Sunlight hours per room: how much sun a window actually gets
What decides the light in a room is not its compass direction but what stands opposite. Azima works the sun’s position minute by minute against the measured horizon around the building — and ends with two figures: how much sun in total, and how much of it in one unbroken stretch.
A south-facing window is not a promise. In a Gründerzeit block the sun spends half the morning behind the firewall opposite, and a north window on the fourth floor above a wide lane gets more light than a south window on the raised ground floor of a narrow courtyard. An orientation figure cannot express that difference — it does not know the building next door.
Try it on one address →What is computed — and where it stops
That is why the longest unbroken window always stands beside the total. Six times thirty minutes and one continuous afternoon add up to the same number and are opposites — anyone looking for a room to work in means the second.
Frequently asked
Where do the neighbours’ heights come from?
From the City of Vienna’s official building model and, where that is thin, from storey counts in OpenStreetMap. The sheet states what share of the neighbours actually carried a measured height — 90 % coverage is a different thing from 30 %, and the reader should be able to see which.
Why a single date rather than the whole year?
Because an annual average misses the question. The edges matter: 21 June says how warm it gets in summer, 21 December whether any sun arrives in winter. Both are in the report, along with the equinox.
Does the figure apply to the flat or to the window?
To the window, and per room the best-placed window is used. A flat rarely has one single exposure — which is why the table is per room rather than one number for everything.
Is this a certified daylight assessment?
No, and the sheet says so. A certificate under ÖNORM B 8110-3 or EN 17037 requires verified input geometry and a person who is liable for it. This is a measurement from open data — reproducible, but not an expert opinion.
Can I try it on one address?
Yes, without signing up. Enter an address and the calculation runs in the browser; a floor plan is not required — without one an example layout is shown inside the measured building volume, and it says so.
All measurements · For agents & developers · The light check
Basis: the City of Vienna building model (data.wien.gv.at, CC BY 4.0) and OpenStreetMap (ODbL). Solar positions computed astronomically. Not a certified daylight assessment under ÖNORM B 8110-3 or EN 17037.