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

What Is CFM in HVAC?

CFM - cubic feet per minute- measures how much air a system moves. It's the currency of every airflow decision in HVAC: how much air a room needs for ventilation, how much a blower must move per ton of cooling, how much a duct can carry, and how much an exhaust fan must pull. Get CFM wrong and even perfectly sized equipment underperforms: airflow is why two identical systems can feel completely different.

Airflow Calculator: CFM, Air Volume, and Air Velocity

Every airflow calculation on this page is the same question asked three ways: how much air, moving how fast, through how big an opening. The general form is CFM = velocity in feet per minute × cross-section area in square feet. A 12 by 8 inch register face is 0.67 sq ft, so air crossing it at 500 FPM is carrying about 333 CFM. That is the version you use when you are measuring something that already exists.

People also ask for CFM per cubic foot, and that one has a tidy answer: it is the air changes per hour divided by 60, nothing else. Ceiling height never enters into it, because the volume figure has already accounted for the height.

The decision rule: match the unit to the thing you are actually sizing. Use CFM per cubic foot for clearing the air in a sealed space like a grow tent or a workshop. Use CFM per square foot when you are comparing rooms in a house. Use velocity times area only when you are checking real airflow at a grille or a duct, because it needs a measurement rather than a target.

The CFM Calculation Formula in HVAC

All four formulas the calculator uses, for working by hand:

• Room ventilation: CFM = (area × ceiling height × ACH) ÷ 60
• System airflow: CFM = tons × 400 (use 350 humid / 450 dry) - get the tons themselves from our AC ton calculator, or read the CFM straight off the tonnage specification chart if you already know the size
• Duct capacity: CFM = cross-section area (sq ft) × velocity (FPM)
• Measured airflow (temperature rise): CFM = output BTU/hr ÷ (1.08 × ΔT°F)

That 1.08 in the last one is not a fudge factor, and knowing where it comes from tells you when it stops applying. It is air density times specific heat times minutes per hour: 0.075 lb per cubic foot × 0.24 BTU per lb per °F × 60 = 1.08. Change any of those conditions and the constant changes with them, which is why high-altitude work uses a smaller number. It also only counts sensible heat, the kind you can measure with a thermometer. For total heat including moisture, the trade uses 4.5 with enthalpy in place of temperature.

How to Calculate CFM for a Room

The room method is the one most people actually need, and it asks for three things: floor area, ceiling height, and how many times an hour you want the air replaced. The reason you divide by 60 at the end is easy to lose track of, so it is worth saying plainly: air changes per hour is a per-hour figure and CFM is a per-minute one, so the 60 is doing nothing more than converting between them.

A 300 sq ft living room with 8 ft ceilings at 6 air changes per hour comes out at (300 × 8 × 6) ÷ 60 = 240 CFM. The Room CFM tab above runs this live for any dimensions, and the table below saves you the arithmetic for the sizes that come up most often.

Not sure which column to read? Take the air changes per hour for your room type from the chart further down, then read across. Rooms that make moisture or smells sit at the high end, because there you are clearing air rather than just conditioning it. Rooms where people mostly sit still sit at the low end.

The decision rule: size for the higher air change rate and control it with fan speed, never the other way round. A fan that moves too much air can be turned down for a few dollars. One that moves too little cannot be turned up, and swapping it later costs far more than the bigger unit would have.

How Many CFM per Room

If the house already has ducts, the question changes shape. There is no fixed CFM per room, because every register draws from one blower with a fixed total, so what a room gets is its share of that total. The shortcut most homeowners can actually apply is floor area: room CFM ≈ (room sq ft ÷ conditioned sq ft) × system CFM.

Take a 2,000 sq ft house on a 3.5 ton system. The blower moves about 1,400 CFM at the standard rate, so a 250 sq ft living room is 12.5% of the floor area and should be seeing roughly 175 CFM. As a sanity range for a typical house, bedrooms land between 75 and 150 CFM and open living or kitchen spaces between 200 and 400.

Area share is a shortcut, not the real answer. It assumes every square foot gains heat at the same rate, and a west-facing room with three windows plainly does not. The accurate version splits the total by each room's share of the cooling load instead of its floor area, which is covered in the FAQ below.

The decision rule: measure before you rebalance. If one room runs hot, find out what it is actually getting at the register before touching a damper. Most complaints about a room not getting enough air turn out to be a crushed flex run or a damper someone closed years ago, and no amount of balancing fixes either of those.

CFM per Square Foot

CFM per square foot is what the room method reduces to once you divide the floor area back out, and only two things set it: the ceiling height and the target air changes per hour. The formula is ceiling height × ACH ÷ 60. An 8 ft bedroom at 5 ACH needs 0.67 CFM per square foot. Raise that ceiling to 10 ft and the same floor needs 0.83, because you added volume without adding any floor to spread it over.

Going the other way, square feet from CFM, is the same relationship read backwards: divide the airflow you have by the rate. A 400 CFM inline fan at 0.80 CFM per square foot covers 500 sq ft of living space with 8 ft ceilings.

The decision rule: the flat 1 CFM per square foot figure is a whole-house sanity check, not a room-sizing method.It holds up fine when you want to know whether a system's total airflow is in the right neighbourhood. It falls apart room by room, because a kitchen at 8 ACH needs about 1.6 times what a bedroom at 5 ACH does over exactly the same floor area.

Recommended Air Changes per Hour by Room

Working the other direction - checking what ACH an existing fan or air purifier achieves? Use the dedicated air changes per hour calculator.

CFM per Ton: System Airflow Chart

This airflow figure is what your ductwork must actually deliver - feed it into the AC duct size calculator to size trunks and branches, and get the tonnage itself from the Manual J load calculator rather than square footage.

How Many CFM for a 3 Ton AC

Read the standard column above and a 3 ton system wants 1,200 CFM. You will also see capacity written as TR, for tons of refrigeration, mostly on commercial gear: it means exactly the same thing, so a 3 TR unit takes the same airflow as a 3 ton one and the Tonnage → CFM tab answers both.

That is the design figure, though, and it is not what most installed systems actually move. Ducts that were never sized for the equipment, a loaded filter, crushed flex, and undersized returns all pull the real number down, commonly by 10 to 25 percent. So the number on the chart is what your ductwork was supposed to deliver, not proof that it does.

The quick field check in cooling is the temperature split: measure supply air and return air at the air handler and look for a difference of 16 to 22°F, which is what a healthy Delta T looks like. A split much wider than that usually means too little air across the coil; a narrow one means too much. In heating you can get an actual number rather than a symptom, using the temperature-rise method below.

The decision rule: fix the airflow before you touch the equipment. A 3 ton system moving 900 CFM behaves like a smaller and less efficient unit, and ices its coil in humid weather. Replacing it with something bigger makes both problems worse, because the new unit inherits the same ducts that caused them.

CFM to BTU and BTU to CFM

These two do not convert directly, and that is the first thing to get straight. Air carries heat, but how much it carries depends on how much warmer or cooler it is than the air it displaces, so nothing converts until you supply a temperature difference. Once you have one, it goes both ways:

• CFM from BTU: CFM = BTU/hr ÷ (1.08 × ΔT°F)
• BTU from CFM: BTU/hr = CFM × 1.08 × ΔT°F

Heating is where this is exact, because a furnace delivers only sensible heat. Here is what a furnace's output works out to across the normal 40 to 60°F rise range printed on its data plate:

Cooling needs one correction, and it is the reason people get confused comparing this against the per-ton chart. The 1.08 constant counts sensible heat only, while an air conditioner spends part of its rated capacity pulling moisture out of the air instead of dropping the temperature. Roughly three quarters of a residential unit's capacity is sensible, so a 3 ton system at 1,200 CFM shows a supply-to-return split near 21°F rather than the 28°F the raw arithmetic would predict.

The decision rule: use this formula to measure, never to size. If the question is what airflow an 18,000 BTU system needs, the answer comes off the per-ton chart above (1.5 tons, so 525 to 675 CFM depending on climate), not from the 1.08 equation. That equation tells you what a system is doing right now, which is a different question from what it should have been built to do.

Bathroom Exhaust Fan CFM Chart

Range Hood CFM: The Two Rules

Gas ranges: total burner output ÷ 100 - a typical 4-burner, 45,000 BTU gas range wants 450 CFM. Electric ranges: 100 CFM per linear foot - a 30-inch range wants 250 CFM. Island installations add roughly 50% because there's no wall to help capture the plume. Two code notes: hoods above 400 CFM require makeup air under the IRC, and duct diameter must match the hood manufacturer's spec - see the exhaust-duct FAQ on our duct calculator.