Can the sun make vitamin D where you are?

How this works

It's our guide to how much sun you need for vitamin D, asked as questions. The guide's point is that the famous “10 to 15 minutes” carries four conditions — summer, midday, some skin showing, and a UK-type latitude — and the check takes them in the order that can move the answer all the way to zero. First the month: in a vitamin D winter nothing else matters, so it goes straight to food and supplements. Then glass, which blocks almost all UVB whatever the month. Then time of day, because early and late light makes very little. Only after those three does it give minutes, and it downgrades them when only your face and hands are out, or when your skin is very dark at a high latitude.

It never adds up a dose. How much vitamin D you actually made today depends on the UV index hour by hour, the cloud, and how long you really stayed out, and a form can't know any of that. What it can tell you honestly is whether the sun is able to do the job this month, and what the research says it takes when it can.

Where the numbers come from

The minutes are from the University of Manchester research by Ann Webb and Lesley Rhodes, as the guide reports it: at UK latitude (about 50–58°N), April to September, midday, forearms and lower legs uncovered, no sunscreen — 10–15 minutes a day for lighter skin, about 25 minutes for darker brown skin, and for very dark skin “longer again”, possibly not realistic from sun alone. Closer to the equator the check calls those figures a ceiling; further north it calls them a floor. It doesn't scale them into new numbers, because the research didn't publish any.

The months are the guide's vitamin D winter table, built on Webb, Kline and Holick's 1988 measurements: no synthesis in Boston (42°N) from November to February, or in Edmonton (52°N) from October to March, and year-round synthesis in Los Angeles and Puerto Rico. The table leaves gaps between its rows — 26–29°, 43–48°, anywhere above 60° — and this page fills each gap from the neighbouring row with the longer winter. That's a choice, and the panel says so where it applies. The 37–39° row comes from the guide's line that Melbourne's weak months are around June to August, flipped for the northern hemisphere. “Edge of the season”, the month either side of a published winter, is also our word, not the research's.

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.

If you're not sure, assume winter

The two mistakes aren't the same size. Taking a standard supplement one month longer than the sun strictly needed costs very little, and in the UK it's what the NHS advises anyway for autumn and winter. Standing outside in February thinking you're making vitamin D when the sun is too low to make any costs you the whole season. That's why every gap in the table rounds towards the longer winter. On the day, the shadow test settles it: if your shadow is longer than you are tall, the sun is too low for much UVB.

• How much sun do you need for vitamin D? — the full guide: UVB, skin type, the vitamin D winter, sunscreen, and why more time isn't more vitamin D
• How to track time in daylight on Apple Watch — what the Watch measures, and why it sometimes shows zero
• The best vitamin D apps — dose estimators, passive daylight trackers and burn warnings, sorted by job