How to use the ventilation calculator
Airtight, well-insulated homes need mechanical ventilation to remove moisture, CO₂ and indoor pollutants. ASHRAE Standard 62.2 is the reference for how much outdoor air a dwelling needs, and building codes base their rules on it. This calculator gives the whole-house rate from the floor area and number of bedrooms, converts it to air changes per hour and lists the kitchen and bathroom exhaust you need on top.
- Choose units. US mode works in square feet and cfm; metric mode works in square metres and litres per second, with m³/h shown too.
- Enter the conditioned floor area of the dwelling, the average ceiling height and the number of bedrooms.
- Set the fan run time if the system runs on a timer rather than continuously.
- Enter the kitchens and bathrooms to total the local exhaust requirement.
Ventilation formulas
ASHRAE 62.2-2016+: Q = 0.03 × A + 7.5 × (Nbr + 1) cfm · Q = 0.15 × A + 3.5 × (Nbr + 1) L/s
IRC / ASHRAE 62.2-2010: Q = 0.01 × A + 7.5 × (Nbr + 1) cfm · Q = 0.05 × A + 3.5 × (Nbr + 1) L/s
Intermittent fan rate ≈ Q ÷ run-time fraction
ACH = cfm × 60 ÷ ft³ · ACH = L/s × 3.6 ÷ m³
A is the conditioned floor area and Nbr is the number of bedrooms. The current ASHRAE rate is the total requirement before any infiltration credit; with a blower door result, some editions let you reduce the fan size. The intermittent figure is a simplified check, since the standard has extra limits on how long a fan may stay off.
| Local exhaust | On demand | Continuous |
|---|---|---|
| Kitchen | 100 cfm (50 L/s) | 5 ACH of kitchen volume |
| Bathroom | 50 cfm (25 L/s) | 20 cfm (10 L/s) |
Worked examples
A three-bedroom house
A 2,000 ft² house with three bedrooms and 8 ft ceilings needs 0.03 × 2,000 + 7.5 × 4 = 60 + 30 = 90 cfm under ASHRAE 62.2. The IRC formula gives 20 + 30 = 50 cfm. The house volume is 16,000 ft³, so 90 cfm is 90 × 60 ÷ 16,000 = 0.34 air changes per hour. If the fan runs only half of each hour, it needs to move about 180 cfm while it is on. One kitchen and two bathrooms add 100 + 2 × 50 = 200 cfm of on-demand exhaust.
A two-bedroom flat in metric
A 150 m² dwelling with two bedrooms and 2.5 m ceilings needs 0.15 × 150 + 3.5 × 3 = 22.5 + 10.5 = 33 L/s, or 118.8 m³/h. The volume is 375 m³, so the rate is 33 × 3.6 ÷ 375 = 0.32 ACH. That is a good size for a heat recovery ventilation unit running on its normal setting, with a boost mode for cooking and showers.
Who this calculator helps
- Architects sizing the ventilation strategy and plant space for new homes and deep retrofits.
- Energy raters and HERS raters checking a design against ASHRAE 62.2 quickly.
- HVAC designers and installers choosing an HRV, ERV, supply or exhaust fan.
- Self-builders and renovators who have made a house airtight and need to plan fresh air.
Practical tips
- Ventilation air is also a heat load. Use the heat loss calculator to see how much it adds, and why heat recovery pays back in cold climates.
- Check the cooling side with the BTU calculator and the envelope with the R-value calculator.
- Fans rarely deliver their rated flow once ducts are attached. Measure the installed flow and choose a fan with some margin.
- Supply fresh air to bedrooms and living rooms and extract from kitchens and bathrooms so air moves through the home.
Presenting a retrofit to a client? Show them the finished interior on a photo of their own home with Room Redesign.
