Purge Ventilation Calculator — Openable Window Area

This calculator determines whether a room meets the purge ventilation requirements of Approved Document F (2021 edition) Table 1.4 by comparing the available openable window area against the minimum requirement.

The requirement

Table 1.4 sets a minimum total open area as a fraction of the room floor area, chosen by the opening angle of the window — purge is required in all rooms (habitable rooms, kitchens, bathrooms, WCs, and utility rooms):

• Floor area ÷ 20 — sliding sash windows, external doors, and hinged or pivot windows opening ≥ 30°
• Floor area ÷ 10 — hinged or pivot windows opening 15–30°
• Not suitable — hinged or pivot windows opening < 15°, whatever their size (para 1.31)

The figures are based on BS 5925 (single-sided ventilation, urban, wind 2.1 m/s, 3°C inside–outside temperature difference) and are calibrated to deliver at least four air changes per hour direct to outside (para 1.27). Meeting the Table 1.4 area is the compliance test — the 4 ACH is the design intent already built into it.

How the check works

Enter the clear opening the window mechanism achieves (for a sliding sash, the half that slides; for a casement, the gap it swings clear). The calculator adds up the free opening of all the windows (and an external door, if used) and compares the total directly against the required Table 1.4 area — there is no further geometry reduction, because Table 1.4 already accounts for opening type through the angle bands above.