Understanding your result
The headline is the continuous outdoor-air rate the house needs, with the air changes per hour it gives and, if the fan runs only part of the hour, the airflow it must deliver while running. The stats repeat the continuous rate, the cycled fan size and the air changes. The table shows the two terms of the formula separately (the area term, which covers emissions from the building and its contents, and the occupant term, which covers people), the result, the 2010 rate for comparison, any infiltration credit, and the rate per person and per square foot.
The figure is outdoor air delivered to the house as a whole, and it can be supplied by a continuously running exhaust fan, a supply fan tied to the air handler, or a balanced energy or heat recovery ventilator. The ERV and HRV size calculator takes this rate and picks a unit; the bathroom exhaust fan calculator sizes the local exhaust that the standard requires separately in each bathroom; and the air changes calculator puts the rate next to the other common targets.
The air-change figure is a check. In a typical house with 8 ft ceilings, 62.2 works out to about 0.3 to 0.4 air changes per hour; a much higher figure means the house is small and full of people, a much lower one that the ceilings are high.
How we calculate this
The formula is the total required ventilation rate of ASHRAE Standard 62.2-2016, Ventilation and Acceptable Indoor Air Quality in Residential Buildings, which most energy codes and green-building programmes reference. It assumes two people in the first bedroom and one in each other; when the household is larger, the standard adds 7.5 CFM for each extra person, and the calculator does the same. The 2010 formula (0.01 per square foot) is shown beside it because many codes and existing houses were designed to it. The rates and the related local-exhaust minimums are collected on the ventilation and exhaust minimums page.
In metric mode the floor area is converted to square feet before the formula and the result shown in cubic metres per hour (1 CFM = 1.7 m³/h).
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Area term | 0.03 CFM per sq ft | ASHRAE 62.2-2016, section 4.1.1 |
| Occupant term | 7.5 CFM per person, bedrooms + 1 people | ASHRAE 62.2-2016 |
| 2010 comparison | 0.01 CFM per sq ft + 7.5 × (bedrooms + 1) | ASHRAE 62.2-2010 |
| Intermittent operation | Rate scaled by run-time fraction, at least once every 3 hours | ASHRAE 62.2 provisions for intermittent ventilation |
| Infiltration credit | 0 unless measured | Requires a blower-door test under the standard’s method |
| Local exhaust (separate) | Bathroom 50 CFM intermittent or 20 continuous; kitchen 100 intermittent or 25 continuous | ASHRAE 62.2 and IRC M1505/M1507 |
Assumptions last reviewed October 8, 2026.
The calculator does not apply the standard’s weighting for unbalanced systems or its credit formula in full, and it does not size the duct, choose the fan or check that the replacement air for an exhaust-only system comes from somewhere clean rather than from a garage or crawl space. Local codes may adopt an older edition with different numbers. The guide to whole-house ventilation and ASHRAE 62.2 explains the strategies, and the guide to humidity and comfort covers why ventilation air changes indoor humidity.
Two worked examples
A three-bedroom house with a continuous fan
2,000 sq ft, three bedrooms, four occupants, 8 ft ceilings, a fan running all the time, no infiltration credit.
- Area term: 0.03 × 2,000 = 60 CFM
- Occupant term: 7.5 × 4 = 30 CFM
- Ventilation rate: 90 CFM, continuous
- Air changes: 90 × 60 ÷ 16,000 = 0.34 per hour
- The 2010 formula would have required 50 CFM
A quiet 90 CFM fan rated for continuous duty in a central bathroom meets the requirement for this house. In a cold climate the 90 CFM of outdoor air costs about 4,900 BTU per hour of heat on a 20 °F day, which is the argument for an HRV.
A larger house with a fan on a timer
2,800 sq ft, four bedrooms, five occupants, 8 ft ceilings, a fan running 30 minutes in each hour.
- Area term: 0.03 × 2,800 = 84 CFM
- Occupant term: 7.5 × 5 = 37.5 CFM
- Ventilation rate: 121.5 CFM, shown as 122 CFM
- Fan size at 30 minutes per hour: 121.5 ÷ 0.5 = 243 CFM while running
- Air changes: 0.33 per hour
Running half the time doubles the fan size, which makes the fan louder and its duct larger. A continuous fan of about 120 CFM, or a cycle of 40 minutes per hour on a 180 CFM fan, is usually the quieter way to meet the same requirement.
Where to find your inputs
Floor area. The conditioned floor area from the plans, appraisal or listing; include a finished, conditioned basement.
Bedrooms. Rooms that are, or could be, used as bedrooms. The standard uses the number to estimate occupants.
Occupants. Only if the household is larger than bedrooms plus one.
Run time. 60 for a continuous fan; the setting on a cycle timer or ventilation controller otherwise.
Infiltration credit. From a blower-door test report that follows ASHRAE 62.2; leave it at zero without one.
Common mistakes
- Using the 2010 formula for a new house. The current standard asks for about 60% more air in a typical house.
- Counting the kitchen hood as ventilation. Local exhaust that runs only while cooking does not meet the continuous whole-house rate.
- Sizing a cycled fan to the continuous rate. A fan that runs half the time must move twice the air.
- A loud fan. A fan that bothers people gets switched off; choose 1 sone or less for continuous duty.
- Taking credit for leaks without a test. The infiltration credit needs a blower-door measurement.
- Over-ventilating. Extra outdoor air costs heating and cooling energy and, in humid climates, adds moisture.
Questions people ask
- How much ventilation does a house need?
- Under ASHRAE 62.2-2016, 0.03 CFM per square foot of conditioned floor area plus 7.5 CFM for each bedroom plus one. A 2,000 sq ft house with three bedrooms needs 0.03 × 2,000 + 7.5 × 4 = 90 CFM of continuous outdoor air, which is about a third of an air change per hour with 8 ft ceilings. A 1,200 sq ft two-bedroom house needs 58.5 CFM; a 3,500 sq ft five-bedroom house needs 150.
- What is the difference between ASHRAE 62.2-2010 and 62.2-2016?
- The area term. The 2010 edition used 0.01 CFM per square foot plus 7.5 CFM per bedroom plus one, so the same 2,000 sq ft three-bedroom house needed 50 CFM. The 2013 and later editions raised the area term to 0.03 and, in exchange, allowed an infiltration credit for leakier houses. Many state codes and the IRC's mechanical ventilation table still reflect the older numbers, so check which edition your code adopts.
- Can a bathroom fan provide whole-house ventilation?
- Yes, if it is quiet, rated for continuous duty, sized to the whole-house rate and runs continuously or on a cycle timer. A 90 CFM fan running all the time in a central bathroom meets the requirement for a typical 2,000 sq ft house. It is an exhaust-only strategy, so the replacement air comes in through leaks, which in a humid climate brings moisture with it; supply or balanced systems such as an ERV avoid that.
- What if the ventilation fan only runs part of the hour?
- It must move more air while it runs, in proportion. A fan that runs 30 minutes in every hour must deliver twice the continuous rate, and one that runs 20 minutes three times the rate. ASHRAE 62.2 allows intermittent operation as long as the fan runs at least once every three hours and the average meets the requirement; the calculator divides the rate by the share of each hour the fan runs.
- What is the infiltration credit?
- A reduction in the mechanical ventilation required for a house whose measured air leakage already supplies part of the fresh air. It needs a blower-door test, and the 2016 standard limits the credit, especially for unbalanced (exhaust-only or supply-only) systems, so most new tight houses get little or none. Enter the credit from the blower-door report if you have one; otherwise leave it at zero.
- Is more ventilation better?
- Up to the standard, yes; beyond it, each extra CFM is outdoor air that has to be heated in winter and cooled and dried in summer. At a 70 °F indoor and 20 °F outdoor temperature every 10 CFM of extra ventilation costs about 540 BTU per hour of heat. An ERV or HRV recovers 60 to 80% of that, which is why balanced recovery ventilation is the usual choice in cold and very humid climates.