About this site
howmuchsun answers two questions about sunlight that depend on where you are: how long you need outside to make vitamin D, and how long before your skin burns. Both are computed from solar geometry for around a thousand cities — nothing here is copied from another site or written by guesswork.
What it is
A calculator, not an article. Every figure comes from the position of the sun at your latitude and elevation, the ozone column above you, and two standard biological weighting functions. The methodology page gives the full chain, with citations.
There is no editorial content, no AI-written prose padding out the pages, and no attempt to tell you what to do about any of it. The numbers are the product.
Why it exists
Because "how much sun do I need?" has a real, computable answer that changes with latitude and season, and most sources either give a single global figure or none at all. The most useful fact on this site — that vitamin D synthesis stops completely for months at a time above roughly 50° latitude — is widely documented in the literature and almost never expressed as a date range for a specific city.
Who wrote it
This site is published anonymously. That is a deliberate choice, and it has a cost worth being upfront about: for health-related content, an unnamed author is a reason to be more sceptical, not less.
So rather than ask you to trust an author, the site is built to be checked:
- Every source is cited on the methodology page, with the specific paper, journal and year — not vague appeals to "studies".
- Every approximation is labelled as one, including the three places where the model is fitted or calibrated rather than derived, and the one place where a formula is knowingly used outside its published validity range.
- The limitations are listed in full, including the ones that make the numbers look worse.
- The solar geometry is independently verifiable. Solstice declinations, the equation of time and noon solar elevations are astronomical facts you can check against any ephemeris.
If a number here disagrees with a source you trust, the source is probably right and I would like to know.
What it is not
It is not medical advice, and it is not written by a clinician. It is a physics calculation about sunlight. Vitamin D status depends on far more than sun exposure — diet, supplements, age, body composition, medication, genetics — and both deficiency and excess carry real risks. Sun exposure itself carries skin cancer risk that no amount of vitamin D offsets.
Use it to understand how sunlight behaves where you live. Talk to a doctor about what to do with that.
Coverage
Around a thousand cities, chosen for search demand and for latitude spread rather than exhaustively. Vitamin D synthesis depends on latitude and barely at all on longitude, so two cities on the same parallel at the same elevation give nearly identical answers — generating every settlement on earth would produce a great many pages saying the same thing. Coverage will expand where it adds something.
Data and credits
- City coordinates, elevations, populations and timezones from
GeoNames (
cities1000), used under CC BY 4.0. - Solar position after the NOAA Solar Calculator, following Meeus, Astronomical Algorithms (2nd ed., 1998).
- Clear-sky UV index after Madronich (2007).
- Total column ozone after van Heuklon (1979).
- Local times via the IANA timezone database.