How big does a window need to be for enough daylight?
The rule of thumb is a glazing area of at least 10 percent of the floor area — but it's the glass, not the wall opening, that counts, and the top of the window decides how far the light reaches in.
Short answer: the classic rule of thumb is that the glazing area should be at least 10% of the room's floor area. A 14 m² room therefore needs roughly 1.4 m² of glass. Watch two things: it's the glass, not the wall opening, that counts — frame and sash typically eat 20-30% of the opening — and the rule says nothing about how far into the room the light actually reaches. That is decided by the window's head height: as a rough guide, daylight reaches two to two and a half times the height from the floor to the top of the window.
The 10% rule — and what it doesn't tell you
The most common rule of thumb for daylight in homes is that the glazing area should equal at least 10% of the floor area of the room the window sits in. It's easy to work with and lands roughly right for an ordinary living space with a vertical window in one exterior wall.
The maths for a room 4.0 × 3.5 metres:
- Floor area: 4.0 × 3.5 = 14.0 m²
- Required glazing area: 14.0 × 0.10 = 1.4 m²
But the rule is only a minimum, and a rough one at that. It takes no account of which way the window faces, how high it sits, how deep the room is, or what casts shade outside it. A room can meet the rule to the letter and still feel dark if the window sits low, the room is deep, and there's a neighbouring house 12 metres away.
If daylight needs to be documented for a building project, local building regulations set their own requirements, and they are stricter and more detailed than a rule of thumb — a professional adviser needs to calculate those. For sizing up a home you're considering buying, the 10% rule is plenty as a first filter.
Glazing area is not the same as the window's dimensions
The mistake that most often throws the maths off is using the wall opening's dimensions instead of the glass's. A window is made up of frame, sash and glazing bars, and none of those are transparent.
An example: a window measures 1.2 × 1.2 metres in the wall opening, so 1.44 m². Frame and sash typically take up 20-30% of that area — for a window this size, often around 25%. That gives an actual glazing area of 1.44 × 0.75 = 1.08 m².
For the 14 m² room that needed 1.4 m² of glass, one such window is therefore not enough. Either the wall opening needs to grow to around 1.4 ÷ 0.75 = 1.87 m² — for example 1.3 × 1.45 metres — or you need two windows.
Lots of glazing bars make the ratio worse. One large undivided pane makes it better. If you're measuring an existing window, measure the glass directly, edge to edge of what you can see through.
The top of the window decides how far the light reaches
A wide, low window and a narrow, tall window can have exactly the same glazing area and produce very different light in a room. The reason is that daylight from the sky comes in at an angle from above, and how far it reaches in depends on the height from the floor to the top of the window, not on the width.
The rule of thumb is that usable daylight reaches 2 to 2.5 times that height into the room:
- Window head at 2.1 metres above the floor: daylight to roughly 4.2-5.3 metres into the room.
- Window head at 1.8 metres above the floor: daylight to roughly 3.6-4.5 metres.
- Window head at 2.4 metres above the floor: daylight to roughly 4.8-6.0 metres.
If the room is deeper than that, the back of it goes dark no matter how wide the window is. The fix is to raise the head height — a taller window or a fanlight above — or to bring light in from another wall.
Width does something different, but just as important: it spreads the light evenly across the room and removes the dark corners beside the window. A wide window makes for a more even room; a tall window makes for a deeper one.
Light from two sides beats light from one
A room with windows in two walls feels noticeably brighter than a room with the same total glazing area gathered in a single wall. The reason is contrast: with light from one side, the wall opposite the window is lit while the side walls stay dark, and the eye reads that difference as gloom. With light from two sides, the shadows even out, and the whole room reads as bright.
That's why corner rooms almost always beat mid-terrace rooms of the same size — and why it's often a better investment to move part of the glazing area into a different wall than to enlarge the existing window.
What's outside can wipe out the whole calculation
The 10% rule assumes an unobstructed sky outside the window. Without a clear view of the sky, the daylight in the room drops without a single dimension on the window changing.
The shadow angle from an object outside is found as arctan(height ÷ distance). A neighbouring house 8 metres tall, 12 metres away, gives arctan(0.67) = 34°. All sky below 34 degrees of elevation in that direction is blocked, and with it all direct sun through the winter half of the year, when at northern latitudes the sun barely climbs above 10-12 degrees at the solstice.
Rule of thumb: if the shadow angle in the direction the window faces is above roughly 25-30 degrees, the glazing area needs to go above 10% for the room to read as bright. Above shadow angles of 45 degrees, the room is effectively relying on secondary, reflected light, and then it's more about light-coloured surfaces outside and in than about a few extra square centimetres of glass.
Five things that add daylight without a bigger window
- Light-coloured surfaces indoors. A white ceiling and pale walls bounce daylight deeper into the room. A dark ceiling absorbs it.
- Splayed reveals. Cutting the window reveal at an angle through a thick wall gives the room a larger glowing surface and a softer transition from glass to wall. In older houses with deep wall openings, this can make a noticeable difference.
- Light-coloured surfaces outdoors. A pale patio, gable end or fence panel outside the window bounces reflected light in. This matters most when the sky is overcast.
- Remove whatever shades it up close. A hedge, a carport post or a covered terrace right outside the glass often costs more light than a building further away.
- Hang the curtain rail wider than the window. So the curtains can be pulled fully clear of the glass instead of covering a quarter of it all day.
How to check an entire home
- Measure the floor area of every living space and work out 10% of it.
- Measure the glass in every window — edge to edge of the transparent part, not the wall opening.
- Measure the height from the floor to the top of the window and multiply by 2 to 2.5 to see how far the light reaches.
- Note which way each window faces, and what shades it in that direction.
If you have a floor plan drawn to scale, the first two steps are already done: the areas are there, and the window widths are there. In HouseSense, rooms and openings are measured when the home is scanned with the iPhone's LiDAR, and once the home is placed on the map, the app also knows its orientation, so the sunlight in the 3D model is calculated for that exact home and the date you choose.