Sunscreen Protection Calculator

This calculator estimates two things from your own inputs: how long a sunscreen's labelled SPF is likely to extend your time in the sun before burning, and how much sunscreen one full-body application uses for a person of your size. It also shows how many full-body applications a bottle of a given size contains. The estimates help you plan reapplication and buy the right bottle size for a beach day or a holiday. This page does not measure UV radiation, does not read the weather, and cannot tell you whether a specific sunscreen will protect your own skin.

Method and formula

The protection-time estimate uses the common rule of thumb that SPF multiplies the time your skin would take to burn without protection:

protection time = skin burn time × SPF

reapplication interval = min(protection time, 120 minutes)

The skin burn time is the number of minutes you can stay in strong midday sun without sunscreen before your skin starts to turn red. Typical values range from about 10 minutes for very fair skin to about 40 minutes for deeply pigmented skin, but your own experience is the better input.

The amount estimate uses the reference dose of 2 mg of sunscreen per square centimetre of skin, which is the amount used in SPF laboratory testing, together with the Mosteller body surface area formula:

body surface area = √(height (cm) × weight (kg) ÷ 3600) square metres

sunscreen per application = body surface area × 20 grams (2 mg/cm² × 10,000 cm²/m² = 20 g/m²)

applications per bottle = floor(bottle size ÷ sunscreen per application)

Worked example

For a person who burns in 15 minutes without sunscreen, using SPF 30, with height 170 cm, weight 65 kg, and a 100 ml bottle:

Body surface area: √(170 × 65 ÷ 3600) ≈ 1.75 m²
Sunscreen per application: 1.75 × 20 ≈ 35 ml
Protection time: 15 × 30 = 450 minutes
Reapplication interval: min(450, 120) = 120 minutes
Applications per bottle: floor(100 ÷ 35) = 2

The numbers match common guidance: an average adult needs roughly a shot glass (about 30–35 ml) of sunscreen per full-body application, and a small travel bottle covers only a couple of full-body uses.

What the result means

The protection time is a rough ceiling, not a safe exposure guarantee. Real-world protection is usually lower than the label suggests because most people apply far less than the 2 mg/cm² test dose, swim, sweat, or rub sunscreen off on clothing. Dermatology organisations therefore recommend reapplying at least every two hours regardless of SPF, which is what the reapplication interval shows.

The per-application amount assumes a full-body application in the 2 mg/cm² test dose. If you cover only arms, face, and neck, you need less; if you apply generously, you need more. "Applications per bottle" counts full-body doses only.

Accuracy and limitations

The burn-time input is self-reported and varies with UV index, altitude, latitude, season, time of day, and your skin's recent sun exposure. The calculator does not adjust for any of these factors.
SPF is tested in laboratories at 2 mg/cm². At half that thickness, measured protection can fall well below the labelled SPF, so under-application shortens real protection time substantially.
The Mosteller formula is an approximation of body surface area; the sunscreen amount it produces is a planning estimate, not a dosing instruction.
The tool does not model water resistance, towel removal, infants' skin, medication photosensitivity, or vitamin-D effects. Sunscreen alone is not complete sun protection: shade and clothing also matter. For medical questions about sun sensitivity, consult a health professional.

Sources

American Academy of Dermatology — How to apply sunscreen — the 2 mg/cm² reference dose, the full-body amount guidance, and the two-hour reapplication advice.
Mosteller RD. Simplified calculation of body-surface area. N Engl J Med. 1987;317(17):1098 — the body surface area formula used for the amount estimate.

Editorial record

Author: SoupCalc Editorial Team
Method review: SoupCalc Engineering Review
Last reviewed: September 2, 2026
Review scope: protection-time rule of thumb, Mosteller amount estimate, examples, source links, and claims about the tool's actual functions