Vitamin D is a hormone your skin makes from UVB light, and many of the women I coach are low in it, especially from October to March. Watch today's UV forecast around the world, then pick a place, a date and a time: the calculator gives an illustrative estimate from a published model, with its uncertainty, for three broad skin categories. It does not tell you how much sun you need, and it never replaces a blood test or your doctor.
Technically, vitamin D is a steroid hormone: nearly every tissue has receptors for it. In practice, low levels tend to show up as tiredness, slow recovery, low mood in winter, frequent colds and body fat that is hard to shift. Your doctor measures 25-OH vitamin D in your blood; we read the result together and adjust food, sun and, when needed, a supplement.
Muscle fibres carry vitamin D receptors. Low levels are linked with weaker muscles, more injuries and slower recovery; correcting a deficiency improves strength and performance in deficient athletes.
Without vitamin D you absorb only 10 to 15 percent of the calcium you eat. It underpins bone density and a long, injury-free training life.
Vitamin D helps immune cells respond. Trials in deficient people show fewer respiratory infections with daily supplementation.
Vitamin D receptors are found in the brain and in reproductive tissue. Winter low mood, poor sleep and irregular cycles often come with low vitamin D, and sun exposure itself anchors your body clock.
Deficiency is more common with higher body fat, because vitamin D is stored in fat tissue. In coaching we correct it first, so that fat loss and training have the hormones they need.
Skin makes vitamin D only when UVB is strong enough, roughly from UV index 3. The same UV also burns and ages skin and causes skin cancer, so more time in the sun is not better. Food gives a little; when a blood test shows you are low, your doctor decides on a supplement and dose.
The globe turns to each location and shows today's forecast peak UV index, with illustrative ranges for light, medium and dark skin to reach the reference amount of vitamin D with face, hands and arms uncovered. Press pause, or tap a place.
One time only. It unlocks the tools in this browser. The tools give general, illustrative estimates for healthy adults, not medical advice.
Choose a place, a date and a local time. The calculator reads the UV index forecast for that hour and gives an illustrative estimate of how long one session of unprotected sun would take to reach a fixed reference amount of vitamin D, with its uncertainty. It is not a recommended, safe or required time. Sunburn risk is shown separately.
Light: burns easily, tans little or not at all (roughly Fitzpatrick types I and II). Medium: sometimes burns, tans gradually to well (roughly types III and IV). Dark: rarely or never burns, naturally brown to dark brown or black skin (roughly types V and VI).
These are three broad groups for comparison, not a clinical skin classification. Skin type is self-reported and varies within each group; a dermatologist assesses individual skin type and sun sensitivity.
Search or pick one of the listed places. The UV index forecast for the date and hour you choose appears here, with the estimate for the skin category you select.
Same date, same local clock time at each place, same uncovered skin as above. Illustrative estimates for one session, in minutes.
| Location | Local date and time | UV index | Light | Medium | Dark |
|---|---|---|---|---|---|
| Loading the UV forecast for all locations… | |||||
“Not estimated” means a reliable minutes estimate is unavailable for that cell; the reason is given in the cell. Data: Open-Meteo UV index forecast (a weather-model forecast that includes clouds, not a measurement).
How the strength of the sun at a given place, date and hour changes the time it takes your skin to make a fixed reference amount of vitamin D, and how that compares across places and broad skin categories. Below a UV index of 3, sun-made vitamin D is minimal and too uncertain to estimate.
Your blood vitamin D level, whether you are deficient, how much vitamin D you need, or a safe amount of sun. Reaching an estimated time does not guarantee adequate vitamin D. Deficiency is assessed with a blood test (25-OH vitamin D) and treated with your doctor; food and supplements are a separate question.
From UV index 3, protection is recommended (WHO): shade around midday, clothing, a hat, sunglasses and broad-spectrum sunscreen. Never aim to tan or redden, and do not stay out longer or skip sunscreen to “make up” for low UV. If you have a history of skin cancer, take photosensitising medication or are pregnant, ask your doctor first.
Deficiency is under 20 ng/mL (50 nmol/L). “Enough” starts at 30 ng/mL. For training, recovery, immunity and mood I aim for 40 to 60 ng/mL (100 to 150 nmol/L), checked by your doctor at the start of coaching and again at the end of the 16 weeks.
Some daily daylight is good for your body clock. For vitamin D, only UV index 3 or more counts, and that is also when skin needs protection. Never stay out until skin turns pink, never aim to tan, and do not skip sunscreen to make vitamin D. Glass blocks the UVB that makes vitamin D.
Above roughly 35 degrees of latitude (most of Europe, Canada, the northern US) the sun is too low for much of October to March to make meaningful vitamin D, whatever the weather. Darker skin may need longer in the same sun; studies disagree on how much, from about 1.3 to 6 times. That is often when a supplement, agreed with your doctor, earns its place.
Vitamin D3, taken with a meal that contains fat, usually 1,000 to 2,000 IU a day in winter for a healthy adult, more only with a blood test and your doctor's agreement. Pair it with magnesium-rich food and vitamin K2 from fermented foods or egg yolk.
Oily fish (salmon, sardines, mackerel), egg yolks, liver and UV-exposed mushrooms give a few hundred IU a day at most. Useful, not sufficient on their own.
Levels move slowly: 8 to 12 weeks to see a change. We retest at the end of the 16 weeks and adjust sun, food and dose to your result, never by guessing.
The consultation is complimentary.
What is estimated. The time of one session of unprotected sun around the chosen hour that would deliver a fixed reference amount of vitamin D-producing UV: the dose Holick (2004) and Webb and Engelsen (2006) equate with roughly 1,000 IU of oral vitamin D. That equivalence comes from a tanning-bed comparison that measured serum vitamin D, not blood 25-OH vitamin D, so it is a reference point, not a daily requirement.
Light skin, whole body uncovered: t = 15.2 × UVI−1.17 minutes, a fit to McKenzie, Liley and Björn (2009) published by Sánchez-Pérez et al. (2024), valid for UV index 3 to 14 (above 14 the calculator uses 14 and says so). The time is divided by the share of skin uncovered (approximate Lund–Browder shares: face, hands and arms 27%, T-shirt and shorts 35%, arms and legs 63%, swimwear 85%); real responses rise less than proportionally with area (Neville et al. 2021).
Skin pigmentation. Studies disagree on how much melanin slows vitamin D synthesis: a factor of about 1.3 to 1.4 between very dark and light skin (Young et al. 2020), about 2.5 to 3 (Farrar et al. 2011; Webb et al. 2018), up to roughly the ratio of sunburn doses (Shih et al. 2018). The calculator therefore uses a range: medium ×1.0 to 2.2, dark ×1.3 to 6, and widens every range by ×0.7 to ×1.5 for model uncertainty (atmosphere, action spectrum, body shape; Webb and Engelsen 2006; Young et al. 2021).
Sunburn is kept separate. Sunburn doses are not turned into vitamin D targets. They are used only to withhold an estimate when the reference amount could not plausibly be reached without a real risk of burning (low-end sunburn doses of 200, 300 and 600 J/m² for light, medium and dark skin; Webb and Engelsen 2006; Fajuyigbe and Young 2016). Sunburn risk itself is shown with the WHO UV index scale; following WHO advice, no “time to burn” is published.
When no estimate is shown. UV index below 3; face and hands only (too little effect in studies; Webb et al. 2018); missing UV data; or when sunburn risk comes first. Exposures at different times of day are not interchangeable: the vitamin D share of UV falls when the sun is low, and production in the skin levels off within an exposure (Holick et al. 1981). Not included: age, body weight, pregnancy, starting vitamin D level, medication, altitude, snow, sand or water reflection, shade, glass or sunscreen. Skin categories are broad and self-reported. These models have not been validated against blood results.
Data. UV index, clear-sky UV index and cloud cover from the Open-Meteo forecast API, hourly, in each location’s time zone. Past dates show the weather model’s values for that date; today and future dates are forecasts. None of them are measurements.
Sources. WHO, Global Solar UV Index: A Practical Guide (2002). Holick MF, Am J Clin Nutr 2004;79:362–71. Webb AR, Engelsen O, Photochem Photobiol 2006;82:1697–1703. McKenzie RL, Liley JB, Björn LO, Photochem Photobiol 2009;85:88–98. Sánchez-Pérez JF et al., Heliyon 2024;10:e30864. Young AR et al., J Invest Dermatol 2020;140:1418–26. Young AR et al., PNAS 2021;118:e2015867118. Farrar MD et al., Am J Clin Nutr 2011;94:1219–24. Webb AR et al., Nutrients 2018;10:457. Shih BB et al., J Invest Dermatol 2018;138:2244–52. Fajuyigbe D, Young AR, Pigment Cell Melanoma Res 2016;29:607–18. Neville JJ, Palmieri T, Young AR, JBMR Plus 2021;5:e10460. Holick MF, MacLaughlin JA, Doppelt SH, Science 1981;211:590–3.
Questions: contact@withmenutrition.com. This tool is general information, not medical advice, and does not diagnose deficiency.
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