Sunburn Calculator
Example: UV 8 + Type II ≈ 21 min to representative MED
Estimate how long unprotected skin may take to burn at a given UV Index, then see how much longer sunscreen buys you in typical use versus ideal lab conditions.
Calculation results updated
UV index
Representative unprotected MED time by phototype
Without Sunscreen
With Sunscreen (Typical Use)
Lab Maximum (Ideal Use)
UVB Filtered
Reapply
UV Risk Level
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How We Calculate This
The model uses representative MED values (J/m²): I=200, II=250, III=300, IV=450, V=600, VI=1000. Erythemally weighted irradiance = UVI × 0.025 W/m², so the unprotected minutes to the representative MED = MED ÷ (UVI × 0.025 × 60). The typical-use time multiplies that by the real-world effective SPF, estimated as √SPF because protection scales as SPF^(amount applied ÷ 2 mg/cm²) and most people apply about half the tested amount (Faurschou & Wulf 2007). The lab-maximum time multiplies by the full labelled SPF, which assumes the ideal 2 mg/cm² coat. UVB filtered = 1 − 1/SPF. Forecast UV Index already accounts for modeled cloud and elevation effects, so no extra cloud factor is applied.
Methodology last reviewed: July 2026. How SparkCalc works
Sources: EPA – UV Index Scale · Faurschou & Wulf 2007 – SPF vs amount of sunscreen applied (Br J Dermatol) · FDA – Sun Protection Factor (SPF) · FDA – Sunscreen application and reapplication directions · FDA – Sunscreen effectiveness testing and product-label directions · EPA – How the forecast UV Index accounts for clouds and elevation · Peer-reviewed representative minimum erythema dose values
Frequently Asked Questions
What is MED (Minimal Erythemal Dose)?
MED is the amount of UV radiation that causes just-noticeable reddening (erythema) of the skin within 24 hours. It's measured in J/m² and varies by skin type.
How does SPF work?
SPF is measured in a lab using a thick, even 2 mg/cm² coat that few people actually apply. Under that test SPF 30 filters about 97% of burning UVB and, in theory, lets skin take about 30 times the dose before reddening. In real life people apply roughly half as much, so effective protection is closer to the square root of the SPF, about 5x for SPF 30. Either way you must reapply at least every two hours, and the FDA warns SPF is not directly related to how long you can stay in the sun.
How often should I reapply sunscreen?
Follow the product label. Reapply at least every two hours, and sooner after swimming, sweating or towel drying. Water-resistant products specify either 40 or 80 minutes of protection while swimming or sweating.
What are Fitzpatrick skin types?
A 1–6 self-reported phototype scale based on burning and tanning response. It was developed for phototherapy dosing and has limitations across diverse skin tones. People of every skin colour can experience UV damage and skin cancer.
How do I find the UV index for my location?
Most weather apps and forecast sites publish a daily UV index, and the EPA offers an hourly UV index forecast by ZIP code. Enter that value here to estimate the representative unprotected MED time. The higher the UV index, the faster the modeled dose accumulates.
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Rough educational estimate only, not a safe exposure limit or medical advice. The typical-use time assumes about half the tested amount of sunscreen is applied; the lab maximum assumes the full 2 mg/cm² coat that few people reach, so neither is a promise. UV damage builds before visible burning, so reapply at least every two hours, and sooner after swimming or sweating, and rely on shade and clothing too. Higher SPF is not a licence to stay out longer.
References
- EPA – UV Index Scale
- Faurschou & Wulf 2007 – SPF vs amount of sunscreen applied (Br J Dermatol)
- FDA – Sun Protection Factor (SPF)
- FDA – Sunscreen application and reapplication directions
- FDA – Sunscreen effectiveness testing and product-label directions
- EPA – How the forecast UV Index accounts for clouds and elevation
- Peer-reviewed representative minimum erythema dose values