Sunburn is caused by ultraviolet light, and the dose that just turns your skin pink, the minimal erythemal dose (MED), depends on your skin type. Very fair type I skin may burn after about 200 J/m² of erythemally weighted UV; darker type V–VI skin tolerates three to five times more. The UV index measures how quickly that dose arrives: each UV index point equals 25 mW/m², so at UV index 8 fair skin burns in well under 20 minutes. Altitude adds about 10% per 1,000 m, and snow can reflect up to 80% of the UV back at you.
Sunscreen stretches the time by its sun protection factor, but only when you apply the tested amount of 2 mg/cm², about a shot glass for an adult’s body. Most people apply half of that, which gives roughly the square root of the label SPF. Sweat, swimming and towelling remove it, so reapply at least every two hours whatever the calculation says.
The first mode answers “how long?” and “which SPF?”. The second mode, kept from our earlier version of this page, works out how much sunscreen you need for a holiday from your body size, the skin you expose and the days in the sun.
How to use the Sunscreen and Sun Exposure Calculator
- What do you want to know?
- Sunlight intensity
- UV index of your location
- Altitude
- Surroundings
- Skin phototype (Fitzpatrick)
- SPF of your sunscreen (1 = none)
- How much you apply
- Planned time in the sun
- Height
- Weight
- Skin exposed
- Hours in the sun per day
- Days
- Bottle size
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose the sunlight (or enter the UV index), altitude, surroundings and skin type. The burn time is the MED divided by the UV dose rate; it is multiplied by the effective SPF. For a planned time outdoors, the needed SPF is (planned time ÷ burn time)^(1/k), rounded up to a standard label.
Worked example
Medium (type III, MED ≈ 350 J/m²) skin at UV index 7 at sea level receives 7 × 0.025 = 0.175 W/m², so it burns after 350 ÷ 0.175 = 2,000 s ≈ 33 minutes. SPF 30 applied properly stretches that to about 16.7 hours in theory, but applied at half the dose (effective SPF ≈ √30 ≈ 5.5) it is about 3 hours. For a 4-hour beach afternoon at that UV index you would need SPF ≥ 7.2 at the full dose (SPF 15 or more) or about SPF 52 at the half dose, which is why applying enough matters more than the number on the bottle.
Burn time without sunscreen (minutes)
| UV index | Type I | Type II | Type III | Type IV |
|---|---|---|---|---|
| 3 | 44 | 56 | 78 | 100 |
| 6 | 22 | 28 | 39 | 50 |
| 8 | 17 | 21 | 29 | 38 |
| 11 | 12 | 15 | 21 | 27 |
Inputs explained
| Field | What to enter |
|---|---|
| What do you want to know? | Choose one: How long can I stay in the sun? Which SPF do I need?, How much sunscreen should I pack? |
| Sunlight intensity | Choose one: Low (winter or very cloudy, UV 2), Moderate (partly cloudy, UV 4), High (sunny spring or autumn, UV 6), Very high (sunny summer, UV 9) … |
| UV index of your location | A number |
| Altitude | Choose one: Up to 1,000 m (3,300 ft), 1,000–2,000 m, 2,000–3,000 m, 3,000–4,000 m … |
| Surroundings | Choose one: Grass, soil or city, Water (lake, pool), Sand or beach, Sea with surf and foam … |
| Skin phototype (Fitzpatrick) | Choose one: Type I (very fair, always burns, never tans), Type II (fair, burns easily, tans minimally), Type III (medium, sometimes burns, tans gradually), Type IV (olive or light brown, rarely burns) … |
| SPF of your sunscreen (1 = none) | A number |
| How much you apply | Choose one: The full test dose (a shot glass for the body, 2 mg/cm²), About half of that, as most people do |
| Planned time in the sun | Units: h, min, s |
| Height | Units: cm, in, ft |
| Weight | Units: kg, lb, g |
| Skin exposed | Choose one: Swimwear (≈ 85%), Shorts and T-shirt (≈ 50%), Face, neck, arms (≈ 20%), Face only (≈ 5%) |
| Hours in the sun per day | A number |
| Days | A number |
| Bottle size | Choose one: 50 mL / 1.7 fl oz, 100 mL / 3.4 fl oz, 150 mL / 5 fl oz, 200 mL / 6.8 fl oz … |
What each result means
- Time to sunburn without protection = [Skin phototype (Fitzpatrick)]÷([UV index used]×0.025)
- Protected time with your sunscreen = [Time to sunburn without protection]×[SPF of your sunscreen (1 = none)]^[How much you apply]
- Effective SPF as applied = [SPF of your sunscreen (1 = none)]^[How much you apply]
- SPF needed for the planned time = max(1, ([Planned time in the sun]÷[Time to sunburn without protection])^(1÷[How much you apply]))
- Reapplications during the planned time (every 2 h) = max(0, ceil([Planned time in the sun]÷7200 – 0.000001) – 1)
- Body surface area = sqrt([Height]×100×[Weight]÷3600)
- One application = [Body surface area]×10000×[Skin exposed]×2÷1000÷1000×1e-3
- Applications per day = max(ceil([Hours in the sun per day]÷2 – 0.000001), 1)
- Sunscreen for the trip = [One application]×[Applications per day]×[Days]
- Bottles to pack = ceil([Sunscreen for the trip]÷[Bottle size] – 0.000001)
Formula
Burn time t₀ = MED ÷ (UVI × 0.025 W/m² × altitude factor × reflection factor) · Protected time = t₀ × SPF^k (k = 1 at 2 mg/cm², ≈ 0.5 at half the dose) · One application = 2 mg/cm² × exposed skin area · Body surface (Mosteller) = √(cm × kg ÷ 3,600)
Worked example
Inputs: What do you want to know?: How long can I stay in the sun? Which SPF do I need?; Sunlight intensity: High – sunny spring or autumn (UV 6); Altitude: Up to 1,000 m (3,300 ft); Surroundings: Grass, soil or city; Skin phototype (Fitzpatrick): Type III – medium, sometimes burns, tans gradually; SPF of your sunscreen (1 = none): 30; How much you apply: The full test dose – a shot glass for the body (2 mg/cm²); Planned time in the sun: 4 h.
Results: UV index used = 6; Time to sunburn without protection = 39 min; Protected time with your sunscreen = 1,167 min; Effective SPF as applied = 30; SPF needed for the planned time = 6.2; Recommendation = SPF 15 or higher; Reapplications during the planned time (every 2 h) = 1.
Key terms
- Minimal erythemal dose (MED)
- The UV dose that just causes visible reddening of the skin.
- UV index
- International scale of sunburn-producing UV radiation; 1 unit = 25 mW/m².
- SPF
- Sun protection factor: how many times longer protected skin takes to burn at 2 mg/cm².
- Fitzpatrick skin type
- A I–VI classification of how skin reacts to sun exposure.
Frequently asked questions
How long can I stay in the sun without burning?
At UV index 6, medium (type III) skin burns after about 39 minutes, fair type II skin after about 28 minutes and very fair type I skin after about 22 minutes.
Does SPF 30 mean I can stay out 30 times longer?
Only in theory and only with 2 mg/cm². Applied at half that thickness it protects roughly like SPF 5–6, and sweat and water wear it off, so reapply every two hours.
Which SPF should I use?
Dermatologists recommend at least SPF 30 for outdoor days; use SPF 50 for fair skin, high UV, altitude, snow or long exposure.
How much sunscreen should I use?
About 30 mL (a shot glass) for an adult's whole body, or half a teaspoon for the face and neck.
Why do I burn faster on snow or at altitude?
Fresh snow reflects up to 80% of UV, and UV rises about 10% for every 1,000 m of altitude.
What is the UV index?
A measure of sunburn-causing UV: 1 unit = 25 mW/m². 3–5 is moderate, 6–7 high, 8–10 very high and 11+ extreme.
Do people with dark skin need sunscreen?
They burn much later, but UV still damages skin and eyes. Use sunscreen for long exposures and high UV.