Air Change Rate (ACH) Calculator
Calculate the air changes per hour (ACH) for a room or building. Determine ventilation rate from airflow volume and room size.
Ventilation CFM Calculator
Calculate the required CFM ventilation rate for a room or building. Determine airflow needs based on room size, occupancy, and ASHRAE standards.
sq ft
ft
%
ASHRAE 62.2 Minimum⧉
90.0 CFM
Based on 4 occupants, 2,000.00 sq ft
0.35 ACH Method⧉
93.3 CFM
Volume: 16,000.00 cu ft
Per-Person (15 CFM)⧉
60.00 CFM
4 occupants x 15 CFM
Recommended CFM⧉
93.3 CFM
Higher of ASHRAE 62.2 and 0.35 ACH
Duct Loss Adder⧉
+9.3 CFM
10% leakage allowance
Fan Size Needed⧉
110.00 CFM
Rounded up to nearest 10 CFM
Actual ACH⧉
0.35 ACH
Air changes per hour at recommended CFM
HRV Heat Recovery⧉
3,024.00 BTU/hr
Energy saved with heat-recovery ventilator at 30F delta
CFM Method Comparison
ASHRAE 62.2
90.0 CFM
0.35 ACH
93.3 CFM
Per-Person
60.0 CFM
Selected Room Spot Check
| Metric | Value |
|---|---|
| Room Type | Living / Family Room |
| Avg Room Volume | 2,286.00 cu ft |
| Supply CFM Needed | 13.3 CFM |
| Exhaust Requirement | None (supply only) |
ASHRAE 62.2 Exhaust Requirements by Room
| Room | Continuous (CFM) | Intermittent (CFM) |
|---|---|---|
| Kitchen | 5 ACH or 100 CFM range hood | 100 |
| Bathroom | 20 | 50 |
| Utility / Laundry | -- | 50 |
| Attached Garage | Continuous exhaust recommended | 100 |
Ventilation Sizing Reference
| Home Size (sq ft) | Bedrooms | ASHRAE 62.2 CFM | 0.35 ACH CFM |
|---|---|---|---|
| 1,000.00 | 2 | 53.00 CFM | 47.00 CFM |
| 1,500.00 | 3 | 75.00 CFM | 70.00 CFM |
| 2,000.00 | 3 | 90.00 CFM | 93.00 CFM |
| 2,500.00 | 4 | 113.00 CFM | 131.00 CFM |
| 3,000.00 | 4 | 128.00 CFM | 158.00 CFM |
| 4,000.00 | 5 | 165.00 CFM | 233.00 CFM |
Planning notes, formulas, and examples
About the Ventilation CFM Calculator
CFM (cubic feet per minute) is the standard unit for measuring airflow in ventilation systems. The right amount of ventilation helps maintain indoor air quality by diluting CO2, moisture, VOCs, and other contaminants. Too little ventilation can create comfort and air-quality problems; too much can waste energy.
ASHRAE residential and commercial ventilation standards are commonly used to size minimum outside-air flow rates. Residential guidance often uses floor area and bedroom count, while commercial calculations usually account for occupancy and floor area by zone.
This calculator determines the required CFM for your space based on those common ventilation rules. It covers both residential and general occupancy calculations, helping you size ventilation fans, ERVs, and fresh-air systems.
When This Page Helps
Proper ventilation sizing helps prevent indoor-air-quality problems while avoiding oversized systems that waste energy. This calculator applies common ASHRAE-based sizing rules to give you a practical CFM target for your space.
How to Use the Inputs
- Enter the floor area of the space.
- Enter the number of bedrooms (residential) or occupants (commercial).
- Select the space type for appropriate standards.
- Review the required CFM ventilation rate.
- Size your ventilation equipment accordingly.
Formula used
ASHRAE 62.2 Residential: CFM = 0.03 × Floor Area + 7.5 × (Bedrooms + 1)
General: CFM = (ACH × Volume) / 60
Per Person: CFM = Occupants × CFM_per_personExample Calculation
Result: 90 CFM required (ASHRAE 62.2)
For a 2,000 sq ft home with 3 bedrooms: CFM = 0.03 × 2,000 + 7.5 × (3 + 1) = 60 + 30 = 90 CFM. This is the continuous mechanical ventilation rate needed for healthy indoor air.
Tips & Best Practices
- ASHRAE 62.2 assumes continuous operation — intermittent fans need higher CFM.
- Kitchen range hoods (100–400 CFM) and bath fans (50–110 CFM) provide local exhaust.
- Balanced ventilation (ERV/HRV) is more efficient than exhaust-only.
- Duct sizing must match CFM — undersized ducts reduce airflow and increase noise.
- In very tight homes (ACH50 < 3), mechanical ventilation is mandatory.
- Verify actual CFM with a flow hood after installation.
ASHRAE 62.2 in Practice
A commonly used residential sizing rule is 0.03 CFM per square foot of floor area plus 7.5 CFM per person (estimated as bedrooms + 1). This formula produces ventilation rates of roughly 60–120 CFM for many homes, which is often achievable with a single ERV or HRV unit.
Ventilation Strategies Compared
Exhaust-only (bath fan on a timer): simple, low-cost, but no heat recovery. Supply-only (fan coil or ERV supply): positive pressure, filters incoming air. Balanced (ERV/HRV): best energy performance, recovers 60–80% of heating/cooling energy. In cold climates, balanced ventilation with heat recovery is a common modern approach.
Oversizing Ventilation
More ventilation is not always better. Oversized systems waste energy, can cause comfort issues (drafts), and in humid climates can bring in excess moisture. Size the system to the relevant minimums and use demand control (such as CO2 sensors) if needed for spaces with variable occupancy.
Sources & Methodology
Last updated:
Frequently Asked Questions
Per ASHRAE 62.2: CFM = 0.03 × floor area + 7.5 × (bedrooms + 1). A 2,000 sq ft, 3-bedroom home needs 90 CFM continuous ventilation. This is the minimum for healthy indoor air quality.
ASHRAE recommends 50 CFM for bathrooms up to 100 sq ft, and 1 CFM per square foot for larger bathrooms. For bathrooms also serving as ventilation for the whole house, the fan must meet the 62.2 requirement.
ASHRAE 62.1 covers commercial and institutional buildings. ASHRAE 62.2 covers residential buildings. Commercial rates are typically higher per square foot due to higher occupancy density and pollution sources.
In tight homes, large exhaust fans (range hoods over 200 CFM) can depressurize the house, causing backdrafting of combustion appliances. A dedicated makeup air system or opening a window provides replacement air. Building codes often require makeup air for hoods over 400 CFM.
No — the CFM requirement stays the same. An ERV recovers heat/moisture from exhaust air, reducing the energy penalty of ventilation by 60–80%. You still need the same CFM for air quality; you just spend less energy conditioning it.
Use a flow hood over registers or exhaust grilles to measure actual airflow. Alternatively, use an anemometer in the duct and multiply velocity by duct cross-section area. Duct leakage often reduces delivered CFM by 10–30% compared to fan rating.
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