Sunburn Calculator

On a clear July day, bare skin starts to redden in about 20 minutes

Put this calculator in New York on a clear day, heading out at noon with bare skin, and skin type III (burns moderately, tans gradually) reaches the reddening threshold 20 minutes later (12:20 PM) on July 15, 45 minutes later on October 15 and 113 minutes later on January 15. Stay out from noon to 4 PM on that July day with SPF 50 applied at the full test amount and reapplied every 2 hours, and you take in only 22% of the threshold by the time you come in. Put on the same sunscreen at half the amount and you start to redden at 2:14 PM. Apply the full amount once and never reapply, and you start to redden at 2:18 PM.

When skin reddens depends on how strong the sun is that day, your skin type, how much sunscreen you put on and when you reapply it, and each of these can change the answer several times over. Enter a city, a date and your times outside, and the calculator adds up the UV your skin takes in minute by minute from that day's hourly UV index. It shows bare skin and your sunscreen plan side by side.

Why work out the time to sunburn before you go out

A sunburn does not show while you are getting it. Patient.info, a reference for doctors, notes that redness usually appears 2 to 6 hours after exposure and peaks at 12 to 24 hours. By the time skin starts to sting, stepping into the shade is already too late, so it helps to decide before you leave how long you will stay out and when you will reapply.

• Beach, lake or pool days: swimming washes sunscreen off, and sand and sea foam bounce UV back at you. Enter your reapplication schedule and the ground around you and you can see how long the plan holds up.
• Hiking and skiing: UV rises with elevation, and snow reflects most of it. Winter is when people least expect to burn.
• Ballgames, outdoor events and job sites: when you will be out for hours, how much sunscreen you put on and how often you reapply change the result more than anything else.
• Kids: the World Health Organization (WHO) says frequent UV exposure and sunburn in childhood and adolescence are an important risk factor for skin cancer, especially the potentially lethal malignant melanoma. For a child with fair skin, try type I or II.

The time this calculator gives you is not a time that is safe to stay out until. WHO warns that people read burn times as a safe level of unprotected sun, and says that sending that message is wrong because cumulative UV exposure plays a major role in skin cancer. Use the result as a cue to get into the shade or reapply before that time, nothing more.

How to use the calculator

1. When and where: pick a city, a date, and the times you head out and come in. Now moves the start to the current time in that city and keeps the same length.
2. Skin type: choose the type that matches how your skin usually reacts to strong sun. Each button also shows how long bare skin of that type lasts at the peak UV of your outing.
3. Sunscreen: choose the SPF, the amount you apply and how often you reapply. Each amount button shows the SPF you actually get from it. Set SPF to None to draw the bare-skin row only.
4. Sky and surroundings: leave the UV box blank to use the clear-sky estimate. On a cloudy day, enter today's max UV index from your weather app. Choose the ground around you for sand, sea or snow, and enter elevation in feet for mountains and ski areas.
5. Reading the timeline: the curve on top is the UV index by time of day, and the two rows below are the dose received on bare skin and with your sunscreen plan. Darker color means closer to the threshold, and hatching marks the part past it. Tap the chart or use the arrow keys to see the UV index and dose at any time.

How it is calculated

The calculator adds up the UV your skin receives each minute and finds the first minute at which the total reaches the reddening threshold for your skin type, the minimal erythema dose (MED).

• UV index and dose: a UV index of 1 equals 0.025 W/m² of erythema-weighted UV (WHO), so one minute (60 seconds) delivers 1.5 J/m².
• Reddening threshold (MED): the calculator uses 200, 250, 300, 450, 600 and 1,000 J/m² for skin types I to VI, from Table 3 of a 2024 paper in Scientific Reports.
• Clear-sky UV index: the sun's height comes from the NOAA solar position equations, and the UV index from Madronich's 2007 formula, 12.5 × cos(solar zenith angle)^2.42. Ozone is set at 300 DU and there are no clouds. If you enter today's peak from a weather app, the whole day's curve is scaled so that its highest point matches your number.
• Amount of sunscreen: SPF is measured with 2 mg per cm² of skin. Faurschou and Wulf (2007) report that applying 1 mg/cm² makes the SPF fall as its square root and 0.5 mg/cm² as its fourth root. The calculator uses that relationship for the actual SPF and rounds it to one decimal. SPF 70 and 100 are counted at face value.
• Reapplying: reapplying every 2 hours (or every 40 or 80 minutes when swimming) is assumed to keep the protection going. With Once only, the calculator counts your skin as bare from 2 hours after you apply, assuming sweat, water and rubbing have worn the sunscreen off. In real life it fades gradually, but no one knows when or how much, so the assumption is cautious.
• The verdict: the dose is converted to a percentage of the threshold and rounded down, and the first minute at 100% or more is the time skin starts to redden. A screen that says 99% on coming in therefore never contradicts a verdict of no reddening.

Minutes of bare-skin exposure until the reddening threshold is reached (the first full minute that ends at 100% of the threshold) if the UV index stayed constant. The skin type descriptions follow the Fitzpatrick scale. Within one skin type, people differ in how much UV it takes to redden them, so treat the table as a guide.

Examples by season and sunscreen plan

This table shows when bare skin of type III starts to redden if you head out at noon on the 15th of each month in New York on a clear day. The same noon hour gives a spread of about nine times between June and December.

Conditions: New York, clear sky, skin type III, heading out at 12:00 PM with bare skin, 2026 dates in local time. Clouds, haze and storms often make the real UV index lower than this clear-sky estimate, so enter the day's peak from your weather app for a closer figure. May is nearly as strong as July.

The next table keeps New York on July 15 and puts a type III person outside from noon to 4 PM, four hours, to show how much the sunscreen plan changes the result.

The amount you apply and whether you reapply matter more than the number on the bottle. SPF 15 applied in full and reapplied on schedule leaves you with less dose than SPF 50 applied at half the amount.

How much sunscreen, and how often to reapply

The SPF on a label is measured at 2 mg of sunscreen per cm² of skin (US FDA, 21 CFR 201.327). The European Commission's Recommendation 2006/647/EC puts that amount at 6 teaspoons of lotion, about 36 grams, for the body of an average adult. Cancer Council Australia advises about 35 mL, around 7 teaspoons, for full-body coverage. The Japanese Ministry of the Environment suggests a pearl-sized dab of cream, or a one-yen-coin-sized pool of liquid, for the face, and then the same amount again on top.

Most people put on much less. Petersen and Wulf (2014) found real-life application between 0.39 and 1.0 mg/cm², and Faurschou and Wulf (2007) say sunbathers use about a quarter of the test amount, 0.5 mg/cm². The European Commission also warns that if the quantity applied is reduced by half, protection may fall by as much as two-thirds.

Actual SPF = labeled SPF^{(amount ÷ 2)}, from the exponential relationship in Faurschou and Wulf (2007). That study used an SPF 4 product, so the size of the drop at high SPF can differ from study to study, and the calculator uses the cautious end. The European Commission notes that sun protection factors above 50 do not substantially increase the protection, and the highest label it recommends is 50+.

• When to apply: the FDA label text says to apply liberally 15 minutes before sun exposure, and Cancer Council Australia says 20 minutes before you go outside. The calculator assumes sunscreen is already on when you step out.
• How often to reapply: the FDA label says to reapply at least every 2 hours, and Cancer Council Australia adds that you should reapply immediately after swimming, sweating or towel-drying.
• Water resistance: the FDA allows the labels Water Resistant (40 minutes) and Water Resistant (80 minutes). If you swim, plan to reapply on that schedule.
• A second coat early on: Petersen and Wulf (2014) advise applying before sun exposure and reapplying once within an hour, which makes up for a thin first layer.

Beach, snow and mountains: how the dose changes

The same UV index delivers more to your skin when the ground reflects UV back up or when you are high above sea level. The calculator adds the top end of each reflectivity WHO gives, so it errs on the side of more UV.

Conditions: clear sky, skin type III, bare skin, 2026 dates. Miami times use a 12:00 PM to 3:00 PM outing and Denver times a 10:00 AM to 3:00 PM outing. The clear-sky model does not know about elevation, so for Denver the 5,280 ft is added on top of the estimate for the city, and the first number in each Denver row is the same outing without any addition. In January in Denver the peak UV is only about 2, yet fresh snow still moves the time to reddening from 115 to 71 minutes.

• Clouds: WHO says up to 80% of solar UV can penetrate light cloud cover. Staying out for long periods bare on an overcast day can still burn you, so use your weather app's peak value.
• Shade: WHO calls shade one of the best defences against the sun's radiation. UV bounced from sand or water still reaches you in the shade, so the calculator does not subtract it.
• In the water: WHO says water offers only minimal protection, and reflections from water can enhance your exposure.
• Behind glass: UVB does not penetrate glass (NIH Office of Dietary Supplements). If you are sitting indoors by a window or in a car, this calculator is not for that situation.

UV index levels and time to burn

The WHO scale groups the UV index into five levels. The table pairs each level's lower boundary with the minutes this calculator gives for bare skin at that UV index, constant over the whole time.

The minutes use the lower boundary of each level, so the true time within a level is longer for the milder part of it. WHO recommends protection above a UV index of 3 and reinforcing the message at 8 and above, and the table is not a safe-time chart.

If you are already burned

The NHS advises getting out of the sun as soon as possible, cooling the skin with a cool shower, bath or damp towel, applying aftersun cream or spray or an unperfumed moisturiser, drinking plenty of water, taking painkillers such as paracetamol or ibuprofen, and covering the burned skin from direct sunlight until it has fully healed. Sunburn usually gets better within 7 days. See a doctor if the skin is blistered or swollen, if your temperature is very high or you feel hot, cold or shivery, if you feel very tired, dizzy or sick, or if a baby or young child has sunburn.

Common mistakes

• "SPF 50 means 50 times longer": that holds only if you applied the 2 mg/cm² test amount and none of it wore off. At half the amount the actual SPF is about 7, and if you never reapply your skin is close to bare after about 2 hours. Multiplying bare-skin time by the SPF gives a far later time than the real one.
• "I will go in when it stings": redness appears hours later, so by the time you feel it you may be well past the threshold.
• "Cloudy days are safe": light cloud lets up to 80% of UV through (WHO).
• "Spring and winter are fine": in New York a clear May noon is almost as strong as July, and snow reflects a large share of the UV that arrives.
• "I am in the shade, so I am fine": you still receive sky UV and UV bounced from sand and water.
• "I only need it on my face": ears, the back of the neck and the tops of the feet are easy to miss. Cover all the skin that will be exposed.

Limits and cautions

• Clear-sky model: the UV estimate assumes no clouds and 300 DU of ozone. Madronich's formula stays within 10% for solar zenith angles from 0 to 60 degrees. On cloudy, hazy or smoky days, enter the peak from your weather app.
• Thresholds vary by person: the MED differs widely even within one skin type, and the calculator uses fixed values per type.
• Sunscreen model: the link between amount and actual SPF comes from a study of a low-SPF product, and the bare-skin-after-2-hours rule for Once only is a simplification. Real products, sweat and rubbing vary.
• Body orientation: the UV index describes a horizontal surface facing the sky. The sides of a standing body get less, and skin facing up while you lie down gets nearly the full amount.
• Not medical advice: if you have a skin condition, or the sunburn is for a baby or young child, act more cautiously than the result suggests and follow the advice of a medical professional.

Sources

• 01WHO. Global Solar UV Index: A Practical Guide (2002)UV index definition, protection above 3 and a stronger message at 8 and above, 80% reflection by fresh snow, 15% by dry sand and 25% by sea foam, about 10% more UV per 1,000 m, UV through light cloud, minimal protection from water, shade, warning against reading burn times as safe times, childhood sunburn and melanoma
• 02US FDA. 21 CFR 201.327, Over-the-counter sunscreen drug productsReapply at least every 2 hours, Water Resistant (40 or 80 minutes), apply 15 minutes before sun exposure, 2 mg/cm² test amount
• 03European Commission. Recommendation 2006/647/EC on sunscreen products2 mg/cm² equals 6 teaspoons (about 36 g) for an average adult, half the amount can cut protection by up to two-thirds, highest label 50+
• 04Faurschou A, Wulf HC. The relation between sun protection factor and amount of sunscreen applied in vivo. Br J Dermatol 2007Exponential relation between amount and SPF, square root at 1 mg/cm² and fourth root at 0.5 mg/cm², sunbathers apply about a quarter of the test amount
• 05Petersen B, Wulf HC. Application of sunscreen: theory and reality. Photodermatol Photoimmunol Photomed 2014Real-life application of 0.39 to 1.0 mg/cm², apply before sun exposure and reapply once within an hour
• 06Cancer Council Australia. Sunscreen adviceAbout 35 mL for an adult's whole body, apply 20 minutes before going out, reapply every 2 hours and right after swimming, sweating or towel-drying
• 07NHS. SunburnHow to treat sunburn at home, usually better within 7 days, when to see a doctor
• 08Patient.info (professional reference). SunburnErythema usually occurs 2 to 6 hours after exposure and peaks at 12 to 24 hours
• 09Japan Ministry of the Environment. UV Environmental Health Manual 2020How much sunscreen to take for the face and layering the same amount again
• 10Scientific Reports 2024, PMC10861575, Table 3MED of 200, 250, 300, 450, 600 and 1,000 J/m² for skin types I to VI
• 11Madronich S. Analytic formula for the clear-sky UV index. Photochem Photobiol 2007;83:1537–1538Clear-sky UVI ≈ 12.5·cos(zenith angle)^2.42, within 10% for zenith angles of 0 to 60 degrees
• 12NOAA Global Monitoring Laboratory. General Solar Position CalculationsSolar zenith angle from date, time and location
• 13NIH Office of Dietary Supplements. Vitamin D, Fact Sheet for Health ProfessionalsUVB radiation does not penetrate glass

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