Walk into any modern Australian commercial building where the mechanical fresh air was undersized, and you notice it within five minutes: heavy air, 3 PM brain fog, and stale smells lingering in meeting rooms. With energy ratings pushing building envelopes tighter than ever, spaces can no longer rely on leaky wall cavities or openable windows for statutory fresh air.
Under the National Construction Code (NCC) and AS 1668.2:2012, mechanical ventilation is legally mandated whenever natural ventilation cannot deliver 5% openable floor area. If your fresh air louvres, toilet exhaust fans, or basement car park shafts fall short of certified airflow targets, private building certifiers will not issue an Occupancy Certificate.
Getting statutory air volume right protects occupant health and prevents costly compliance defects. Whether you are balancing an office outdoor air intake, sizing duct runs for a 4-pan commercial amenities block, or verifying basement car park ventilation, this guide breaks down the exact formulas, velocity limits, and compliance thresholds used across Australian job sites. If you are also running ducted heating and cooling lines, pair your ventilation rates with our Duct Sizing & Airflow Calculator and whole-building Residential Heat Load Calculator.
AS 1668.2 Minimum Outdoor Air Supply Rates
Australian Standard AS 1668.2 Table A1 sets statutory minimum outdoor air supply rates to dilute carbon dioxide, body odours, and indoor volatile organic compounds (VOCs). The total required outdoor air supply is calculated as the greater of the occupant-based rate and the minimum floor-area rate.
For general commercial office spaces, the benchmark requirement is 10 L/s per person, backed by an absolute floor-area minimum of 1.0 L/s per square metre. Meeting rooms and high-density training facilities require identical per-person rates but feature higher floor allowances (1.5 L/s·m²) to accommodate packed peak headcounts.
| Occupancy Type | Outdoor Air Rate (L/s·person) | Minimum Floor Rate (L/s·m²) | NCC Default Density (m²/person) | Typical Airflow for 100m² Space |
|---|---|---|---|---|
| Commercial Office | 10 L/s | 1.0 L/s·m² | 10 m² / person (10 staff) | 100 L/s (360 m³/h) |
| Meeting / Boardroom | 10 L/s | 1.5 L/s·m² | 2.0 m² / person (50 people peak) | 500 L/s (1,800 m³/h) |
| Restaurant / Dining Area | 10 L/s | 1.5 L/s·m² | 1.5 m² / person (66 diners) | 660 L/s (2,376 m³/h) |
| Gymnasium / Fitness Centre | 12 L/s | 2.5 L/s·m² | 4.0 m² / person (25 athletes) | 300 L/s (1,080 m³/h) |
| School Classroom | 10 – 12 L/s | 1.5 L/s·m² | 2.5 m² / person (40 students) | 400 – 480 L/s |
| Retail Shop / Showroom | 10 L/s | 1.0 L/s·m² | 3.0 m² / person (33 shoppers) | 330 L/s (1,188 m³/h) |
| Auditorium / Cinema | 10 L/s | 1.5 L/s·m² | 1.2 m² / person (83 patrons) | 830 L/s (2,988 m³/h) |
Toilet & Bathroom Mechanical Exhaust Rates
Mechanical exhaust systems for sanitary compartments remove moisture, aerosols, and odours while maintaining continuous negative pressure so air never flows back into habitable rooms or hallways. AS 1668.2 Clause 5.4 and NCC 2022 Part 10.8 specify strict exhaust volumes for commercial and residential bathrooms.
In commercial buildings, shopping centres, and schools, exhaust ventilation is sized on fixture counts:
- Toilet Pans (WC Cubicles): Mandatory 25 L/s per pan.
- Urinals: Mandatory 25 L/s per bowl (or per 600mm length of continuous wall trough).
- Minimum Room Threshold: A commercial amenities block must never drop below 50 L/s total exhaust or 10 Air Changes per Hour (ACH), whichever is higher, regardless of how few cubicles exist.
Residential Bathroom & Laundry Exhaust Rates (NCC 2022)
In residential dwellings (NCC Class 1 houses and Class 2 apartments), statutory mechanical exhaust rates depend on whether the ventilation runs continuously or intermittently with a timer:
- Bathrooms with Shower or Bath: Minimum 50 L/s intermittent (typically wired to the light switch with a run-on timer) or 25 L/s continuous.
- Separate Water Closets (WC / Powder Rooms): Minimum 25 L/s intermittent.
- Laundries: Minimum 40 L/s intermittent or 20 L/s continuous to eliminate moisture from unvented tumble dryers.
- Mandatory Direct External Discharge: NCC 2022 strictly prohibits discharging bathroom exhaust air into unconditioned ceiling roof spaces; all ducting must vent directly outdoors via an eave vent, external wall louvre, or roof cowl.
Basement Car Park Ventilation (Section 4)
Enclosed and underground car parks present extreme life safety risks due to carbon monoxide (CO) from petrol engines and nitrogen dioxide (NO₂) from diesel vehicles. AS 1668.2:2012 Section 4 governs statutory car park ventilation across Australia.
Under the deemed-to-satisfy requirements of Clause 4.4, mechanical car park extraction requires:
- Baseline Area Airflow: A continuous extraction rate of 5.0 L/s per square metre of total enclosed car park floor area.
- Alternative Vehicle Space Method: Sizing based on operating car bays and travel distance (Clause 4.4 formula Q = 2000 × N₁ + C), typically averaging 400 L/s per active parking bay.
- Supply Air Inflow: Make-up air must enter either through natural access ramps and transfer grilles (with maximum air velocity of 2.5 m/s across pedestrian pathways) or dedicated mechanical supply fans delivering at least 75% to 85% of the exhaust capacity.
Clause 4.8 Carbon Monoxide (CO) Monitoring Energy Savings
Running massive 5.0 L/s·m² car park exhaust fans 24/7 consumes tens of thousands of kilowatt-hours in unnecessary electrical power each year. Under AS 1668.2 Clause 4.8, installing a compliant multi-point CO monitoring sensor array allows the ventilation system to modulate down to 1.5 – 2.0 L/s·m² during quiet hours when CO concentrations remain below 30 ppm.
When morning or evening commuter vehicles enter or exit, the automated sensor system instantly ramps the multi-speed or VSD fans back up to 100% capacity (5.0 L/s·m²), ensuring full occupant life safety while slashing annual facility power bills by up to 60%.
How to Calculate Outdoor Airflow (Worked Example)
To demonstrate how commercial mechanical engineers and HVAC contractors calculate fresh air requirements under AS 1668.2, consider a standard commercial office tenancy in Melbourne:
Worked Engineering Example: Level 2 Commercial Office Suite
• Conditioned Floor Area: 120 m²
• Ceiling Height: 2.7 metres → Room Volume: 120 × 2.7 = 324 m³
• Design Headcount: 14 desk workstations
• Step 1 (Occupant Rate): 14 people × 10 L/s = 140 L/s
• Step 2 (Floor Minimum Rate): 120 m² × 1.0 L/s·m² = 120 L/s
• Step 3 (Governing Flow): Maximum(140 L/s, 120 L/s) = 140 L/s
• Step 4 (Volume Conversion): 140 L/s × 3.6 = 504 m³/h (297 CFM)
• Step 5 (Air Changes Check): 504 m³/h ÷ 324 m³ = 1.56 ACH fresh outdoor air turnover
• Step 6 (Duct Sizing at 4.0 m/s): Area = (0.14 m³/s) ÷ 4.0 = 0.035 m² → 250mm Round Spigot
AS 1688 vs AS 1668.2: The Standard Number Explained
Across Australian search engines, hundreds of HVAC apprentices, certifiers, and building managers search monthly for a "ventilation rate calculator as1688" or "as1688 ventilation calculator". This is a very common transposition typo for AS 1668.2.
In Australian engineering legislation, there is no building ventilation code numbered AS 1688. The official Standards Australia publication governing mechanical ventilation is AS 1668.2:2012 (incorporating Amendment 2:2016), titled:
"The use of ventilation and airconditioning in buildings — Part 2: Mechanical ventilation in buildings."
This standard works in tandem with AS 1668.1 (Fire and smoke control in multi-compartment buildings) and NCC Section F4. If an architectural specification or council RFI requests compliance with ventilation rates, you are designing strictly to AS 1668.2.
Converting Air Changes per Hour (ACH) to L/s
Air Changes per Hour (ACH) measures how many times the entire volume of air in an enclosed room is completely replaced with outdoor or treated air in a 60-minute period. While European and US specifications frequently quote ACH, Australian mechanical systems are specified in Litres per second (L/s).
To convert between the two metrics:
Airflow Rate (L/s) = [Room Volume (m³) × Target ACH] ÷ 3.6
Airflow Rate (m³/h) = Room Volume (m³) × Target ACH
Actual ACH Achieved = [Airflow (L/s) × 3.6] ÷ Room Volume (m³)
Typical Australian ACH engineering guidelines across common commercial spaces include:
- General Office Spaces: 4 – 8 ACH total circulation (with 1.5 – 2.0 ACH pure outdoor fresh air).
- Commercial Bathrooms / Restrooms: 10 – 15 ACH exhaust extraction.
- Commercial Kitchens: 20 – 35 ACH high-velocity grease and steam exhaust.
- Underground Car Parks: 4 – 6 ACH emergency smoke/exhaust purge.
- Laboratories & Cleanrooms: 15 – 30+ ACH with HEPA filtration.
Make-Up Air, Door Undercuts & Pressure Balance
A mechanical exhaust fan cannot extract air from a room unless an equal volume of replacement make-up air can enter. When bathroom or kitchen exhaust fans are installed without dedicated transfer grilles or adequate door undercuts, rooms enter deep negative pressure.
This negative pressure starves fan impellers, drops airflow delivery by over 50%, creates annoying door whistles, and makes bathroom doors physically difficult to push open. Under AS 1668.2 Clause 5.2 and good trade practice:
- Door Undercut Clearance: For domestic and small commercial toilets discharging 25 to 50 L/s, provide a minimum 15mm to 20mm clear undercut beneath the bottom of the door leaf across a 820mm standard door (providing ~0.012 m² free open area).
- Acoustic Door Transfer Grilles: Where toilet exhaust exceeds 75 L/s, door undercuts become noisy; install an acoustic chevron transfer grille in the lower door panel or ceiling baffle plenum.
- Make-Up Air Velocity: Keep make-up air passing under doors or through relief grilles under 1.5 to 2.0 m/s to eliminate whistling and air turbulence.