Understanding your result
The headline is the system’s total airflow, the tonnage multiplied by the CFM per ton, with the number of rooms and the total register count in the sub-line. The bars show each room’s share so the heavy rooms stand out; the stats give the three largest. The table lists every room with its CFM, the weight it was given (area × exposure factor) and the registers suggested, then the average airflow per square foot, which is the sanity check on the whole system.
Each room’s CFM is the number to carry to the duct size calculator, which sizes the branch duct that feeds it, and to the register size calculator, which picks the registers. The system total is what the return grille calculator must bring back. If the average CFM per square foot looks off, the notes say so and the central AC size calculator is the place to check the tonnage.
Rooms that come out under 40 CFM get a note: a register delivering that little tends to dump air straight down, and such rooms often do better sharing a register with the next room or with a transfer grille.
How we calculate this
The method is an area-weighted split with a correction for how much more heat some rooms gain than others; it is the quick version of what ACCA Manual J does properly by computing each room’s heat gain from its walls, windows, orientation and occupants, and then Manual D does by sizing ducts to deliver that air. The exposure factors are judgement values in the range those room loads typically fall; a corner bedroom with two exterior walls and west glass commonly carries 20 to 30% more load per square foot than an average room, and a kitchen’s appliances add a few thousand BTU per hour on top of its envelope load.
The 120 CFM per register is a comfortable figure for a common 4 × 12 or 6 × 10 floor register at a face velocity around 500 FPM; the register calculator does the exact sizing. The duct size chart lists the branch sizes that carry each room’s airflow.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Airflow per ton | 400 CFM (350 humid, 450 dry) | Residential design practice; equipment ratings at nominal airflow (AHRI) |
| Exposure factors | 0.8 / 1.0 / 1.2 / 1.3 | Judgement values reflecting typical room load differences in Manual J calculations |
| Airflow per register | 120 CFM | A common floor register at about 500 FPM face velocity |
| Typical CFM per sq ft | 0.9 to 1.2 | A 1 ton per 400 to 500 sq ft system at 400 CFM per ton |
Assumptions last reviewed October 8, 2026.
The calculator does not compute room loads; it apportions the system airflow by a proxy for them. It does not account for a room’s distance from the air handler (long runs deliver less unless the duct is sized for it), for rooms on different floors, which in a single-zone system often need the upstairs weighted higher in summer, or for the return path. The guide on return air explains the half of the system this calculator does not touch, and the guide to duct sizing basics takes each room’s CFM to a duct size.
Two worked examples
A five-room house on a 3-ton system
3 tons at 400 CFM per ton; a 320 sq ft living room (average), a 200 sq ft kitchen, a 180 sq ft and a 150 sq ft bedroom (average), and a 60 sq ft interior bathroom.
- System airflow: 3 × 400 = 1,200 CFM
- Weights: 320, 200 × 1.3 = 260, 180, 150, 60 × 0.8 = 48; sum 958
- Living room: 1,200 × 320 ÷ 958 = 401 CFM, 4 registers
- Kitchen: 326 CFM, 3 registers; bedrooms 225 and 188 CFM, 2 each; bathroom 60 CFM, 1
- Average: 1.32 CFM per sq ft; 12 registers in all
The kitchen’s 1.3 factor lifts it to a share larger than a bedroom of the same area, which is what its appliances and traffic deserve. At 1.32 CFM per square foot the system is on the generous side for 910 sq ft of rooms, which is fine if these are the only rooms served; if there is also a 300 sq ft family room on the same system, add it and the shares fall into the normal band.
A three-room apartment on a 2-ton system in a humid climate
2 tons at 350 CFM per ton; a 260 sq ft living room with west glass (sunny), a 140 sq ft bedroom (average) and a 120 sq ft interior bedroom.
- System airflow: 2 × 350 = 700 CFM
- Weights: 260 × 1.2 = 312, 140, 120 × 0.8 = 96; sum 548
- Living room: 700 × 312 ÷ 548 = 399 CFM, 4 registers
- Bedrooms: 179 CFM (2 registers) and 123 CFM (2 registers)
- Average: 1.35 CFM per sq ft
The sunny living room takes 57% of the air for 50% of the area. Four registers across a 260 sq ft room is more than most apartments have; two larger registers at a lower face velocity would be the practical compromise, which the register calculator can size by raising the maximum per register.
Where to find your inputs
Tons. From the outdoor unit’s model number (24 = 2 tons, 36 = 3 tons) or the central AC size calculator for a new system.
CFM per ton. 400 unless your climate is humid (350) or hot and dry (450); a contractor’s Manual S selection states the design airflow.
Room areas. Length × width of each room the system serves; leave out rooms on a separate system or a mini-split.
Exposure. Judge each room by its exterior walls, glass and use; when in doubt, average.
Common mistakes
- Splitting by area alone. A sunny corner bedroom and an interior bedroom of the same size do not need the same air.
- Leaving rooms out. Every room the system serves takes a share; forgetting the family room oversupplies the rest.
- One big register per room. Above 150 CFM a single register is drafty; spread the air.
- Ignoring the return. Bedrooms behind closed doors need a return or a transfer path, or the supply you planned never arrives.
- Treating the shares as fixed. Dampers at the trunk takeoffs let a technician balance the real system; the shares are the starting point.
- Skipping the Manual J. Room-by-room loads replace the exposure factors for a new duct design.
Questions people ask
- How many CFM does each room need?
- Roughly 1 CFM per square foot of floor area as a first cut, with a 3-ton system moving 1,200 CFM across about 1,200 sq ft of conditioned space. A 320 sq ft living room with average exposure on that system gets about 400 CFM; a 150 sq ft bedroom about 190; a 60 sq ft interior bathroom about 60. Sunny, corner or kitchen rooms need 20 to 30% more than their area suggests, interior rooms about 20% less. ACCA Manual J computes each room's load exactly.
- How many CFM per ton should I use?
- 400 CFM per ton is the standard design figure. Use 350 in humid climates, where a slower airflow across a colder coil removes more moisture, and 450 in hot, dry climates, where latent load is small and more airflow improves efficiency. The figure is set on the air handler's blower and should match the equipment's rating; the calculator uses it to find the total that is then split between rooms.
- How many supply registers does a room need?
- One register comfortably delivers 100 to 150 CFM without a draft; above that the air comes out too fast or the register has to be large. The calculator divides each room's CFM by 120 and rounds up, so a 400 CFM living room gets four registers and a bedroom one or two. Place them under windows or along exterior walls for a floor system, and spread them across the ceiling for a ceiling system.
- What does exposure mean here?
- How much heat a room gains or loses compared with an average room of its size. An interior room with no exterior wall gains little and gets a 0.8 factor; an average room with one exterior wall and normal windows is 1.0; a corner room, a room with large west or south windows, or a room over a garage gains more and gets 1.2; a kitchen, with its appliances, gets 1.3. These are rough weights; a room-by-room Manual J replaces them.
- Why does the calculator say the CFM per square foot is low or high?
- Typical houses run 0.9 to 1.2 CFM per square foot of conditioned area. A figure well below that means the system may be undersized for the area you listed or that rooms are missing; a figure well above it means the system is oversized or that the listed rooms are not the only ones served. Either way, it is worth checking the system size with the central AC calculator before sizing ducts to the split.
- Does the return air need the same split?
- No. Supply air is split room by room; return air is collected centrally, usually from a hallway and the main living area, with transfer grilles or door undercuts letting bedroom air back out. The total return airflow must equal the total supply (1,200 CFM for a 3-ton system), and it is sized with the return grille calculator. Rooms behind closed doors with no return path will be starved of supply air however well the supply is split.