This is a packing estimate for an average adult, not a measured personal dose.
Body size, exposed area, product format, missed spots, swimming, sweating, and towel drying can require more.
Sunscreen is one protection layer and should not be used to extend tanning or outdoor exposure.
Source-backed sunscreen tool
Sunscreen Calculator
Use this sunscreen calculator to estimate product for one adult session. AAD says most adults need at least 1 ounce (about 30 mL) to cover exposed skin and at least 1 teaspoon for the face. FDA says to apply 15 minutes before sun, reapply at least every two hours, and follow 40- or 80-minute water-resistance directions after swimming or sweating. Your product label always wins.
AAD + FDA label guidance
Sunscreen amount calculator
Estimate product for one adult outdoor session. Product label, body size, exposed area, swimming, sweating, and towel drying can require more.
Per application at least 1 oz (about 30 mL)
Applications for session 1
Minimum to pack at least 1 oz (about 30 mL)
Apply 15 minutes before sun. If you stay beyond 2 hours, reapply then; reapply sooner after swimming, sweating, or towel drying.
Estimate, not a dose guarantee. Use broad-spectrum, water-resistant SPF 30+ and follow the exact directions on your product. Sunscreen does not make UV exposure risk-free or extend a tanning window.
1 oz+ Exposed skin: AAD minimum for most adults; body size may require more.
1 tsp+ Face: AAD minimum for face coverage.
2 hr Dry reapply: FDA minimum interval while staying outdoors.
Before you start a session
What can change the tan window, SPF timing, or stop cue.
How the sunscreen calculator works
The tool multiplies a source-backed minimum amount per application by the number of label-based application intervals in your session. Dry outdoor time uses a two-hour interval. Swimming or heavy sweating uses the 40- or 80-minute water-resistance statement selected from the product label.
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Initial application counts
Every plan starts with one application about 15 minutes before sun exposure. A two-hour session ending at the interval needs one application; staying beyond it needs another.
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Towel drying overrides the clock
FDA label language calls for reapplication immediately after towel drying. Add another application each time that happens.
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Pack more when uncertain
Sprays, sticks, uneven coverage, larger bodies, and more exposed skin are not converted into a fake precise dose. Use enough for visible, even coverage and follow the label.
What this calculator does not promise
It does not calculate safe time in the sun, effective SPF on your skin, or a guarantee against sunburn. Choose broad-spectrum, water-resistant SPF 30 or higher, combine it with shade and clothing, and stop or cover up instead of using reapplication to extend intentional UV exposure.
Questions
Short answers for the exact search intent, with the cautions that keep the plan usable.
How much sunscreen should an adult use?
AAD says most adults need at least 1 ounce, about a shot-glass amount or 30 mL, to cover skin not covered by clothing. Body size and exposed area may require more.
How much sunscreen should I use on my face?
AAD recommends at least 1 teaspoon for the face. Also cover easy-to-miss exposed areas such as ears, neck, hairline, and lips with an appropriate SPF product.
Does water-resistant sunscreen last all day?
No. FDA labels state 40 or 80 minutes of water resistance, then require reapplication, plus immediate reapplication after towel drying and at least every two hours.
Does higher SPF mean I can reapply less often?
No. FDA warns that SPF is not a multiplier for time outdoors. Follow the same label reapplication directions and use shade, clothing, hats, and sunglasses too.
Related TanPilot pages
Move from the UV number to tan timing, burn risk, skin type, and app setup.