Sunbathing Calculator (time to sunburn from UV index and skin type)

Understanding the sunbathing calculator — concept and principle

This calculator estimates how soon unprotected skin may start to burn at a given UV index, depending on skin type, and how much sunburn-weighted UV you would receive in your planned time. It is meant to help plan shade and protection, not to find a "safe" tanning time.

UV index → dose per hour: with the default inputs the result is 29 min (30 min planned = 105%).

How it is calculated

Erythemal irradiance (W/m²) = UV index × 0.025

Dose per hour (SED) = UV index × 0.025 × 3,600 ÷ 100

Time to possible sunburn (s) = MED (SED) × 100 ÷ (UV index × 0.025)

1 SED (standard erythema dose) = 100 J/m².
Minimal erythema dose (MED): about 1–3 SED for skin types I–II, 3–7 for III–IV, 7 to more than 12 for V–VI. The low and high ends give the range.

The principle

The UV index is a scaled measure of sunburn-weighted UV: each index point equals 0.025 W/m². Multiplying by time gives the dose your skin receives. Skin reddens once the dose reaches its minimal erythema dose, which is higher for darker skin with more melanin. Dividing the burn dose by the dose rate gives the time to reach it.

Worked example

Default inputs: UV index 7, skin types III–IV, 30 minutes planned.

Dose rate = 7 × 0.025 = 0.175 W/m² → 6.3 SED per hour.
Time to 3 SED = 3 × 100 ÷ 0.175 = 1,714 s = 29 min; to 7 SED = 4,000 s = 1 h 07 min.
Dose in 30 min = 6.3 × 0.5 = 3.15 → 3.2 SED.
Planned dose ÷ lowest burn dose = 3.15 ÷ 3 = 105%; UV category High (6–7).

How to read the result

Where does your result fall?

Plan with the first, shorter time: fair skin within the range can burn from the low end. A planned dose above 100% of the lowest burn dose means sunburn is possible. There is no safe tan; any tan is a sign of UV damage. Health agencies advise shade, clothing, a hat, sunglasses and broad-spectrum sunscreen whenever the UV index is 3 or more.

Common mistakes

Using sunscreen to stay longer in the sun.
Ignoring reflection from sand, water and snow, which adds UV.
Assuming clouds block most UV; many let much of it through.

Keep in mind: This is an estimate for planning protection, not a safe exposure time. Children, people taking photosensitising medicines and those with a history of skin cancer need extra care.

Frequently asked questions

What does UV index 7 mean?

It is in the "high" category; unprotected fair skin can burn in under half an hour.

Does a tan protect against sunburn?

Only slightly, roughly like a low SPF, and the tan itself reflects skin damage.

When is UV strongest?

Around midday, especially in summer and at high altitude.

Sources: World Health Organization, Global Solar UV Index: A Practical Guide, 2002 · CIE, Standard erythema dose: a review, CIE 125-1997

Checked 9 Nov 2026. Lucas RM, Neale RE, Madronich S, McKenzie RL, Photochem Photobiol Sci 2018 (UVI ≡ 40 × erythemal irradiance in W/m²; 1 SED = 100 J/m²; MED of skin type II about 2–3 SED); indicative MEDs by phototype from Royal Society of Chemistry book, Table 1.1 “A classification of skin phototypes based on susceptibility to sunburn…”; UV index categories from the US EPA “UV Index Scale”.

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