About the Vitamin D Sun Exposure Calculator
Your skin makes vitamin D when UVB light strikes it, and the amount depends on three things: how strong the UV is (the UV Index), how sensitive your skin is (its minimal erythemal dose, which rises with natural pigmentation) and how much skin is exposed. This calculator combines them to estimate how many minutes of midday-style sun you would need to make a chosen amount of vitamin D, such as 1,000 IU.
It is built on the widely cited estimate from vitamin D researcher Michael Holick that one minimal erythemal dose (just enough sun to turn skin faintly pink) on a swimsuit-clad body produces roughly as much vitamin D as swallowing 20,000 IU. Smaller areas of skin and shorter exposures scale that down proportionally. The result is compared with your estimated time to sunburn so you can see whether the target is realistic without burning.
These are order-of-magnitude estimates for healthy adults, not a prescription. Real production varies with age (older skin makes less), body fat, latitude, season, time of day, glass (which blocks UVB) and sunscreen. Production also plateaus: beyond about one MED, extra sun mostly adds skin damage. If you are concerned about deficiency, a blood test of 25-hydroxyvitamin D is the only reliable check.
With the default inputs, the sun time for your target is 5.2 min. Change any value above to recalculate instantly.
How to use the vitamin d sun exposure calculator
- 1Enter the current or forecast UV Index for when you will be outside.
- 2Pick your skin type based on how easily you burn and tan.
- 3Choose how much skin will be bare (no sunscreen on that skin).
- 4Enter a vitamin D target, for example 1,000 IU.
- 5Compare the minutes needed with your time to sunburn and stay well below it.
Formula and method
Time to one minimal erythemal dose (t_burn) comes from the WHO UV Index definition — one index unit is 25 mW/m² of erythemally weighted UV — and a typical MED for each Fitzpatrick skin type (200, 250, 300, 450, 600 and 1,000 J/m² for types I–VI). Because darker skin needs more UV both to burn and to make vitamin D, scaling by MED captures the effect of pigmentation.
Vitamin D output assumes one MED on about 85% of the skin (a swimsuit) yields roughly 20,000 IU, scaled linearly by the share of skin exposed and the fraction of a MED received. Linearity holds reasonably well for sub-burning doses; above about one MED, previtamin D is converted to inactive by-products, so extra sun adds little. The calculator flags targets that would need more than half your burn time.
- MED
- Minimal erythemal dose for the skin type (J/m²)
- UVI
- UV Index
- exposed %
- Share of body surface uncovered
- t_needed
- Minutes of exposure to reach the target IU
Worked examples
Skin type III in T-shirt and shorts at UV 5
Type III skin (MED 300 J/m²) at UV 5 reaches one MED in 300 ÷ (5 × 0.025) = 2,400 seconds = 40 minutes. With 33% of skin bare, one MED would give 20,000 × 33/85 ≈ 7,765 IU, so 1,000 IU needs 1,000 ÷ 7,765 × 40 ≈ 5.2 minutes — about 13% of the burn dose.
Very dark skin (type VI), face and forearms, UV 4
A type VI MED of 1,000 J/m² at UV 4 takes 1,000 ÷ 0.1 = 10,000 seconds (about 167 minutes). Only 13% of skin is bare, so one MED would make about 3,059 IU and 1,000 IU needs roughly 54 minutes — reachable, but much longer than for fair skin.
Fair skin (type II) in a swimsuit at UV 8, target 2,000 IU
Type II skin burns in 250 ÷ 0.2 = 1,250 seconds (about 21 minutes) at UV 8. A swimsuit exposure gives the full 20,000 IU per MED, so 2,000 IU takes one tenth of a MED — around 2 minutes.
Frequently asked questions
How much sun do I need for vitamin D?+
For many fair-skinned adults, 5–15 minutes of midday summer sun on the arms and legs a few times a week is enough to make a useful amount. People with darker skin may need two to six times longer. The calculator estimates this for your UV Index and skin type.
Can I get vitamin D through a window?+
No. Ordinary window glass blocks almost all UVB, the wavelengths that make vitamin D, while letting through UVA that can still age the skin. You need direct outdoor exposure.
Does sunscreen stop vitamin D production?+
Correctly applied sunscreen greatly reduces vitamin D production in the covered skin, though in real-world use people still make some because application is usually thinner than in tests. The calculator assumes the exposed skin has no sunscreen.
Why can’t I make vitamin D in winter?+
At higher latitudes in winter the sun is low, so UVB has a long path through the atmosphere and is largely absorbed by ozone. When the UV Index stays below about 3, skin production is minimal, which is why many health bodies suggest supplements in winter.
How much vitamin D do adults need?+
The US recommended dietary allowance is 600 IU (15 mcg) a day for adults up to 70 and 800 IU (20 mcg) over 70. The UK advises 400 IU (10 mcg) a day, especially in autumn and winter. These guidelines assume minimal sun exposure.
Is more sun always better for vitamin D?+
No. Production levels off after roughly one minimal erythemal dose because excess previtamin D is converted to inactive compounds. Beyond that point additional sun only increases sunburn and skin cancer risk.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.