About the Sunburn Time Calculator
This sunburn calculator estimates how long unprotected skin can stay in the sun before it receives one minimal erythemal dose (MED) — the amount of UV that causes just-visible reddening about a day later. It combines the UV Index from your local forecast with a typical MED for your Fitzpatrick skin type, then adjusts for ground reflection (snow, sand, water) and altitude.
It also shows how much longer sunscreen might protect you. Labels state SPF measured with a thick 2 mg/cm² layer, but most people apply a half to a quarter of that, and research shows protection then falls roughly to the square root or fourth root of the label SPF. Choosing your realistic application amount gives a more honest estimate than simply multiplying by the SPF.
Treat the result as a rough upper limit, not a safe allowance. Individual sensitivity varies two- to three-fold within each skin type, the UV Index changes through the day, and skin damage begins well before redness appears. Use the number to plan shade breaks and reapplication — sunscreen should be reapplied at least every two hours and after swimming or sweating.
With the default inputs, the time to sunburn without sunscreen is 24 min. Change any value above to recalculate instantly.
How to use the sunburn time calculator
- 1Look up today’s peak UV Index for your location and enter it.
- 2Choose the Fitzpatrick skin type that best matches how you burn and tan.
- 3Enter your sunscreen SPF (0 for none) and how thickly you really apply it.
- 4Pick the ground surface and enter your altitude if you are in the mountains.
- 5Read the minutes to sunburn and plan shade and reapplication well before that point.
Formula and method
One UV Index unit equals 25 mW/m² of erythemally weighted UV (the WHO definition uses a factor of 40 m²/W). Dividing your skin type’s minimal erythemal dose (in J/m²) by that irradiance gives the seconds until one MED. Typical MEDs used here are 200, 250, 300, 450, 600 and 1,000 J/m² for skin types I to VI.
The forecast UV Index is increased for ground reflection (WHO: fresh snow up to 80%, sea foam about 25%, dry sand about 15%) and by 10% per 1,000 m of altitude. Sunscreen time is the bare-skin time multiplied by an effective SPF. Faurschou and Wulf (2007) found that applying 1 mg/cm² gives roughly the square root of the label SPF and 0.5 mg/cm² roughly the fourth root, which is modelled as SPF raised to the power of (amount ÷ 2).
- MED
- Minimal erythemal dose for the skin type (J/m²)
- UVI
- Forecast UV Index
- UVIeff
- UV Index adjusted for reflection and altitude
- SPFeff
- Effective SPF for the amount applied (mg/cm²)
Worked examples
Fair skin (type II) at UV 7 with SPF 30, moderate application
Type II skin has a typical MED of 250 J/m². UV Index 7 is 7 × 0.025 = 0.175 W/m², so 250 ÷ 0.175 = 1,429 seconds, about 24 minutes. At 1 mg/cm² SPF 30 acts like √30 ≈ 5.5, stretching the time to about 130 minutes, far short of the 714 minutes the label implies.
Very fair skin skiing on snow at 2,500 m
Fresh snow adds up to 80% and 2,500 m adds 25%, so UV 6 becomes 6 × 1.8 × 1.25 = 13.5. Type I skin (MED 200 J/m²) receives one MED in 200 ÷ (13.5 × 0.025) = 593 seconds — under 10 minutes without protection.
Olive skin (type IV) at the beach, UV 9, SPF 50 applied thinly
Dry sand reflects about 15%, so UV 9 becomes 10.35. Type IV skin (MED 450 J/m²) burns in about 29 minutes unprotected. A thin 0.5 mg/cm² layer of SPF 50 behaves like the fourth root, about SPF 2.7, giving only around 77 minutes.
Frequently asked questions
How long does it take to get sunburned?+
It depends mainly on the UV Index and your skin type. At UV 8, very fair skin can burn in about 15–20 minutes, while darker skin may take an hour or more. Snow, water and altitude shorten the time considerably.
Does SPF 30 let me stay out 30 times longer?+
Only in the lab, where sunscreen is applied at 2 mg/cm². Most people apply a quarter to a half of that, which cuts protection to roughly the fourth or square root of the label value, so SPF 30 often acts like SPF 2–6. Applying generously and reapplying matters more than a higher SPF.
What UV Index requires sun protection?+
WHO and national agencies recommend sun protection whenever the UV Index is 3 or higher: seek shade around midday, wear clothing, a broad-brimmed hat and sunglasses, and use broad-spectrum sunscreen on exposed skin.
Can you burn on a cloudy day?+
Yes. Thin or broken cloud lets most UV through, and scattered light from bright clouds can even raise UV levels briefly. Always use the forecast UV Index rather than how warm or bright it feels.
What are the Fitzpatrick skin types?+
They classify skin by how it reacts to the sun: type I always burns and never tans, type II usually burns, type III sometimes burns and tans gradually, type IV rarely burns, and types V and VI are naturally brown or black skin that very rarely burns.
Is it safe to stay out until just before the calculated time?+
No. The estimate marks the point of visible reddening, but DNA damage and skin ageing start with smaller doses, and personal sensitivity can differ greatly from the typical value. Aim to stay well below the calculated time.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.