Vitamin D Sun Exposure Calculator

On a clear midday, a few minutes of sun can cover the daily vitamin D recommendation

The U.S. recommended dietary allowance for vitamin D is 15 μg (600 IU) a day for ages 1 to 70. For New York at 12:30 p.m. on a clear day, in short sleeves (16.5% of the body uncovered) with skin type III, this calculator gives about 4 minutes in July, 12 minutes in October and 33 minutes in January to make that amount. The burn threshold for the same skin is about 18 minutes in July and 1 hour 40 minutes in January.

The time can differ several-fold with the strength of the UV that day, skin color and how much skin is uncovered. Pick a city, date and time above and the calculator estimates the clear-sky UV index; type in the UV index from a weather app and it calculates with that value instead. With a 25(OH)D result from a blood test, the Dose from a blood test mode estimates how much more to take each day to reach a target.

When it is worth working out the sun time

Skin makes vitamin D when it takes in ultraviolet B (UVB), and food and supplements cover the rest. How much sun it takes therefore swings with the season, the latitude and the day. A calculation helps in cases like these.

• A desk job with little daylight: if the commute and a lunch break are the only time outside, the calculator shows whether a short midday walk is enough, and in which months it is not.
• Living in the north: on a clear January day at 12:30 p.m., the same outfit takes about 11 minutes in Miami, 33 in New York and 71 in Seattle. The year-round map shows the months when your own city falls short.
• Wanting enough sun without a burn: the time to the target and the burn threshold appear together, so the point to move to shade, clothing or sunscreen is clear.
• Darker skin or more coverage: the deeper the skin tone and the less skin uncovered, the longer it takes. With face and hands only, the target can be out of reach before the burn threshold.
• Age 71 or older: the NIH allowance rises to 20 μg (800 IU). Choosing 800 IU as the daily target shows how much longer that takes.

How to use the calculator

1. When and where: choose a city, date and time. Now fills in the city's current time, and tapping the hourly UV curve also changes the time. Clock times are local, daylight saving time included.
2. UV index: leave it empty to use the clear-sky estimate. On a cloudy or hazy day, the UV index from a weather app is more accurate. Behind a window indoors or in a car, turn on Behind a window.
3. Skin uncovered: tap a clothing button, or tap body parts on the diagram to switch them on and off. Switch off any part covered with sunscreen.
4. Skin type: choose by how skin reacts to the sun. Always burning is I or II, sometimes burning and tanning evenly is III, and rarely burning with an easy tan is IV.
5. Daily target and time in the sun: pick the target that fits your age and the time to reach it appears in large type. Enter a time or drag the bar to see how much is made, how much is missing and which foods would cover the gap.
6. Year-round map: the colors show the minutes to the target for each month and hour. Tapping a cell sets the calculation above to that month and hour.

How the calculation works

The calculation runs in three steps. It turns the UV index into the dose the skin receives, compares that dose with the skin type's burn threshold (the minimal erythema dose, MED), and converts it into an amount of vitamin D for the skin uncovered.

• UV index and dose: the WHO defines the UV index as 40 m²/W times the erythemally weighted irradiance, so an index of 1 is 0.025 W/m². In one minute (60 seconds) that is 1.5 J/m².
• 16,000 IU: the NILU FastRT calculator uses a standard vitamin D dose equal to an oral 1,000 IU, reached by exposing a quarter of the skin (hands, face and arms) to a quarter of a personal MED. Scaled to the whole body at one full MED, that is 16,000 IU. It agrees with a 2025 review that puts one MED in a swimsuit at roughly 15,000 to 20,000 IU.
• Sun-height factor: the UV that makes vitamin D has shorter wavelengths than the UV that burns, so it falls off faster as the sun gets lower. McKenzie and colleagues (2009) showed that the ratio of the two weightings is about 2 when the sun is high and close to 1 at a solar zenith angle of 70°. The calculator joins those two points, so a low sun makes less vitamin D at the same UV index.
• Stop at the burn threshold: once the dose passes one MED, the calculation stops. Past that point only the risk of a burn grows.
• Clear-sky UV estimate: the solar zenith angle comes from the NOAA solar position equations, and the UV index from Madronich's (2007) formula, 12.5 × cos(zenith angle)^2.42, with ozone fixed at 300 DU and no clouds. Category names follow the WHO guide: Low up to 2, Moderate 3 to 5, High 6 to 7, Very high 8 to 10, Extreme 11 and above.

The two right-hand columns are for New York, October 15 at 12:30 p.m., clear sky (UV 4.5), short sleeves (16.5%) and a daily target of 600 IU. Darker skin lengthens the time to the target and the time to the burn threshold in the same proportion.

The share of skin uncovered follows the adult Lund–Browder chart used to estimate burn area (U.S. Joint Trauma System form). Of the 7% for the head, only the front half, the face, is counted as exposed.

Examples by season and clothing

New York, the 15th of each month at 12:30 p.m., clear sky, skin type III, daily target 600 IU. Face and hands (with the neck) is 10.5% of the body, short sleeves 16.5% and short sleeves with shorts 30.5%.

The hour of day matters even more than the season. On the same January day at 9:30 a.m. the clear-sky UV index is 0.8 and short sleeves need 1 hour 45 minutes, more than three times the midday figure of 33 minutes. A higher target takes longer too: for 800 IU in short sleeves, midday takes 44 minutes in January and 6 minutes in July.

Daily vitamin D intake recommendations

The NIH Office of Dietary Supplements lists a recommended dietary allowance (RDA) and a tolerable upper intake level. 1 μg is 40 IU.

Other countries set different numbers. The European Food Safety Authority (EFSA) sets an adequate intake of 15 μg for adults, a value that assumes minimal skin synthesis. Japan's 2025 intake standards give 9.0 μg for adults, and the 2025 Korean standards give 10 μg for adults and 15 μg from age 65. The calculator's FastRT standard 1,000 IU is the standard dose that the NILU calculator uses.

Estimating a dose from a blood test

The 25(OH)D result of a blood test reflects the vitamin D stored in the body. The NIH divides it as follows (1 ng/mL = 2.5 nmol/L).

The extra amount is calculated from the finding that 100 IU more a day raises the blood level by 0.7 to 1 ng/mL. Heaney and colleagues (2003) measured a rise of about 0.70 nmol/L for each additional 1 μg a day (0.7 ng/mL per 100 IU), and another review puts it near 1 ng/mL per 100 IU. The calculator uses both values, shows the result as a range and rounds up to the next 100 IU.

The default target is 20 ng/mL, the lower limit the NIH calls adequate; 30 ng/mL can also be chosen because some lab reports use it as their cutoff. The Endocrine Society's 2024 guideline, however, advises against routine 25(OH)D testing in the general population and finds no evidence for a best target level for disease prevention. This mode is meant for reading a result that has already been taken.

Making up the gap with food

Few foods are rich in vitamin D, mostly fatty fish. These are the amounts in the NIH fact sheet.

The calculator's food list uses the first five rows. Fortified products vary by brand, so check the label.

Common mistakes

• Sun through a window: UVB does not pass through glass, and the NIH notes that sunshine indoors through a window does not produce vitamin D. With the window option on, the calculator counts zero.
• Using the clear-sky estimate on a cloudy day: the estimate contains no clouds. On an overcast day, enter the UV index from a weather app.
• Assuming longer is always more: the calculation stops at the burn threshold. Skin regulates its own production, and Japan's intake standards state that sun exposure does not cause vitamin D excess. What more sun adds is burn risk.
• Treating a low winter sun like midday: a low sun means a smaller UV index and also a smaller share of vitamin D-producing UV, which is why the early and late cells in the year-round map run so much longer than midday.
• Mixing IU and μg: 1 μg is 40 IU. A 1,000 IU supplement is 25 μg, which is 1.7 times the 15 μg RDA for ages 1 to 70.
• Counting skin that has sunscreen on it: sunscreen is made to block UVB. Switch those areas off on the body diagram to stay close to reality.

Limits and cautions

• The 16,000 IU figure, the sun-height factor and the MED values are averages from research, and the amount made differs from person to person even in the same conditions. Treat results as estimates.
• The clear-sky estimate fixes ozone at 300 DU and leaves out clouds, haze, altitude and reflection from snow or water. On days with more ozone it can read high.
• The angle at which each body part receives light is not modeled; the horizontal-surface UV index is used as is, and the movement of the sun during the exposure is not included.
• The slope in the blood-test mode is an average, so the rise varies with body weight, constitution and absorption. The NIH notes that people with a body mass index of 30 or more tend to have lower blood levels. Settle doses in the deficient range, or above the upper limit, with a doctor.
• This calculator is not a diagnosis or a prescription. People with skin conditions, or taking photosensitizing medicines, should discuss sun exposure with a clinician first.

Reference date The sources below were checked on 2026-10-19.

Sources

• 01NIH Office of Dietary Supplements. Vitamin D – Fact Sheet for Health ProfessionalsRDA and upper limit, 25(OH)D ranges, UVB does not pass through glass, the 5 to 30 minute statement, the food table
• 02EFSA. Dietary reference values for vitamin D. EFSA Journal 2016;14(10):4547Adequate intake for adults, 15 μg a day
• 03Japan Ministry of Health, Labour and Welfare. Dietary Reference Intakes for Japanese (2025)Adults 9.0 μg; vitamin D excess does not occur from sun exposure
• 04Korean Ministry of Health and Welfare and Korean Nutrition Society. 2025 Dietary Reference Intakes for KoreansVitamin D adequate intake of 10 μg for ages 19 to 64 and 15 μg from age 65
• 05NILU FastRT. Vitamin D and erythema exposure times – READMEStandard vitamin D dose equal to an oral 1,000 IU; a quarter of the skin exposed to a quarter of a personal MED
• 06Scientific Reports 2024, PMC10861575, Table 3MED for skin types I to VI: 200, 250, 300, 450, 600 and 1,000 J/m²
• 07Nutrients 2025 review, PMC11821240One MED in a swimsuit is roughly equal to 15,000 to 20,000 IU taken by mouth
• 08McKenzie RL, Liley JB, Björn LO. UV radiation: balancing risks and benefits. Photochem Photobiol 2009;85:88–98Ratio of vitamin D to erythemal weighting: about 2 with the sun high, close to 1 at a zenith angle of 70° (Fig. 4b)
• 09Madronich S. Analytic formula for the clear-sky UV index. Photochem Photobiol 2007;83:1537–1538UVI ≈ 12.5·μ0^2.42·(Ω/300)^−1.23, within 10% for zenith angles of 0 to 60° and 200 to 400 DU
• 10WHO. Global Solar UV Index: A Practical Guide (2002)UV index = 40 m²/W × erythemally weighted irradiance (1 = 0.025 W/m²); categories Low, Moderate, High, Very high, Extreme; reported as a whole number
• 11NOAA Global Monitoring Laboratory. General Solar Position CalculationsEquation of time, declination and hour angle for the solar zenith angle
• 12Heaney RP et al. Human serum 25-hydroxycholecalciferol response to extended oral dosing with cholecalciferol. Am J Clin Nutr 2003;77:204–210Rise of about 0.70 nmol/L per 1 μg (about 20 weeks of dosing to a steady level)
• 13PMC3427198About 1 ng/mL rise in 25(OH)D for every 100 IU a day
• 14Demay MB et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab 2024No routine testing in the general population; no evidence for an optimal target level
• 15Joint Trauma System. Adult Lund-Browder Burn Estimate DiagramAdult head 7, neck 2, trunk front 13 and back 13, upper arm 4, forearm 3, hand 2.5, thigh 9.5, lower leg 7, foot 3.5% (one side)

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