Natural Light Calculator — Glazing Ratio & Daylight Factor

In England and Wales there is no mandatory glazing-to-floor ratio for daylight in the Building Regulations — daylight adequacy is assessed using BS EN 17037 daylight-factor or illuminance methods. Scotland’s Technical Handbook does set a rule: habitable rooms and kitchens should have glazing of at least 1/15th (about 6.7%) of the floor area. Older guidance used larger fractions (the historic model byelaw was 1/10th, and ADL1A 2010 non-mandatory guidance noted glazing “much less than 1/20th” may give poor light). A common rule of thumb for a 15 m² room is therefore around 1 m² of glazing at 1/15th, but this is guidance, not a legal minimum.

The glazing-to-floor area ratio is the total glazed window area divided by the room floor area, expressed as a percentage. For example, 3 m² of windows in a 15 m² room gives a ratio of 20%. There is no England/Wales legal minimum; Scotland’s guidance is 1/15th (6.7%). Note that the ratio is a sizing rule of thumb only and does not by itself prove a room is well daylit — obstructions, orientation and room depth all matter.

Approved Document O (Overheating) was introduced in 2021 (in force from June 2022) and applies to new residential buildings in England. It limits solar gains, which is most relevant for south and west-facing rooms with large glazing. Its simplified method does not use a single percentage: it caps the maximum glazing area by orientation and by whether the dwelling has cross-ventilation — broadly in the region of 11–18% of floor area for high-risk single-aspect rooms. A new dwelling generally needs a full Part O assessment; large glazing on south/west elevations is the main trigger for mitigation such as solar shading or larger openable areas.

The daylight factor is the ratio of indoor to outdoor illuminance under a standard overcast sky, expressed as a percentage. BS EN 17037:2018 replaced the old single average-daylight-factor approach; its UK National Annex gives per-room median daylight-factor targets of about 0.7% for bedrooms (100 lux), 1.1% for living rooms (150 lux) and 1.4% for kitchens (200 lux), achieved over 50% of the room. The daylight factor shown by this tool is a rough rule-of-thumb estimate, not an EN 17037 compliance result — an accurate average daylight factor uses total room surface area, surface reflectance and a maintenance factor (the BRE/CIBSE method).

A BRE rule of thumb is that the daylit depth of a room is roughly 2.5 × (window-head height − working-plane height), where the working plane is taken at about 0.85 m above the floor in dwellings. For a window head at 2.1 m this gives about 2.5 × (2.1 − 0.85) ≈ 3.1 m of well-daylit depth. Rooms deeper than this tend to have poor daylight at the back and benefit from higher window heads, roof lights or light wells.