HVAC CFM Calculator: Airflow by Room, Tonnage, or Duct Size

Multiply the room's area by its ceiling height to get volume in cubic feet, multiply by the target air changes per hour (ACH), and divide by 60. Example: a 200 sq ft bedroom with 8 ft ceilings at 5 ACH needs (200 × 8 × 5) ÷ 60 ≈ 133 CFM. Use the Room CFM tab in the calculator above - it includes the recommended ACH for each room type.

There are three, depending on what you're solving. Room ventilation: CFM = (area × ceiling height × ACH) ÷ 60. System airflow: CFM = tons × 400 (350 in humid climates, 450 in dry). Duct capacity: CFM = duct cross-section area in square feet × air velocity in feet per minute. A fourth, for measuring real airflow: CFM = BTU/hr ÷ (1.08 × temperature rise°F).

The design standard is 400 CFM per ton of cooling. In humid climates, techs set 350 CFM per ton - slower air across a colder coil wrings out more moisture. In hot-dry climates, 450 CFM per ton maximizes sensible cooling since humidity isn't a concern. So a 3-ton system moves roughly 1,050-1,350 CFM depending on setup.

The common rule of thumb is about 1 CFM per square foot of conditioned space for system airflow - a 2,000 sq ft house with a 3-ton system at 400 CFM/ton gets 1,200 CFM, or 0.6 CFM/sq ft; older leakier homes trend toward 1. Individual room CFM varies much more (a kitchen needs several times a bedroom's rate per square foot), so use the ACH method per room instead of a flat number.

The HVI standard: 1 CFM per square foot of bathroom, with a 50 CFM minimum. A typical 5×8 bathroom (40 sq ft) needs a 50 CFM fan. Bathrooms over 100 sq ft are sized per fixture instead: 50 CFM each for a toilet, shower, or tub, and 100 CFM for a jetted tub. Long duct runs and elbows reduce delivered airflow, so size up one step if the run exceeds about 25 feet.

For gas ranges: total burner BTU ÷ 100 - a 45,000 BTU range needs 450 CFM. For electric ranges: 100 CFM per linear foot of range width - a standard 30-inch range needs 250 CFM. Island hoods need about 50% more because cross-drafts steal capture. Note that hoods over 400 CFM trigger makeup-air requirements in the IRC.

Base rate: exchange the full tent volume every 1-3 minutes. A 4×4×6.5 ft tent is 104 cubic feet, so 35-105 CFM base. Then add for restrictions: +25% for a carbon filter, +20% for each significant duct bend, and +10-20% if lights run hot. That lands a typical filtered 4×4 tent at a 100-150 CFM inline fan. Oversize and run the fan on a speed controller rather than undersizing.

Multiply the duct's cross-sectional area (in square feet) by the air velocity (FPM). A 10-inch round duct has 0.545 sq ft of area, so at 900 FPM supply velocity it carries about 490 CFM; at 700 FPM return velocity, about 380 CFM. The Duct CFM tab above does this for round and rectangular ducts - and our duct size calculator works the reverse direction with friction accounted for.

The temperature-rise method is the standard field check for furnaces: run the heat, measure supply air temperature minus return air temperature, then CFM = output BTU ÷ (1.08 × temperature rise). Example: an 80,000 BTU-output furnace with a 50°F rise is moving about 1,481 CFM. For individual registers, techs use an anemometer (average face velocity × register area) or a flow hood.

Multiply CFM by 1.699 for m³/h (CMH), by 0.472 for L/s, or by 28.32 for L/min. Going the other way, divide: 1,000 m³/h ≈ 589 CFM, and 100 L/s ≈ 212 CFM. The Convert Units tab above handles all of these at once.

CFM measures actual airflow volume at whatever temperature and pressure exist; SCFM (standard CFM) restates that flow at standardized conditions (68°F, 14.7 psia, 36% RH) so equipment can be compared fairly. The distinction matters a lot for air compressors and blowers under pressure; for residential HVAC ductwork at near-atmospheric pressure, CFM and SCFM are close enough that the difference is usually ignored.

Total return CFM must equal total supply CFM - the blower can't push more air than it pulls back. So a 3-ton system needs about 1,200 CFM of return capacity. Size return grilles for 400-500 FPM face velocity to stay quiet: 1,200 CFM ÷ 500 FPM ≈ 2.4 sq ft of free grille area. Undersized returns are one of the most common airflow problems in homes - see the return-air section of our duct size calculator.

Distribute the system total in proportion to each room's share of the cooling load: room CFM = system CFM × (room load ÷ whole-house load). A bedroom that's 10% of the load on a 1,200 CFM system gets 120 CFM. That's exactly why room-by-room Manual J numbers matter for duct design - run our HVAC load calculator for the whole-house figure first.

ACH (air changes per hour) is how many times a room's entire air volume is replaced in one hour. The two convert directly: CFM = (ACH × room volume) ÷ 60, and ACH = (CFM × 60) ÷ room volume. A 100 CFM fan in a 960 cubic-foot bathroom delivers (100 × 60) ÷ 960 ≈ 6.3 ACH. Residential targets range from about 4 ACH in basements to 8+ in kitchens and bathrooms.