UV Index Calculator – How much sunlight you need for Vitamin D

The sun’s UVB rays help the skin produce vitamin D, which is essential for bone health, immune function, and MS management. While Overcoming MS recommends taking vitamin D supplements, natural sunlight also has important benefits.

In some parts of the world, at certain times of the year, you may be able to get enough vitamin D from sunlight alone. This page will tell you how to calculate how much sun exposure you need.

How much time should I spend in the sun?

You need 10 to 15 minutes of all-over sun on a ‘UV index 7’ day. You will need longer if the UV index is lower (e.g. 20 to 30 minutes if the index is 3.5) and less time in the sun if higher (e.g. 5 to 7.5 minutes if the index is 14).

Staying out longer than that won’t make any more vitamin D. Expose as much skin as modesty and temperature allows. The more skin exposed in a sitting, the more vitamin D is made, but only up to a certain maximum (about 10,000 to 15,000 IU). No more is made by staying out longer, and excess exposure raises the risk of sunburn and skin cancer. There are a number of factors involved in calculating this:

• UV index – The UV index measures the intensity of UV rays at a specific location and time. A higher UV index means less time needed for vitamin D production.
• Time of day – Midday sun is best for vitamin D production as UVB levels are highest. The UV index varies throughout the day, peaking at midday before dropping. Example: If the UV index is 14 at midday, it will be around half that at 10 am and 2 pm.
• Skin exposure – The more skin exposed, the more vitamin D your body can produce. Long sleeves and clothing block UVB rays, reducing vitamin D production. Exposing a small area of skin for longer doesn’t help, as once a patch of skin makes vitamin D, it stops producing more for that day.
• Skin tone – Darker skin tones (e.g. African, Afro-Caribbean, South Asian) need more sun exposure to produce the same amount of vitamin D as lighter skin tones.
• Weather and UVB availability – Cloudy weather significantly reduces UVB exposure. Windows block UVB rays, meaning you cannot produce vitamin D indoors, even in direct sunlight. Sitting in a sunroom or conservatory won’t work.
• Season and location – In cooler climates, the sun is too weak between October and March to produce enough vitamin D. If you live in a country with long winters, supplementation is essential.

It is also worth noting that sunscreen blocks the absorption of UVB, which is responsible for vitamin D production. So, if you’re spending a short amount of time in the sun, avoid sunscreen. If you’re out for longer, it is sensible to get sun on unprotected skin for the first 10-15 minutes (depending on the UV index), and then apply sunscreen.

UV Index and time needed for vitamin D production

The UV index identifies the strength of the sun (UV radiation) from a specific location on a particular day. The higher the UV index, the less time you need in the sun:

Example

Swimming outdoors for 10 to 15 minutes, two or three times a week (with most skin exposed) can produce 10,000 to 15,000 units of vitamin D per session.

This estimate is based on a UV index of 7, typical for spring in hotter regions (e.g. Australia) or summer in cooler regions (e.g. the UK).

Sunbeds and vitamin D

We strongly recommend that you do not use a sunbed to increase your vitamin D levels due to the risk of unnecessary skin damage and skin cancer. There is no safe level of sunbed use.

Check the UV index for your location

To find out how much sun exposure you need, check the UV index forecast for your area:

• In the US, the Environmental Protection Agency has UV indexes.
• In Australia, see the Bureau of Meterology’s data.
• In the UK, you can find forecasts from the Met Office online.