Who this is for: For installers balancing a residential system — who needs each branch's target CFM before cutting duct — and DIYers sizing an exhaust fan by ACH.
Room airflow three ways: share of the whole-home CFM by area, air changes per hour, or tonnage × 400 — all in one place.
Quick answer: Room CFM three ways: area share (room sqft ÷ home sqft × system CFM), air changes (room volume × ACH ÷ 60 — bathrooms target 8 ACH), or tonnage (400 CFM per ton; a 3-ton system = 1,200 CFM). A 120 sq ft bedroom in a 1,600 sqft, 3-ton house targets 90 CFM.
sqft
sqft
tons
Climate
This room's CFM
90
120 ÷ 1600 × 1200
System CFM
1200
3 tons × 400
Per sqft
0.75
CFM per sqft of home
Feed each room’s target CFM into the duct size calculator to get the branch diameter — that chain (tons → CFM → room split → duct) is the whole residential airflow design.
Area share is the residential default; bathrooms carry a 50 CFM / ~8 ACH code floor for exhaust; 400 CFM/ton is the standard cooling ratio (350 in humid climates).
Core facts
Standard
400 CFM per ton (350 in humid climates)
Area share
room sqft ÷ home sqft × system CFM
ACH method
CFM = sqft × height × ACH ÷ 60
Code floor
Bath fan ≥50 CFM (~8 ACH)
Example
120/1,600 sqft of a 3-ton → 90 CFM
Status
Estimating tool — formal design per ACCA Manual D. Compiled September 2026.
How do you calculate CFM for a room?
Three routes, one answer each. Area share: a room's fraction of floor area takes the same fraction of the whole-home CFM — a 120 sq ft bedroom in a 1,600 sq ft house with a 3-ton system (1,200 CFM) gets 90 CFM. Air changes per hour: room volume × ACH ÷ 60 — a 100 sq ft bedroom with 8 ft ceilings at 6 ACH needs 80 CFM; bathrooms target 8 ACH (50 CFM minimum by code), kitchens higher. Tonnage method: whole-home CFM is tons × 400 (standard dry conditions; 350 in humid climates where latent load takes a bigger cut). Enter what you know in each panel and the calculator does the arithmetic — then use the duct size calculator to turn the target CFM into a duct diameter.
Common uses
Setting damper positions to target CFM room by room
Sizing a bathroom exhaust fan to the 8 ACH / 50 CFM code floor
Checking whether a 3-ton system's 1,200 CFM splits sensibly across rooms
Converting an odd room's needs into a duct diameter
Frequently Asked Questions
What is CFM in HVAC?
Cubic feet per minute — the volumetric airflow a duct carries or a system moves. A typical residential system moves 400 CFM per ton of cooling; a 3-ton system moves about 1,200 CFM total.
How many CFM per ton?
The standard rule is 400 CFM per ton of cooling. Humid climates often run 350 (slower air = more moisture removal), and heat-pump heating mode sometimes wants more. Equipment data plates sometimes specify — when they do, the plate wins.
How do I get each room's CFM?
By floor-area share: room sq ft ÷ house sq ft × total system CFM. It's the residential default because it tracks where the load roughly is. Rooms with unusual glass or occupancy deserve an up-adjustment.
What's ACH and what should it be?
Air changes per hour — how many room-volumes of air pass through per hour. Bedrooms target ~5–6, living areas ~6–8, bathrooms 8 (50 CFM code minimum for the fan), kitchens 15 for hood-sized needs. Higher ACH = faster dilution of odors and moisture.
How do I check the result?
Measure with a flow hood at the register, or estimate velocity × grille free area (a 6-inch round at 600 fpm carries ~118 CFM). Balancing is iterate-and-measure — the calculator gives the target, the hood tells you when you've arrived.