Window to Floor Ratio Calculator
Glazing area as a share of floor area — the first check on whether a room gets enough daylight, and the first warning that it will overheat.
The rule of thumb is that glazing should be 10 to 20% of the floor area of the room it serves.
How the window to floor ratio calculator works
The rule of thumb is that glazing should be 10 to 20% of the floor area of the room it serves. Below 10% a room is dim in daytime; above 25% it gains and loses heat faster than most envelopes can manage.
The ratio is a screening tool, not a daylight calculation — orientation, obstruction and glass type all matter — but a room that fails it will not be rescued by them.
Formula: ratio = glazing area / floor area
Worked examples
| Inputs | Window to floor ratio | Note |
|---|---|---|
| Two windows in a 27 m² room | 15.56% | 15.6% — comfortable |
| One small window | 4.44% | under-glazed |
| A glass wall | 44.44% | heavily glazed |
FAQFrequently asked questions
What window to floor ratio is right?
Ten to twenty per cent for most rooms. Living rooms toward the top of that; bedrooms and service rooms toward the bottom.
Is more glass always better for daylight?
Up to about 25% the daylight gain is real. Beyond it the room is already bright and the extra glass mainly adds heat gain and loss.
Does orientation matter?
Enormously — north-facing glass in the southern hemisphere is an asset; west-facing is a liability. This ratio is the check before orientation, not instead of it.
What is the frame factor?
The share of a window opening that is actually glass. Frames take 15 to 30%, more on small windows.
Does this satisfy a code daylight requirement?
Some codes use exactly this ratio as the deemed-to-satisfy path, often at 10%. Others require a daylight-factor calculation, which the daylight page does.
Where these figures come from
- Sabine (1922) — Collected Papers on Acoustics — the reverberation-time equation
- CIBSE Guide A — Environmental design — daylight, comfort and ventilation criteria
- Neufert — Architects' Data — the standard reference for space and dimension
- International Code Council — International Building Code — occupant load and egress factors
- Building Regulations — Approved Documents — the UK building regulations
Last checked: September 2026. Formulas are the standard building-science and code relationships; default thresholds are common international values and should be replaced with the ones in your code.