Updated September 2026
Before you can size a duct, you need a CFM number — and the fastest field way to get a ballpark figure is from the system's cooling capacity in tons. This page works through the standard rule of thumb and what it produces, size by size, using the same formula as the calculator.
A widely used field estimate for residential cooling systems is about 400 CFM per ton of nominal cooling capacity for the main supply trunk — one ton of cooling capacity is 12,000 BTU/hr, so a 3-ton system is roughly 36,000 BTU/hr. This is a rough field number, not a substitute for an actual Manual J load calculation: real airflow requirements shift with climate, duct static pressure, equipment sensible heat ratio, and altitude, and can reasonably range from about 350–450 CFM per ton depending on those factors. Use it for a sanity check or a rough trunk estimate, not as the final design figure for a real installation.
Each row below applies the 400 CFM/ton estimate, then runs that airflow through the equal-friction formula at the standard 0.1 in.wg/100 ft residential target — exactly what the calculator does for a single trunk figure.
| System size | Est. airflow | Round trunk (0.1 friction) | Velocity |
|---|---|---|---|
| 1.5 ton | 600 CFM | 12" | 764 fpm |
| 2 ton | 800 CFM | 14" | 748 fpm |
| 2.5 ton | 1,000 CFM | 14" | 935 fpm |
| 3 ton | 1,200 CFM | 16" | 859 fpm |
| 3.5 ton | 1,400 CFM | 16" | 1,002 fpm |
| 4 ton | 1,600 CFM | 18" | 905 fpm |
| 5 ton | 2,000 CFM | 20" | 916 fpm |
These are main-trunk estimates from the tonnage rule of thumb — branch runs to individual rooms should be sized from each room's actual load share, not this table directly.
The table above sizes one trunk carrying the whole system's air — most systems then split that into several branch runs feeding individual rooms or zones. To size a branch, take the room's share of the total load (from a Manual J calculation, or roughly proportional to floor area and exposure as a rough stand-in), convert that fraction of the total CFM, and run it through the calculator the same way. A 3-ton system's 1,200 CFM split evenly across, say, 6 rooms would put roughly 200 CFM per branch — but real room loads are rarely equal, so treat an even split as a starting estimate only.
The actual CFM a system needs to move per ton of capacity depends on the equipment's sensible heat ratio, the design temperature split across the coil, and the climate — a humid climate typically runs a lower CFM/ton to maximize dehumidification, while a dry climate can run higher. Manufacturers publish blower performance tables for specific equipment that give the real number for that unit at a given external static pressure; the 400 CFM/ton figure is a reasonable placeholder when you don't yet have that data, not a value to hard-code into a final design.
If a Manual J has already been done for the job, use its actual CFM output instead of the tonnage estimate — it accounts for the building's specific insulation, window area, orientation, and local climate rather than a flat multiplier. The tonnage rule of thumb is most useful earlier in a project, for a quick trunk size check before a full load calculation exists, or as a sanity check that a completed Manual J's total CFM is in a reasonable ballpark for the equipment tonnage selected.
Does this apply to heating-only systems too? The 400 CFM/ton rule is specifically a cooling-capacity estimate; heating airflow requirements (especially for gas furnaces sized independently of the AC tonnage) should come from the furnace's own rated CFM, not the cooling tonnage.
What if my system is a variable-speed or multi-stage unit? Those units modulate airflow across a range rather than running one fixed CFM — size the duct to the unit's maximum rated airflow (highest stage/speed), not the average, so the duct isn't undersized at full output.
Where do return ducts fit into this? They carry the same total system CFM but are sized to a different (lower) velocity target — see return air duct sizing.