Sunscreen Application Calculator
Use our free Sunscreen application Calculator for quick, accurate results. Get personalized estimates with clear explanations.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Amount (g) = BSA (m2) x 20 g/m2
Where BSA is Body Surface Area calculated using the DuBois formula: BSA = 0.007184 x Weight(kg)^0.425 x Height(cm)^0.725. The 20 g/m2 factor comes from the dermatological recommendation of 2 mg/cm2 of sunscreen on exposed skin. Total usage multiplies per-application amount by the number of reapplications needed.
Worked Examples
Example 1: Beach Day Application
Problem:A 70 kg, 170 cm adult spends 6 hours at the beach with SPF 50 sunscreen, swimming occasionally. UV index is 9. How much sunscreen is needed?
Solution:BSA = 0.007184 x 70^0.425 x 170^0.725 = 1.81 m2\nPer application = 1.81 x 20 = 36.2 grams\nSwimming: reapply every 80 min = 5 applications in 6 hours\nTotal = 36.2 x 5 = 181 grams (6.4 oz)\nSPF 50 blocks 98% of UVB
Result:Total needed: 181g (6.4 oz) across 5 applications of 36.2g each
Example 2: Daily Commute Protection
Problem:A 60 kg, 160 cm person applies SPF 30 for a 2-hour outdoor period during moderate UV (index 5), no swimming.
Solution:BSA = 0.007184 x 60^0.425 x 160^0.725 = 1.63 m2\nPer application = 1.63 x 20 = 32.6 grams\nNo swimming: reapply every 120 min = 1 application for 2 hours\nTotal = 32.6 grams (1.15 oz)\nBurn time without sunscreen: ~40 min\nBurn time with SPF 30: ~1,200 min
Result:Total needed: 32.6g (1.15 oz) for a single application with SPF 30
Frequently Asked Questions
How much sunscreen should I apply per application?
Dermatologists recommend applying 2 milligrams of sunscreen per square centimeter of exposed skin, which translates to roughly 1 ounce (about a shot glass full) for the average adult body. Most people apply only 25-50% of the recommended amount, which dramatically reduces the actual protection received. For the face alone, you need about a nickel-sized dollop or approximately half a teaspoon. Under-application is one of the most common sunscreen mistakes, effectively reducing your SPF protection by half or more. Sunscreen Application Calculator uses your body surface area to give you a personalized recommendation.
How often should I reapply sunscreen during the day?
The standard recommendation is to reapply sunscreen every two hours during continuous sun exposure, regardless of the SPF level. If you are swimming or sweating heavily, you should reapply every 80 minutes even if the product is labeled water-resistant. No sunscreen is truly waterproof, and the FDA banned that term from labels in 2011. Physical activity, towel drying, and water exposure all remove sunscreen from the skin faster than normal. Even on cloudy days, up to 80% of UV radiation penetrates cloud cover, so reapplication schedules should remain consistent whenever you are outdoors.
Does sunscreen expire and how should I store it?
Yes, sunscreen does expire and using expired sunscreen can leave you dangerously unprotected against UV radiation. Most sunscreens have a shelf life of three years from the date of manufacture, and the FDA requires an expiration date on the label if the product has not been stability tested to last three years. Heat, humidity, and direct sunlight can degrade active ingredients faster than the labeled expiration suggests. Store sunscreen in a cool, dry place and never leave it in a hot car where temperatures can exceed 120 degrees Fahrenheit. If the texture, color, or smell has changed, discard the product immediately regardless of the printed date.
What is the UV index and why does it matter for sunscreen?
The UV index is a standardized international scale from 0 to 11+ that measures the intensity of ultraviolet radiation reaching the ground at a given location and time. A UV index of 1-2 is considered low risk, 3-5 is moderate, 6-7 is high, 8-10 is very high, and 11+ is extreme. Higher UV index values mean you will burn faster without protection and should use higher SPF sunscreen with more frequent reapplication. The UV index peaks between 10 AM and 4 PM during summer months and is higher at greater altitudes, near the equator, and around reflective surfaces like water, sand, and snow. Checking the daily UV forecast helps you plan appropriate sun protection.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy