Rex says
Same equations the textbooks use — just way less awkward. Pop in your stats and I'll tell you what the number actually means for you.
Try a scenario
Click to load — tweak from there.Inputs
Result
Est. burn time with sunscreen
266
Burn time with no protection
17
Suggested reapplication interval
120
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter skin type (fitzpatrick).
- 2Enter uv index.
- 3Enter sunscreen spf (0 = none).
- 4Enter surface reflection.
- 5Read your est. burn time with sunscreen on the right — it updates as you type.
- 6Hit Share to keep the scenario or send it to someone.
About this calculator
Time to sunburn depends primarily on UV index and skin phototype (the Fitzpatrick scale), then gets extended by sunscreen's SPF factor and shortened by reflective surfaces like snow, sand, or water. This calculator uses published baseline minimal erythema dose (MED) times for each Fitzpatrick skin type at UV index 1, scales inversely with the actual UV index, then multiplies by your sunscreen's SPF (properly applied sunscreen at SPF 30 roughly multiplies burn time by 30, though real-world application is usually thinner than the tested standard, cutting effective protection significantly). Use this as a planning tool for reapplication timing, not a guarantee — cloud cover, altitude, and surface reflection all shift real UV exposure.
Worked example
Using the values the calculator loads with:
Inputs
- Skin type (Fitzpatrick): Type III — medium, sometimes burns
- UV index: 8
- Sunscreen SPF (0 = none): 30
- Surface reflection: Normal ground / grass
Results
- Est. burn time with sunscreen: 266
- Burn time with no protection: 17
- Suggested reapplication interval: 120
What each field means
Inputs
- Skin type (Fitzpatrick)
- Pick the option that matches your situation — the maths changes per option. Choices: Type I — very fair, always burns, Type II — fair, burns easily, Type III — medium, sometimes burns, Type IV — olive, rarely burns, Type V — brown, very rarely burns, Type VI — dark brown/black, almost never burns.
- UV index
- The uv index used in the calculation. Starts at 8 so you have a working example on load. Accepted range: 1–14.
- Sunscreen SPF (0 = none)
- The sunscreen spf (0 = none) used in the calculation. Starts at 30 so you have a working example on load. Accepted range: 0–100.
- Surface reflection
- Pick the option that matches your situation — the maths changes per option. Choices: Normal ground / grass, Water (boosts UV ~30%), Sand (boosts UV ~50%), Fresh snow (boosts UV ~85%).
Results
- Est. burn time with sunscreen
- Returned as a length of time and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Burn time with no protection
- Returned as a length of time. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Suggested reapplication interval
- Returned as a length of time. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Why does the calculator assume sunscreen performs below its labeled SPF?
SPF testing uses 2 mg/cm² of product, roughly a shot-glass amount for a full body, applied uniformly. Real-world users typically apply 20-50% of that amount, which cuts effective protection well below the labeled number — this tool applies a rough real-world discount instead of the lab-rated multiplier.
How much does UV index change throughout the day?
UV index peaks within about 2 hours of solar noon and can be 3-4 times higher at midday than in early morning or late afternoon at the same air temperature, so the same activity at 10am vs. 1pm can have very different burn risk even on the same day.
Why do reflective surfaces matter so much?
Fresh snow reflects up to 80-90% of UV back onto skin (which is why snow blindness and skiers' sunburns happen even in cold weather), water reflects roughly 10-100% depending on angle, and dry sand reflects around 15-25%, all adding to direct overhead exposure.
Does cloud cover eliminate UV risk?
No — up to 80% of UV radiation passes through light cloud cover, and some cloud conditions can even intensify UV briefly through scattering effects. Overcast days still require sun protection at moderate-to-high UV index readings.
Accuracy and limitations
- Population formulas describe averages; individual bodies vary, especially at the extremes of height, age, or muscle mass.
- Results do not account for medical conditions, medication, or pregnancy.
- Use this as a directional reference, not a diagnosis — talk to a clinician before acting on it.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). UV Exposure Burn Time Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/uv-exposure-burn-time
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/uv-exposure-burn-time" target="_blank" rel="noopener">UV Exposure Burn Time Calculator — RevenueLab</a> (2026).</p>
Source: [UV Exposure Burn Time Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/uv-exposure-burn-time) (2026).