How much sun you need for vitamin D has a short answer and a long one. The short one: in summer, at midday, with your forearms and lower legs uncovered, about 10 to 15 minutes a day is enough for people with lighter skin, and about 25 minutes for people with darker brown skin — that's what researchers at the University of Manchester found when they modelled UK weather and daily life. The long answer is that four things move that number: the UV index, your skin type, your latitude and the season, and how much skin is showing. In a northern winter, the answer isn't "more minutes" — it's that the sun can't make vitamin D at all.
Why it's UVB, and why that changes everything
Your skin makes vitamin D when UVB light hits it. Not daylight in general, not warmth, not brightness — one band of ultraviolet. That single fact explains most of what surprises people:
- UVB is strongest when the sun is high. Its path through the atmosphere is shortest at midday in summer, and much longer in the early morning, late afternoon and winter, when most UVB is absorbed before it reaches you.
- Glass blocks it. Window glass stops almost all UVB. A sunny windowsill or a car commute makes essentially no vitamin D.
- It's the same light that burns you. The UVB that makes vitamin D is also the main cause of sunburn, which is why the useful dose and the harmful dose are closer together than anyone would like.
That's why the UV index is the most useful single number here. As a rough working rule, vitamin D production is meaningful at a UV index of about 3 or more, and small below it. A quick check without an app: look at your shadow. If it's longer than you are tall, the sun is too low for much UVB.
The four things that change the answer
| Factor | Less time needed | More time needed |
|---|---|---|
| UV index | High (6+) — summer midday, low latitudes, altitude | Low (under 3) — early, late, cloudy, winter |
| Skin type | Lighter skin (Fitzpatrick I–III) | Darker skin (IV–VI): more melanin filters UVB |
| Latitude & season | Near the equator, or summer anywhere | Above ~40°, especially October–March (north) / April–September (south) |
| Skin showing | Arms and legs, or more | Face and hands only |
| Age | Younger | Older: skin makes less vitamin D from the same sun |
Sunscreen reduces UVB too, and in a lab a high-SPF sunscreen applied thickly cuts vitamin D production sharply. In real life people apply far less than the lab amount, and studies of everyday sunscreen use have generally not found it causes deficiency. The practical advice from UK researchers is short, unprotected exposure — never long enough to redden — and sunscreen for anything longer.
How long by skin type
The Manchester research by Ann Webb and Lesley Rhodes is the most careful everyday guidance available, and it's worth knowing exactly what it assumed: UK latitude (roughly 50–58°N), UK weather, midday exposure, April to September, with forearms and lower legs uncovered and no sunscreen. Under those conditions:
- Lighter skin (burns easily to tans gradually): about 10–15 minutes a day is enough to build stores that last through winter. Fair skin that burns quickly should treat the lower end as the target — the aim is a small daily dose, never pink skin.
- Darker brown skin (such as South Asian): about 25 minutes a day.
- Very dark skin (Fitzpatrick VI): longer again, and at UK-type latitudes it may not be realistic to get enough from sun alone. The NHS advises people with dark skin — for example of African, African-Caribbean or South Asian origin — to consider a vitamin D supplement all year round, not just in winter.
Closer to the equator, or at altitude, the same dose comes faster; in Australia's summer, a few minutes can be enough and the burn risk arrives quickly. Further north, or outside midday, it takes longer — and in winter, it stops being possible.
The vitamin D winter: when minutes stop counting
Above roughly 40° latitude, there are months when the sun never gets high enough for UVB to make vitamin D in your skin, however long you stay out. The classic measurement is a 1988 study by Webb, Kline and Holick: skin exposed to sunlight in Boston (42°N) made no vitamin D from November through February, and in Edmonton (52°N) none from October through March, while production carried on all year in Los Angeles (34°N) and Puerto Rico (18°N).
| Where | Roughly | Vitamin D winter (no sun synthesis) |
|---|---|---|
| Tropics — Singapore, Puerto Rico, Hawaii | 0–25° | None — year-round |
| Los Angeles, Tokyo, Sydney, Cape Town | 30–36° | Little to none; weak in midwinter |
| Boston, New York, Chicago, Rome, Madrid | 40–42° | About November–February |
| London, Amsterdam, Berlin, Edmonton, Vancouver | 49–53° | About October–March |
| Edinburgh, Copenhagen, Stockholm, Oslo | 56–60° | Longer again — roughly October to early April |
Southern-hemisphere cities flip the calendar: Melbourne's weak months are around June to August. During a vitamin D winter the realistic sources are food (oily fish, eggs, fortified foods) and supplements — which is why the NHS advises everyone in the UK to consider a 10 microgram (400 IU) daily vitamin D supplement in autumn and winter. The US recommended intake for adults is 600 IU (15 micrograms) a day, and 800 IU over age 70. Summer sun matters in this picture too: vitamin D is stored, so the months with strong UVB build the reserve you draw down through winter.
More time doesn't mean more vitamin D
A common assumption is that if 15 minutes is good, an afternoon is better. It isn't. Once your skin has made a certain amount of vitamin D's precursor, further UVB converts the excess into inactive by-products instead of more vitamin D, so production levels off within a fraction of the time it takes to burn. That's also why you can't overdose on vitamin D from the sun — but you can certainly burn. Short, regular exposure beats long, occasional sessions on every count: more vitamin D over a week, and far less skin damage. If your skin has gone pink, you've gone well past the useful dose.
Why "enough" is hard to judge on your own
The guidance above is honest but awkward to follow day to day: the UV index changes hour by hour, your skin type sets both the minimum and the burn limit, and nobody remembers on Friday how many of this week's lunch breaks were actually spent outside. Only a blood test (25-hydroxyvitamin D) tells you your actual status; what you can track is the input. That's the gap we built SunshinePal for (listed on the App Store as Sunshine — Vitamin D Tracker). With an Apple Watch Series 6 or later it picks up your time in daylight on its own through HealthKit, then estimates the vitamin D you likely made from the live UV index, your Fitzpatrick skin type and your time outdoors — with a personal burn-time window from the same inputs, so "long enough" and "too long" come from one number. Supplements log alongside, so a grey winter week shows up as a supplement doing the sun's job. It's an estimate and labelled as one; it tells you whether today was likely enough, not what your blood level is.
If you want to understand the minutes themselves, our guide to tracking time in daylight on Apple Watch explains what the Watch measures and why it sometimes shows zero; for a comparison of the apps in this space, see the best vitamin D apps.
Frequently asked questions
How much sun do you need for vitamin D?
Less than most people think, but only at the right time of year. Research from the University of Manchester modelled UK conditions and found that 10 to 15 minutes of midday sun a day between April and September, with forearms and lower legs uncovered and no sunscreen, gives people with lighter skin enough vitamin D to avoid deficiency all year. People with darker brown skin, such as those of South Asian origin, need about 25 minutes. Closer to the equator the times are shorter; in winter at higher latitudes, the sun is too low to make any vitamin D however long you stay out.
Can you get vitamin D from sun through a window?
No. Vitamin D is made by UVB light, and ordinary window glass blocks almost all UVB while letting through visible light and much of the UVA. Sitting in a sunny window, or driving with the sun on your arm, can still tan or burn you over time and still counts as daylight for your body clock, but it makes essentially no vitamin D. You need to be outside, with skin uncovered.
What time of day is best for vitamin D from the sun?
Around solar noon, roughly 11am to 3pm in most places, when the sun is highest and UVB is strongest. Early morning and late afternoon light is mostly UVA and makes very little vitamin D. A quick check is your shadow: if it's longer than you are tall, the sun is too low for much UVB. Midday is also when you burn fastest, so the best approach is short, regular exposure rather than long sessions.
Can you get vitamin D from the sun in winter?
It depends on your latitude. A 1988 study by Webb, Kline and Holick found that skin exposed to sunlight in Boston, at about 42 degrees north, made no vitamin D from November through February, and in Edmonton, at 52 degrees north, from October through March, while synthesis continued year round in Los Angeles and Puerto Rico. Most of Europe, Canada, the northern United States and northern Asia have a vitamin D winter. During those months, food and supplements are the realistic sources, which is why the UK's NHS advises everyone to consider a 10 microgram vitamin D supplement in autumn and winter.
Was today enough?
SunshinePal estimates the vitamin D you made from the live UV index, your skin type and your time outside — and tells you when to head back in.
Download SunshinePal on the App Store →Related: How to Track Time in Daylight on Apple Watch and The Best Vitamin D Apps. This is general information, not medical advice; if you're worried about your vitamin D level, ask your doctor about a blood test before starting high-dose supplements. Questions? Email us.