What a sunburn time actually measures
A sunburn is a dose response, not a clock response. Your skin reddens when it has absorbed a certain quantity of ultraviolet energy, and the time it takes to get there is simply that quantity divided by the rate the sun is delivering it. The quantity is the minimal erythema dose, or MED: the smallest ultraviolet dose that produces a just-perceptible reddening 16 to 24 hours later. The rate is what the UV index reports.
The UV index is not an arbitrary scale. One index unit is defined as 25 milliwatts per square metre of ultraviolet irradiance after it has been weighted by the CIE erythema reference action spectrum, standardised as ISO 17166. That weighting is what makes the index meaningful: raw ultraviolet power would be dominated by UVA, while the biological damage that produces erythema is driven overwhelmingly by the much weaker UVB band. Weight the spectrum by how effectively each wavelength reddens skin, integrate, divide by 0.025 W/m², and you have the index number on your weather app.
So a UV index of 8 means 0.2 W/m² of erythemally effective ultraviolet is landing on a horizontal surface. Skin type II, with a MED near 250 J/m², absorbs that in 1,250 seconds — a little under 21 minutes. Everything else on this page is a modification of that one division.
Doses are also quoted in standard erythema doses (SED), where 1 SED is exactly 100 J/m². The SED is skin-independent, which makes it the right unit for comparing a day at the beach with a day in the office; the MED is skin-dependent, which makes it the right unit for asking whether you will burn.
The formula, term by term
Start with irradiance. Multiply the UV index by 0.025 to get watts per square metre. Multiply by seconds to get joules per square metre. Set that equal to the MED and solve for time, then divide by 60 for minutes.
MED by skin type is the term with the widest published range, because it is measured on volunteers and individuals vary. This calculator uses mid-range values from the commonly reported Fitzpatrick ranges: 200 J/m² for type I, 250 for II, 350 for III, 450 for IV, 600 for V and 1,000 for VI. Treat them as the centre of a band, not a personal constant. Recent tanning raises your own MED; medication that causes photosensitivity lowers it sharply, sometimes by more than a factor of two.
Reflection adds to the downward beam. The WHO UV index guide reports typical reflectances of about 3% for grass, 10% for water, 15% for dry sand and up to 80% for fresh snow. The multipliers in the surface selector are built from those figures and are deliberately conservative for snow, because reflected UV arrives at angles that overhead shade does not block.
Shade multiplies in the other direction. A beach umbrella removes the direct beam but not the diffuse sky, which on a clear summer day carries roughly half the erythemal ultraviolet at ground level. That is why the exposure-share input defaults to 100% and why dropping it to 50% for an umbrella is a defensible estimate rather than a generous one.
SPF is a laboratory ratio. ISO 24444 measures it at 2 mg of product per square centimetre of skin, which for an adult body is about 36 g — roughly a shot glass, per application. Applying half that does not give you half the protection: it gives you far less. Faurschou and Wulf measured the relationship in vivo and found protection rises roughly as SPF raised to the power d/2, where d is the applied thickness in mg/cm². At 1 mg/cm² that makes an SPF 30 behave like SPF 5.5. The proportional option in the selector reproduces instead the familiar rule of thumb that typical application delivers about a third of the labelled number, and is the gentler of the two estimates.
Worked example: UV index 8, skin type II, SPF 30 applied thinly
You are on grass at midday with a forecast UV index of 8, no shade, skin type II, and you put on SPF 30 at about 1 mg/cm² — half the test thickness. You plan to be out for two hours.
- Irradiance. 8 × 0.025 = 0.2 W/m² erythemally weighted.
- Unprotected burn time. MED 250 J/m² ÷ 0.2 W/m² = 1,250 s = 20.83 minutes.
- Effective SPF. 30 raised to the power 1 ÷ 2 = √30 = 5.477, not 30.
- Protected burn time. 20.83 × 5.477 = 114.1 minutes.
- Dose collected. 0.2 W/m² × 7,200 s = 1,440 J/m² = 14.4 SED.
- Share of threshold used. Your protected budget is 250 × 5.477 = 1,369 J/m², so 1,440 ÷ 1,369 = 105%.
That last line is the point of the exercise. Two hours in UV index 8 under a thinly applied SPF 30 puts you just past your erythema threshold — a light burn, not a severe one, but a burn. Applying the same product at the full 2 mg/cm² would raise the budget to 250 × 30 = 7,500 J/m² and drop the same session to 19% of threshold. The variable that moved the answer by a factor of five was thickness, not SPF number.
How to read the result
Read the share of threshold used first and the burn time second. The burn time answers a hypothetical — how long could you stay — while the share answers the question you actually have, which is whether the outing you have planned is within budget.
Anything at or above 100% means visible erythema is expected. Between about 60% and 100% you are in the range where the skin has absorbed a real dose without necessarily showing it, and where a second session later the same day tips you over, because erythemal dose accumulates across the day rather than resetting. Below 60% you have headroom for the forecast to be wrong.
The WHO exposure categories give a useful sanity check on the input rather than the output: index 0–2 is low, 3–5 moderate, 6–7 high, 8–10 very high and 11 or above extreme. Protection is advised from category 3 upward. Note that the index is a clear-sky midday figure in most forecasts, so a morning or late-afternoon reading is lower — roughly half the daily total arrives in the four hours around solar noon.
The reapplication figure is a separate constraint and does not scale with SPF. US labelling permits only 40- and 80-minute water-resistance claims, verified by immersion testing, and general guidance is to reapply at least every two hours regardless. Whenever the calculated burn window exceeds the reapplication interval, the reapplication interval is what governs your afternoon.
Minutes to a sunburn by UV index and skin type
| UV index | Type I 200 J/m² | Type II 250 | Type III 350 | Type IV 450 | Type V 600 | Type VI 1000 |
|---|---|---|---|---|---|---|
| 3 (moderate) | 44 | 56 | 78 | 100 | 133 | 222 |
| 5 (moderate) | 27 | 33 | 47 | 60 | 80 | 133 |
| 8 (very high) | 17 | 21 | 29 | 38 | 50 | 83 |
| 11 (extreme) | 12 | 15 | 21 | 27 | 36 | 61 |
Multiply any cell by the effective SPF to get the protected time. Fresh snow can push the effective index well above the forecast figure, shortening every cell.
What this calculator does not account for
- Cumulative dose across days. Erythema resolves; DNA damage and photoageing accumulate. A week of sessions each at 80% of threshold produces no burn and a substantial total dose.
- Altitude and latitude. Erythemal ultraviolet rises roughly 10% per 1,000 m of elevation. If your forecast index is for a valley town and you are on a ridge, the index at your skin is higher than the number you entered.
- Photosensitising drugs and conditions. Tetracyclines, some diuretics, retinoids, St John's wort and several others can cut the effective MED dramatically. If you are taking anything photosensitising, this calculator overstates your burn time.
- Body site. The MED figures come from back or buttock testing. Facial skin, and skin that has been under clothing all winter, reddens sooner.
- Sunscreen that is no longer there. The effective SPF assumes an intact film. Towelling, swimming past the water-resistance rating, sand abrasion and heavy sweating remove it, and nothing in the arithmetic knows that.
- UVA and long-term risk. SPF measures erythema, which is largely a UVB effect. Broad-spectrum labelling exists because a high SPF alone says little about UVA protection.
Where this sits among other sun-safety guidance
The shadow rule — if your shadow is shorter than you are, the sun is strong — is a solar-elevation proxy for the same physics and is a good field check when you have no forecast. It fails on snow, where reflection matters more than elevation, and near water in late afternoon.
Personal UV dosimeters and phone-based UV meters report the same erythemally weighted quantity this page uses, in SED, and can be compared directly against the dose output above. Vitamin D guidance is expressed in the same currency too: the exposures usually discussed for synthesis are small fractions of an MED on limited skin area, well below the numbers that matter for burning.
If you are planning a whole day outdoors, the other variable worth pricing is heat rather than light. For a road trip, work out the driving hours first with the road trip drive time calculator, then decide which of those hours put you outside at solar noon. Gardeners sizing a bed with the square foot garden plant spacing calculator and homeowners running a heated pool costed with the pool heating cost calculator are both making decisions about time spent under exactly the irradiance modelled here. And if the day involves tyres on hot asphalt, the tire pressure temperature change calculator covers the other thing the sun is doing to your afternoon.
This is a planning estimate, not a medical instrument
Minimal erythema dose varies between individuals of the same Fitzpatrick type by a factor of two or more, and varies within one individual across the year. Use the output to decide when to seek shade, not to justify staying out to the last modelled minute. Anyone with a history of skin cancer, a photosensitising condition, or a photosensitising medication should follow clinical advice rather than a dose model.