Roof Window Sizing Calculator — What Size Velux Do I Need?

This calculator estimates the size and number of roof windows for a loft room from the room floor area and room type. It applies a daylighting rule of thumb and then picks the smallest standard VELUX size that meets it, before prompting you to check Approved Document B escape provision.

The method

Target glazing area = Floor area × Glazing fraction (1/6 for habitable rooms)

Window size = smallest VELUX size whose effective daylight area meets the target (or the fewest of the largest size if no single window is big enough).

Is the 1/6 figure a regulation?

No. England’s Building Regulations set no minimum daylight glazing area — Part L sets fabric U-values and Part O sets glazing maxima for overheating, but neither sets a minimum. The 1/6 (habitable) and 1/8 (bathroom) ratios are widely-used good-practice rules of thumb, not a clause from any current standard. For a rigorous assessment use BS EN 17037:2018+A1:2021 (daylight-factor / illuminance method). Scotland is the only UK nation with a statutory ratio: 1/15 of floor area (Scottish Technical Handbook s.3.16).

Glazing fractions (rules of thumb)

• Habitable rooms (bedroom, living, kitchen, study): ~1/6 of floor area
• Bathrooms: ~1/8 of floor area

Standard VELUX sizes (effective daylight area)

• CK02: 550 × 780mm — 0.22 m²
• MK04: 780 × 980mm — 0.47 m²
• MK06: 780 × 1180mm — 0.59 m²
• SK06: 1140 × 1180mm — 0.95 m²
• UK08: 1340 × 1400mm — 1.40 m²

Areas are VELUX’s published effective daylight areas (the glass area that admits light), not the larger geometric pane area.

Standards

Daylighting good practice: BS EN 17037:2018+A1:2021 (Daylight in buildings). Emergency escape windows: Approved Document B Vol 1 §2.10 (openable area ≥ 0.33 m², no dimension < 450mm, bottom of openable area ≤ 1100mm above floor). Roof-window installation: follow the manufacturer’s fitting instructions (e.g. VELUX) together with BS 5534:2014+A2:2018 (slating and tiling — code of practice) for the roof-covering interface. Note BS 8213-4 (survey and installation of windows and external doorsets) does notapply to roof windows or rooflights — its scope is limited to windows installed vertically into the external face of a building. England’s Part L sets glazing U-values, not daylight area.