THE Free Area Calculator

Aerodynamic free area, written Aa, is the effective area for airflow once turbulence and edge losses are accounted for. It is calculated as Aa = geometric area × Cv, where Cv is the discharge coefficient measured under BS EN 12101-2. Aa is the value that must satisfy BS 9991, BS 9999 and Approved Document B.

Approved Document B Volume 1 requires 1.5 m² free area in the common corridor or lobby (para 3.51(a)) and 1.0 m² at the head of the stair (para 3.52). Per ADB Appendix D5, this ‘free area’ can be measured as either geometric area or declared aerodynamic free area (Aa) per BS EN 12101-2. The SCA Guidance on Smoke Control (Rev 3.1) interprets these as equivalent to 0.9 m² Aa (corridor) and 0.7 m² Aa (stair). This calculator uses the SCA equivalence to ensure compliance.

Yes. A bottom-hung or side-hung AOV window only achieves its full geometric area at 90°. At 30° the projected opening is roughly half. The calculator applies sin(angle) to the geometric area for hinged vents, which mirrors the convention used in most UK AOV datasheets.

Hot smoke rises, so the opening needs to be at the top of the window for it to escape. A bottom-hung window (hinged at the sill) opens outward at the top, putting the aperture where the smoke layer collects. Top-hung windows (hinged at the head) open at the bottom: fine for everyday natural ventilation, not effective for smoke clearance.

BS EN 12101-2 covers natural smoke and heat exhaust ventilators (NSHEVs): typically louvred outlets at the head of a smoke shaft or external smoke louvres. BS EN 12101-8 covers smoke control dampers: motorised dampers inside smoke shafts, on corridor extract grilles, or in mechanical extract ducts. Both are sized on their certified Aa, but the test regimes and leakage classifications differ. For sign-off use the certified figure from the relevant Declaration of Performance.

The calculator uses sensible defaults based on UK manufacturer datasheets (Cv 0.50 for bottom-hung, 0.55 for side-hung, 0.65 for louvres and dampers, 0.70 for roof vents). These generic defaults apply the coefficient to the full face or projected opening and are conservative for estimation. For louvres and dampers especially, always verify against your product’s certified Aa in the Declaration of Performance: Vent Trade louvres typically achieve Cv 0.83–0.85 against the throat area. For final sizing and sign-off, use the product-picker mode or the manufacturer’s certified Aa directly.

The SCA Guidance on Smoke Control to Common Escape Routes (Rev 3.1) sets aerodynamic free area targets: 0.9 m² Aa for the ADB 1.5 m² common-corridor figure (para 3.51(a)), and 0.7 m² Aa for the 1.0 m² stair-head figure (para 3.52). This calculator uses these SCA-endorsed Aa equivalences as the baseline targets. Always use the manufacturer’s certified Aa from their BS EN 12101-2 or BS EN 12101-8 Declaration of Performance, not the generic Cv defaults shown here.

Generic Cv values are typical-case figures used when the actual vent isn’t known yet: they’re conservative on purpose. A certified Cv comes from a BS EN 12101-2 / -8 test conducted on a specific product configuration. When you toggle the calculator to ‘Pick a Vent Trade product’ mode, the result uses the certified Aa directly and computes the implicit Cv for reference. For louvres and dampers especially, generic Cv 0.65 may understate actual performance: Vent Trade louvres typically achieve Cv 0.83–0.85. For final sizing, always use the product-picker mode or refer to your product’s Declaration of Performance.

Yes: pick the ‘Basement’ scenario chip. Basement smoke ventilation is typically sized at 2.5% of the basement floor area (per BS 7346-4 / common UK convention). The calculator multiplies your floor-area input by 0.025 to set the target Aa.