Free CFM Calculator
Calculate cubic feet per minute (CFM) airflow needed for HVAC ventilation sizing — enter room dimensions and air changes per hour.
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Estimates based on ASHRAE Standard 62.1 guidelines. Consult a licensed HVAC contractor for system design.
Results are estimates. Consult a professional.
What is CFM and why does it matter?
CFM stands for Cubic Feet per Minute — the standard unit for measuring airflow volume in HVAC systems. It tells you how quickly air moves through a duct, fan, or room. Getting the CFM right is the first step in sizing any ventilation system: too little airflow leaves a room stuffy and damp; too much wastes energy and creates uncomfortable drafts or excessive noise.
Ventilation rates are set not by guesswork but by the number of air changes per hour (ACPH or ACH) — how many times the full volume of a room's air is replaced each hour. A bathroom exhaust fan, a range hood, a fresh-air HRV, or a central AHU all move air in CFM. Knowing how many CFM a room needs lets you choose the right fan or confirm your existing equipment is adequate.
CFM formula
The CFM calculation converts an ACPH target into a per-minute flow rate. Because ACPH counts full-room air replacements per hour and CFM measures flow per minute, you divide by 60 to convert hours to minutes.
Step-by-step CFM calculation
A homeowner wants to add a fresh-air supply duct to a 12-foot-wide, 15-foot-long bedroom with 8-foot ceilings. The target is 6 air changes per hour — the upper end of the ASHRAE recommended range for a living or sleeping space.
Step 1 — Calculate the room volume
Step 2 — Apply the CFM formula
ACPH guidelines by room type
ASHRAE Standard 62.2 (residential) and 62.1 (commercial) set minimum ventilation rates. The table below gives typical ACPH targets for common residential and light-commercial spaces. Use the midpoint for well-insulated modern construction; go higher for older homes, high-occupancy, or high-moisture spaces.
| Room type | Typical ACPH | Notes |
|---|---|---|
| Bedroom / living room | 4–6 | ASHRAE 62.2 residential minimum; 4 for light-use rooms, 6 for occupied sleeping areas |
| Home office | 6–8 | Slightly higher to offset CO₂ buildup from occupancy |
| Bathroom (exhaust) | 8 | IRC requires exhaust fans rated for 50 CFM intermittent or 20 CFM continuous |
| Garage (attached) | 10 | ASHRAE 62.1 Table 6-1; dilutes CO and combustion gases |
| Kitchen (general) | 15 | Separate from range hood — covers background cooking vapours |
| Range hood (active cooking) | 100+ | Spot exhaust CFM rated by hood size, not ACPH-based |
| Laundry room | 10–15 | Lint, humidity, and detergent vapours |
| Server room / IT closet | 20–60 | Heat removal drives this; may need dedicated cooling CFM calculation |
Sources: ASHRAE Standard 62.1 (Table 6-1) and ASHRAE Standard 62.2 for residential. IRC R303 for bathroom fans. Actual system design may differ.
Choosing the right fan or duct
Once you have the required CFM, you need to select a fan or duct that can deliver it against the static pressure of the ductwork. A fan's published CFM is typically its free-air rating; add ductwork and the actual flow drops. Use the fan's performance curve to find the CFM at your estimated duct resistance.
- Bathroom exhaust fans — HVI (Home Ventilating Institute) rates fans at 0.10 in.wg. An 80 CFM fan at free air may deliver 70 CFM with 6 ft of flex duct. The IRC requires at least 50 CFM for intermittent use.
- Range hoods — Size by hood width and cooking style, not room ACPH. General rule: 100 CFM per 10 in of range width for light cooking; up to 150–300 CFM per 10 in for high-BTU ranges.
- Supply ducts (HVAC) — Residential supply velocity targets 700–900 ft/min for trunk ducts, 400–600 ft/min for branch runs. Duct CFM = velocity (ft/min) × area (ft²).
- HRV/ERV — Heat or energy recovery ventilators are typically sized to deliver 0.35 ACH for the whole house (ASHRAE 62.2), then adjusted for the number of bedrooms.
Key terms explained
Accuracy and limitations
This calculator gives the minimum required CFM based on room volume and ACPH. It is a planning estimate — not a full HVAC design. Several real-world factors can change the actual CFM needed:
- Room geometry. The formula assumes a rectangular room. Vaulted ceilings, sloped soffits, or irregular walls change the actual volume — measure all segments and sum them.
- Infiltration credit. ASHRAE 62.2 allows some credit for envelope air leakage in tightly built homes, potentially reducing the required mechanical ventilation CFM.
- Occupancy load. Commercial applications (ASHRAE 62.1) base ventilation on people per zone plus area, not ACPH alone. Use the CFM/person + CFM/ft² method for offices, classrooms, and retail.
- Source-control exhaust. Range hoods, bathroom fans, and laundry exhausts are sized separately and do not substitute for general fresh-air ventilation.
- Duct losses. Even a perfectly sized fan loses CFM to duct friction, filter resistance, and leakage. Real-world delivery is typically 20–30% lower than the nameplate rating in a long duct run.
About this calculator
This CFM calculator is provided free by Calculators Cloud. It uses the standard HVAC airflow formula — volume × ACPH ÷ 60 — as described in ASHRAE Standard 62.1. No personal data is collected; all calculations run locally in your browser.
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ASHRAE Standard 62.1 – Ventilation and Indoor Air Quality (ANSI/ASHRAE 62.1-2022).ASHRAE Standard 62.2 – Ventilation and Acceptable Indoor Air Quality in Residential Buildings.Home Ventilating Institute (HVI) – Certified Products Directory and test standards.