UVB Vitamin D Synthesis Calculator
Your skin makes vitamin D from UVB — but how long that takes depends on two things you can know right now: today's UV index and your skin type. This tool estimates a sensible midday exposure window from a peer-reviewed model. It suggests sunlight minutes only — never supplement doses.
Check any weather app for your current or midday UV index
Based on how your skin typically responds to sun
Enter your values above to see your results.
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How it works
The model
The estimate is based on a peer-reviewed UV index model (Kallioğlu et al., Scientific Reports 2024), calibrated against Holick's widely cited rule: about a quarter of body surface (arms and legs) receiving a quarter of a minimal erythema dose produces roughly 1,000 IU of vitamin D. Time scales linearly with your skin type's minimal erythema dose (MED) and inversely with UV index.
Why skin type changes the answer so much
Melanin competes with vitamin D precursors for UVB photons. The model uses Fitzpatrick-type MEDs from 200 J/m² (Type I) to 1000 J/m² (Type VI) — a 5× spread. Classic experimental work (Clemens et al., Lancet 1982) found deeply pigmented skin may need up to ~6× the UV dose for the same vitamin D response; later studies debate the exact figure, so treat multiples as approximate.
What this estimate assumes — read this
Midday sun, arms and legs uncovered, no sunscreen (SPF 15+ blocks ~95% of UVB), outdoors — ordinary window glass blocks UVB entirely. Cloud cover, altitude, age (skin synthesis drops substantially by age 70), and body composition all shift the real number. The estimate is a starting point, not a prescription.
Safety boundary
More sun is not better: the estimate is already calibrated to meaningful synthesis, and every result stays below the time unprotected skin takes to redden. Chronic overexposure is an established skin cancer risk. This tool never recommends supplement doses — whether to supplement, and how much, is a decision for your physician with your lab results.
Frequently asked questions
How much sun do I need for vitamin D?
It depends on UV index and skin type. In strong midday summer sun, lighter skin types may need only a few minutes of arms-and-legs exposure, while the deepest skin types may need several times longer. Below UV index 3 — common in winter and at high latitudes — synthesis is negligible at any duration.
Where do I find the UV index?
Any weather app shows it, and the US EPA and National Weather Service publish daily forecasts. Use the midday value for the most realistic estimate, since UV index peaks around solar noon.
Can I get vitamin D through a window?
No. Ordinary glass blocks essentially all UVB — the wavelength your skin needs. Sitting in a sunny spot indoors does not count.
Does sunscreen stop vitamin D production?
Largely yes — SPF 15 or higher blocks around 95% or more of UVB. The estimate on this page assumes unprotected skin for the stated short window, followed by normal sun protection for any longer time outdoors.
Should I take a vitamin D supplement instead?
That is a decision for your physician, ideally guided by a 25(OH)D blood test. This tool deliberately estimates sunlight exposure only and never suggests supplement doses.
Sources
- Kallioğlu MA, Sharma A, Kallioğlu A, et al. UV index-based model for predicting synthesis of (pre-)vitamin D3 in the Mediterranean basin. Scientific Reports, 2024.
- Holick MF. Vitamin D deficiency. N Engl J Med, 2007.
- Clemens TL, Adams JS, Henderson SL, Holick MF. Increased skin pigment reduces the capacity of skin to synthesise vitamin D3. Lancet, 1982.
This calculator is for informational purposes only and is not medical advice, diagnosis, or treatment. Reference ranges are general population guidelines — always discuss your lab results with a qualified healthcare professional.