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Purge Ventilation Calculator — Openable Window Area
Calculate the minimum openable window area for purge ventilation per Approved Document F 2021 Table 1.4. Checks compliance for all habitable room types.
Width of the clear opening (not the whole frame)
Height of the clear opening
Number of identical openable windows
French / patio doors can contribute (Table 1.4)
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How We Calculate This
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.
Frequently Asked Questions
Purge ventilation is the ability to rapidly ventilate a room to dilute and remove high concentrations of pollutants — for example, after painting, applying chemicals, or burning food. Approved Document F requires all habitable rooms to have purge ventilation capability, typically provided by openable windows. It is not intended for continuous use; rather, it provides the ability to quickly flush out a room when needed.
Approved Document F 2021 Table 1.4 sets the minimum openable area: 1/20 of the room floor area for sliding sash windows, external doors, or hinged/pivot windows opening at least 30°; 1/10 for hinged/pivot windows opening 15–30°; windows opening less than 15° are not suitable for purge. For example, a 15m² room with a casement (≥30°) needs at least 0.75m² of openable area (≈4 air changes per hour).
Table 1.4 works on the free opening area a window actually clears to outside, and the required fraction depends on the opening angle, not the window style. A hinged or pivot window opening at least 30° (and sliding sash windows and external doors) need 1/20 of the floor area; a hinged or pivot window opening only 15–30° needs 1/10 — twice as much — because it clears a smaller gap for a given frame size; a window opening less than 15° is not suitable at all. Measure the clear opening the mechanism achieves (for a sliding sash, that is the half that slides), then compare it directly against the Table 1.4 area — no further percentage reduction is applied.
Yes. Approved Document F allows external doors (such as French doors, patio doors, or back doors) to contribute to the purge ventilation openable area. A standard 900mm x 2100mm door provides 1.89m² of openable area when fully open. However, building control officers may prefer that the purge requirement is met by windows alone, as doors may be locked or obstructed.
Yes. Approved Document F 2021 requires purge ventilation in all rooms, including kitchens, bathrooms, and WCs. These rooms still need the Table 1.4 openable area (1/20 of floor area for sash/door/≥30° hinged; 1/10 for 15–30° hinged). Purge is separate from mechanical extract — a bathroom needs both an extract fan (15 l/s intermittent or 8 l/s continuous) AND the Table 1.4 openable area. Internal rooms without external walls may use an alternative route through an adjacent room.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.