Natural Ventilation Calculator

Calculate purge vent area, background ventilation and extract rates per UK Approved Document F.

Room Type

Room Dimensions (mm)

Length

Width

Floor-to-ceiling height

Openable window area (optional, for ACH estimate)

m²

Results

Room Volume

—

m³

Floor Area

—

m²

ACH Estimate

—

changes/hr

Purge vent area —

Background vent —

Extract rate —

Part F Requirements

Purge vent (habitable) 1/20 of floor area

Purge vent (wet room) 1/20 of floor area

Background (habitable) 8000 mm² ea

Background (wet room) 4000 mm² ea

Extract — Kitchen 60 l/s (or 30 adj)

Extract — Bathroom 15 l/s

Extract — WC 6 l/s

Extract — Utility 30 l/s

Trickle Vent Sizes

Small trickle vent 2500 mm²

Standard trickle vent 4000 mm²

Large trickle vent 8000 mm²

Slot vent (over-frame) 5000 mm²

ACH Estimate

Air changes per hour (ACH) is estimated from openable window area using ACH = (opening area × wind speed × 3600) / volume. Assumes 1 m/s average wind speed. Real ACH depends on wind, stack effect and building airtightness.