Sunscreen Duration Calculator
Find your safe reapply time based on SPF, skin type & UV index
Sunscreen Settings
Skin type reference guide
Results
Protection Time Comparison
Important Reminders
- Reapply every 2 hours regardless of SPF level
- Reapply immediately after swimming, sweating, or toweling off
- Apply enough sunscreen (about 2mg/cm²) to achieve labeled SPF
- SPF 30 blocks ~97% UVB, SPF 50 blocks ~98% - minimal difference
- This calculator is for reference only. Results may vary
Sources & formula verification
reapply = min(baseBurnTime[skinType] × SPF × (10/UVI) × 0.6, 120) minutes.Sources: FDA sunscreen labeling guidance, NCBI Fitzpatrick Scale reference, Omnicalculator sun-exposure model.
Verified 2025-12-03 · 5/5 test cases · error 0%.
About the Sunscreen Duration Calculator
This calculator estimates sunscreen protection time based on SPF (Sun Protection Factor), Fitzpatrick skin type classification, and UV Index (UVI). Formulas reference US FDA guidelines, dermatological research, and major sunscreen brand recommendations.
How to Use
- Select your sunscreen's SPF level (commonly 30 or 50)
- Choose your Fitzpatrick skin type based on how your skin reacts to sun
- Enter the current UV Index (check your weather service)
- Review results, especially the "Recommended Reapplication Time"
- Share or export PDF to save your results
What is SPF?
SPF (Sun Protection Factor) measures a sunscreen's ability to block UVB rays. SPF 30 blocks about 97% of UVB, SPF 50 blocks about 98%. Note: Higher SPF does not mean proportionally longer protection - it indicates slightly better blocking ability.
What is the Fitzpatrick Scale?
The Fitzpatrick skin type classification was developed in 1975 by Harvard dermatologist Thomas Fitzpatrick. It categorizes skin into six types (I-VI) based on how skin responds to UV exposure (burning vs. tanning tendency).
Frequently Asked Questions
Q: Why reapply every 2 hours maximum?
A: The FDA recommends reapplying sunscreen every 2 hours regardless of SPF. Sunscreen effectiveness decreases due to sweating, rubbing, and real-world application being less than tested amounts.
Q: Is SPF 50 much better than SPF 30?
A: The difference is minimal. SPF 30 blocks ~97% UVB, SPF 50 blocks ~98% - only 1% difference. Proper application amount and regular reapplication matter more.
Q: Why is the calculated time so long but 2-hour reapplication still recommended?
A: Theoretical calculations assume ideal conditions (enough product, no sweating, no rubbing). In reality, these conditions are rarely met, so FDA's 2-hour recommendation is safer.
Q: Do I need sunscreen on cloudy days?
A: Yes. Clouds only block some UV rays - up to 80% of UV can penetrate cloud cover. Check the day's UV Index to determine protection needs.
Medical Disclaimer
This calculator is for reference only and does not replace professional medical advice. Results are based on scientific formulas and general statistics. Actual sunscreen effectiveness may vary due to individual skin type, application amount, activity level, and environmental factors. Please consult a dermatologist for skin concerns or special needs.
