Sun Safety Calculator
Calculate how long you can stay in the sun before burning.
Enter UV index, skin type, and SPF to get your estimated safe sun exposure time in minutes.
How long you can stay in the sun without burning depends on two things: how intense the UV radiation is (UV index) and how sensitive your skin is (Fitzpatrick skin type).
The UV Index is a dimensionless number, usually 0-11+, developed by the World Health Organization. UV 3-5 is moderate; 6-7 is high; 8-10 is very high; 11+ is extreme. In summer at midday in Australia or the American Southwest, UV 12-14 is not unusual.
How the burn time is actually derived
Unprotected burn time comes from the Minimal Erythemal Dose (MED), the amount of ultraviolet energy that just visibly reddens skin. Each Fitzpatrick type has a different MED, measured in joules per square meter.
The UV Index is defined as erythemally weighted irradiance in watts per square meter, multiplied by 40. So UV Index 1 means 0.025 W/m². Time to reach your MED is then MED divided by that irradiance, and converting seconds to minutes gives a formula clean enough to check by hand:
Minutes to burn = MED ÷ (UV Index × 1.5)
| Fitzpatrick type | Description | MED (J/m²) | Burn time at UV 6 |
|---|---|---|---|
| I | Always burns, never tans. Very fair, often red hair and freckles. | 200 | 22 min |
| II | Burns easily, tans minimally. Fair skin, light eyes. | 250 | 28 min |
| III | Burns moderately, tans uniformly. Medium skin. | 350 | 39 min |
| IV | Burns minimally, tans easily. Olive skin. | 450 | 50 min |
| V | Rarely burns, tans darkly. Brown skin. | 600 | 67 min |
| VI | Rarely burns, deeply pigmented. Dark brown to black skin. | 1000 | 111 min |
Burn time is inversely proportional to the UV Index, so the same Type II skin that has 28 minutes at UV 6 has 17 at UV 10 and 84 at UV 2.
A note on Type VI. “Never burns” is the traditional wording and it is wrong. Deeply pigmented skin has a much higher MED, not an infinite one, and sunburn in Type V and VI skin is well documented, particularly at altitude or on water. The higher number here is a longer runway, not immunity.
SPF, and why the label number is not what you get
SPF multiplies your unprotected time in theory. SPF 30 should mean thirty times as long.
In practice almost nobody applies the 2 mg/cm² used in SPF testing. Typical real-world application is a quarter to a half of that, and protection falls faster than the thickness does. Studies consistently find real-world protection running at 20 to 50 percent of the labeled figure. This calculator shows both the label figure and a realistic estimate at 40 percent, because the gap between them is the whole practical point.
Two consequences worth internalising. A well-applied SPF 30 beats a thinly applied SPF 100. And reapplication every two hours matters more than the number on the bottle, because sunscreen comes off on towels, clothes, and sweat regardless of what it was rated for.
This is an estimate. Actual burn times vary with altitude, reflective surfaces such as snow, sand and water, cloud cover, and medication that increases photosensitivity. Consult a dermatologist for personal skin health guidance.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.