Sunscreen Calculator
Calculate how much sunscreen you need and when to reapply.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Request a ToolHow to Use the Sunscreen Calculator
Get your personalized sunscreen amount in seconds:
- Choose your SPF. Higher SPF provides more UV protection. SPF 30 blocks about 97% of UVB rays; SPF 50 blocks about 98%.
- Select your activity level. Swimming and heavy sweating breaks down sunscreen faster, requiring earlier reapplication.
- Check the body areas you are covering. The calculator weighs each area proportionally to give you an accurate total amount.
- Read your results. The calculator shows how much to apply, how often to reapply, and how many total applications you need for a full day outdoors.
About Sunscreen Application
Dermatologists recommend applying 1 ounce (about 2 tablespoons or a shot glass full) of sunscreen for full-body coverage. Most people apply only 25-50% of the recommended amount, which means they get significantly less protection than the labeled SPF. Applying too little effectively reduces SPF 50 to the protection level of SPF 15 or less.
Sunscreen should be applied 15-30 minutes before sun exposure to allow it to bind to the skin. Reapply every 2 hours during outdoor activities, or every 80-90 minutes when swimming or sweating. Water-resistant sunscreens maintain their SPF for the labeled time (usually 40 or 80 minutes) in water, but reapplication is still required.
Frequently Asked Questions
What is 1 oz of sunscreen in practical terms?
One ounce of sunscreen for full-body coverage is roughly the amount that fills a shot glass (2 tablespoons). In product terms, a typical 3.4 oz tube of sunscreen contains about 3-4 full-body applications. A 6 oz spray bottle contains about 5-6 applications if used correctly. Most people use a 3.4 oz tube on a single beach day when applied and reapplied properly, which is why sunscreen seems to run out so fast when used correctly.
Does a higher SPF mean I can stay in the sun longer?
Technically, higher SPF extends the theoretical time before burning, but dermatologists do not recommend using SPF to extend sun exposure time. SPF 30 blocks about 97% of UVB; SPF 50 blocks about 98%. The difference in protection between SPF 30 and SPF 100 is much smaller than people assume. Reapplication timing remains the same regardless of SPF: every 2 hours, or more often when swimming or sweating heavily.
Does sunscreen expire?
Yes. Sunscreen ingredients degrade over time, reducing their effectiveness. The FDA requires sunscreen manufacturers to maintain stability for at least 3 years, and most products have an expiration date printed on the packaging. Sunscreen stored in hot conditions (like a car or beach bag in direct sun) can degrade faster. Discard sunscreen that is past its expiration date, has changed color, or has separated.
Should I use a different amount for spray vs. lotion sunscreen?
Spray sunscreen requires more product to achieve the same coverage as lotion because much of the spray misses the skin or evaporates. The recommendation for sprays is to apply until the skin glistens, then rub it in. For lotion, the 1-ounce rule applies directly. Studies show spray users consistently apply far less than the recommended amount. Lotion is generally easier to apply correctly, especially for self-application on the back.