82 m², giving roughly 36 mL — about 7 teaspoons, or a full shot glass — for one full-body application. That is close to figures cited by dermatology groups (commonly rounded to "about 1 ounce" or "a golf-ball-sized amount" for the whole body).

Apportioning coverage area

Not every application covers the whole body. This calculator offers preset shares based on the standard body-surface-area breakdown used in medicine (the "rule of nines"): head, neck, and ears are about 9% of total BSA; each arm is about 9% (18% for both); each leg is about 18% (36% for both); and the torso, front and back combined, is about 36%. Selecting one of these applies that fraction to the full-body dose instead of the whole 100%.

Reapplication and bottle life

Sunscreen wears off — through swimming, sweating, toweling, and simple abrasion — regardless of its SPF number, so dermatology guidance generally calls for reapplying at least every two hours during sun exposure, and immediately after getting wet. The "total per day" result multiplies the per-application amount by however many times you plan to reapply. The "bottle life" result divides a standard 100 mL (3.4 fl oz) bottle by the per-application amount, which is often a smaller number than people expect — a single full-body application at the correct dose can be a meaningful fraction of a whole bottle.

Assumptions and limits

This is a planning estimate, not a medical or regulatory prescription. It assumes average lotion density (~1 g/mL — sprays, sticks, and thicker mineral formulas vary), a fixed 2 mg/cm² target rather than a lower amount some people find practical, and BSA shares based on average adult proportions rather than a measurement of your specific body. Children's proportions differ substantially from adults' and are not modeled here.

Frequently Asked Questions

How much sunscreen should I actually apply?
The FDA's SPF testing protocol, and general dermatology guidance for real-world use, is based on 2 milligrams of sunscreen per square centimeter of skin. For an average adult with a body surface area of about 1.8 square meters, that works out to roughly 35 to 40 grams (about 1.2 to 1.4 fluid ounces, or 7 to 8 teaspoons) to cover the entire body in one application — noticeably more than most people apply.
How is body surface area calculated?
This calculator uses the Mosteller formula: BSA in square meters equals the square root of (height in centimeters times weight in kilograms, divided by 3600). It is the same equation used clinically to estimate body surface area from height and weight.
How often should I reapply sunscreen?
General guidance is to reapply at least every two hours, and immediately after swimming, heavy sweating, or towel-drying, regardless of the SPF printed on the label. SPF measures protection at a fixed dose applied once, not how long that dose lasts on skin.
Why does a bottle run out faster than expected?
Because the correct per-application amount (2 mg per square centimeter of skin) is larger than most people assume. A single full-body application can use a meaningful fraction of a standard 100 mL bottle, so a bottle bought for one trip may only cover a handful of full-body applications, not weeks of daily use.

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