Vitamin D Sun Exposure Calculator - UV Index, Dose and Time

Sun time for vitamin D, checked against what the experts actually say

How much sun do you need for vitamin D? There is no single number, and this vitamin D sun exposure calculator does not pretend otherwise. It takes the UV index for the hour you plan to be out, the minutes, the skin you leave uncovered, sunscreen, how often you do it, skin type and age, and returns four things: the WHO category for that UV index with the protection WHO asks for, the UV dose in standard erythema doses (SED), how your session compares with the 5 to 30 minute pattern cited by the NIH Office of Dietary Supplements, and the recommended vitamin D intake for your age.

Quick start: UV index 7, a 15 minute walk at lunch in shorts and a T-shirt, no sunscreen, 5 times a week: category High, dose 1.58 SED (158 J/m²), 6 of 6 conditions of the ODS pattern, and a recommended intake of 600 IU a day for ages 1 to 70.

What it will not give you is a "safe time before you burn" or a number of IU made by your skin. WHO, together with the WMO, UNEP and ICNIRP, recommends against reporting burn times, because people read them as a safe amount of unprotected sun. And the ODS fact sheet says plainly that the sun exposure needed for vitamin D is uncertain and varies from person to person.

Nine answers on the form

UV index - the value for the hour you will be outside, from a weather app or a national forecast. Decimals are fine; the category uses the whole number, as WHO reports it.
Minutes in the sun - 1 to 720.
Outdoors or behind glass - UVB does not pass through window glass.
Time of day - between 10 a.m. and 4 p.m. or outside those hours.
Uncovered skin - face and hands, add arms, or add arms and legs.
Sunscreen - none, below SPF 8, or SPF 8 and higher.
Sessions a week - 0 to 14.
Skin type - I to VI from the WHO table, from "always burns" to naturally black skin.
Age group - for the recommended intake.

Six days outside, entered as they are

Each card is one set of inputs and what the calculator returns for it. The dose is the same physics every time; what changes is how close the day comes to the ODS pattern.

Lunch walk in summer clothes

UV index 7, 15 min, 10 a.m. to 4 p.m., face, arms, hands and legs, no sunscreen, 5 times a week: High, 1.58 SED, 6 of 6.

Early run at 8 a.m.

UV index 3, 30 min, arms and legs bare, no sunscreen, 4 times a week: Moderate, 1.35 SED, 5 of 6. The hour is outside the window ODS names.

Desk next to a sunny window

UV index 5 outside, 30 min behind glass, every workday: no dose and 5 of 6, with the one that matters missing. UVB does not get through the glass, so no vitamin D.

Two hours on the beach at 1 p.m.

UV index 10, 120 min, SPF 30, once a week: Very high, 18 SED, 3 of 6, and the WHO message for extra protection.

Winter weekend walk in a coat

UV index 2, 20 min at noon, face and hands only, twice a week: Low, 0.6 SED, 5 of 6. Most of the skin ODS lists is covered.

Gardening at 11 a.m. with sunscreen

UV index 8, 30 min, face, arms and hands, SPF 15, 3 times a week: Very high, 3.6 SED, 4 of 6.

Where the dose number comes from

The UV index is defined by WHO as erythemally weighted UV irradiance multiplied by 40 m²/W. One index point is therefore 0.025 W/m². The CIE defines one standard erythema dose as 100 J/m² of the same weighted UV. Put together, each minute at UV index 1 delivers 1.5 J/m², or 0.015 SED:

Dose (SED) = 0.015 × UV index × minutes

At UV index 6 for 15 minutes that is 1.35 SED. Two caveats come with it. The index describes a horizontal surface, so a face turned to the sun, shade, cloud, water or snow will change what the skin really receives. And SED is a standardized unit of erythemal radiation, as the CIE defines it, not a measure of how a given skin reacts, so the calculator does not turn it into a burn time.

The five UV categories and the advice tied to them

The categories and messages below are the ones in the WHO practical guide. Below 3, WHO says that even for very sensitive fair skin the risk is limited and no protection is needed under normal circumstances; from 3 protection is needed, and the message is reinforced from 8.

UV index	Category	WHO message	SED in 15 min
1-2	Low	No protection required	0.23-0.45
3-5	Moderate	Protection required	0.68-1.13
6-7	High	Protection required	1.35-1.58
8-10	Very high	Extra protection	1.8-2.25
11+	Extreme	Extra protection	2.48 and up

How much vitamin D a day, by age

These are the intakes in the ODS fact sheet, set by the Food and Nutrition Board. ODS notes they were set on the assumption that people get minimal sun, so they do not shrink because you spent time outside.

Age	IU a day	mcg a day
0-12 months (adequate intake)	400	10
1-70 years, including pregnancy and lactation	600	15
Over 70	800	20

Sun and vitamin D questions

How long should you stay in the sun for vitamin D?

The figure ODS cites from some expert bodies and researchers is about 5 to 30 minutes, mainly between 10 a.m. and 4 p.m., daily or at least twice a week, on the face, arms, hands and legs without sunscreen. ODS presents it as an example and adds that season, time of day, cloud cover, smog, skin melanin and sunscreen all change the result.

Does sun through a window give you vitamin D?

No. ODS states that UVB does not penetrate glass, so sitting in the sun indoors does not produce vitamin D. The calculator marks a window session as a miss on the first condition and leaves the dose blank, because the outdoor UV index does not describe the room.

Does sunscreen stop vitamin D?

ODS says sunscreen of SPF 8 or more appears to block the UV that makes vitamin D, and also that in practice people apply too little and miss spots, so the skin probably still makes some. The same agencies advise SPF 15 or higher whenever you are in the sun; FDA adds reapplying at least every two hours.

Why doesn't the calculator show how long until I burn?

Because WHO recommends against it: burn times read as a safe level of unprotected sun, while cumulative UV also counts for skin cancer and for the eyes. FDA makes a related point about sunscreen, that SPF describes an amount of UV and not a time you can stay out.

Is the UV index enough to know the dose on my skin?

It is a good start, not the full answer. The index is measured or modelled for a horizontal surface, so the SED figure is what lands on flat ground in the open. Shade, posture, clouds and reflection all move it up or down.

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