Window Sunlight Calculator
Set a window’s direction, size and sill height to estimate when direct sun enters and how far it reaches across a room.
Your window in the light.
Sun angle 52.4° · direct sun now · Use the timeline or edit orientation to update
Illustrative scene; drawn objects are decorative · 0.6 m modeled floor reach (8 m room) · flat ground and your own obstacles. Change window dimensions and orientation under “Edit dimensions”.
Light on this day
Europe/Lisbon · Select any event to move the map to that instant. Times are displayed to the minute; selection keeps the calculated instant.
Sunrise and sunset use an open horizon and atmospheric refraction. Dawn/dusk use the sun’s center at −6°, −12° and −18°. Golden light uses −4° to +6°: a planning convention, not a weather forecast. Calculation notes →
7 h 28 m modeled direct sun at this point · 0 user-entered obstacles · two-minute samples · clear sky assumption.
FROM DIRECTION TO DURATION
When does direct sunlight reach here?
2026-09-18 · Lisbon, Portugal · Europe/Lisbon
Each bar shows direct sun within that local clock hour. Clock changes can skip or repeat an hour; repeated-hour totals may exceed 60 minutes.
Read hourly values
| Local hour | Direct sun |
|---|---|
| 00:00 | 0 h 00 m |
| 01:00 | 0 h 00 m |
| 02:00 | 0 h 00 m |
| 03:00 | 0 h 00 m |
| 04:00 | 0 h 00 m |
| 05:00 | 0 h 00 m |
| 06:00 | 0 h 00 m |
| 07:00 | 0 h 00 m |
| 08:00 | 0 h 00 m |
| 09:00 | 0 h 00 m |
| 10:00 | 0 h 00 m |
| 11:00 | 0 h 00 m |
| 12:00 | 0 h 52 m |
| 13:00 | 1 h 00 m |
| 14:00 | 1 h 00 m |
| 15:00 | 1 h 00 m |
| 16:00 | 1 h 00 m |
| 17:00 | 1 h 00 m |
| 18:00 | 1 h 00 m |
| 19:00 | 0 h 36 m |
| 20:00 | 0 h 00 m |
| 21:00 | 0 h 00 m |
| 22:00 | 0 h 00 m |
| 23:00 | 0 h 00 m |
Show direct-sun time windows (1)
- 12:08 WEST–19:36 WEST
Clear-sky geometry, not a weather forecast. Two-minute sampling; time-window edges are approximate. Sun must face the selected window. 0 modeled obstacles; no terrain horizon loaded. Missing geometry is not proof of an unobstructed site.
What reaches your window?
Facing direction changes when direct sunlight can enter. The floor reach below uses your window height and sill.
Illustrative 8 m deep room. The far edge uses sill + window height. Width affects the patch’s width, not this distance; furnishings and overhangs are not included.
| Date | Direct sun | Explore date |
|---|---|---|
| march Equinox2026-03-20 | 7 h 26 m | |
| june Solstice2026-06-21 | 8 h 02 m | |
| september Equinox2026-09-23 | 7 h 24 m | |
| december Solstice2026-12-21 | 6 h 50 m |
Two-minute midpoint samples. 0 modeled obstacles. Counts sunlight facing the window. Clear-sky geometry, not a weather forecast.
A different season. A different light.
Keep your place and jump through the year.
Find your alignment.
Choose what you want to see from Lisbon, Portugal.
1 Choose your target
The pin is where you stand. Mark the landmark or direction you want the sun to line up with.
2 Choose the light
Set precise angles and tolerance
3 Pick your date range
Select a map target first, or open precise angles to enter a direction.
The target sets a compass direction, not the height of a landmark. Windows show when the Sun’s center is within the selected angular tolerance. Terrain, buildings and weather are not included in this alignment search. Near-horizon geometry is not a guarantee of a visible sunrise.
Window Sunlight and Indoor Sun-Reach Calculator
The window calculator links an outdoor sun position to a simple cutaway room. It estimates whether the sun faces the window and, when it does, how far the direct-light patch can travel across the floor.
Use this when: Use this when the primary question is direct sun entering a window or reaching across a simple room, not a plan-view sun path.
Inputs this tool uses
- A location, date, local time and IANA time zone. Repeated or skipped DST times must resolve to a specific UTC instant.
- Window width, height and sill height in metres, plus the window’s facing bearing in degrees clockwise from true north.
- Optional measured outdoor screens and receiver height. These simplify outside obstructions; they do not import a measured facade.
What this tool calculates
- Whether the sun is in front of or behind the window plane
- Approximate floor penetration from solar altitude and window geometry
- Schematic beam position for an oblique solar direction
- Blocking from the outdoor obstacles already drawn on the map
How the estimate is made
The space simulator first checks that the sun is above the horizon, within 90° of the window bearing, and not blocked by an enabled map obstacle. For a clear beam it computes reach = ((sill + window height) / tan(altitude)) × cos(angular difference), then clamps the result to 0–8 m. The page labels this output direct-beam reach. Window width is stored and shown as a scene input, but does not currently change this numeric reach equation.
How to use it
- 01
Aim the window
Set the outward-facing bearing from true north: a south-facing window is 180°. This is the direction you look while looking straight out through the window.
- 02
Enter its dimensions
Measure clear glazed width, height and the sill’s height above the floor.
- 03
Add the outside context
Add verified outside blockers as screens. Roof overhangs and balcony slabs are not represented by this vertical-screen model.
Worked example
With a 0.9 m sill, 1.5 m window height, a window facing the sun, and solar altitude 30°, the reach is 2.4 / tan(30°) = 4.2 m. At a 60° bearing difference the cosine factor halves that to about 2.1 m. If the sun is behind the window or a selected obstacle blocks it, the displayed result is 0.0 m.
How to read the result
Sun reach
This is the estimated perpendicular distance of direct light across a level floor. It is not the overall brightness of the room.
No direct sun
A zero can mean night, the sun is behind the window, or a modeled obstacle blocks the solar ray.
What to do next
Measure the clear window opening and sill, set the facing bearing, and test the dates and times when direct light matters. Use a site visit to check curtains, reveals, glazing and overhangs that this simple room model does not include.
Limits you should know
- Glass coatings, curtains, deep reveals and roof overhangs are not yet modeled.
- Diffuse daylight and reflected light can brighten a room even when direct sun reach is zero.
- Measurements assume a level floor and a vertical window plane.
Questions about this calculator
Does a north-facing window always get no sun?
No. The answer depends on hemisphere, season, latitude and obstructions. Near the tropics, the sun can pass north or south of a location during different months.
Is sun reach the same as daylight?
No. Sun reach tracks the direct beam. Daylight also includes diffuse sky light and reflections from surrounding surfaces.
Methodology version 2.0 · Reviewed September 20, 2026