A central system needs about 400 CFM of airflow per ton, and ducts sized to move it quietly. Enter your system size and duct role for the round diameter and rectangular equivalent.
Estimate only — a full ACCA Manual D design accounts for total run length and fittings.
Ducts are sized from airflow, not tonnage directly:
For the whole picture — total run length, fittings and register selection — a full ACCA Manual D design is the standard.
Typical main-trunk sizes at ~400 CFM/ton and a quiet ~700 fpm:
| System | Airflow (CFM) | Round trunk |
|---|---|---|
| 1 ton | 400 | 10–12" |
| 1.5 ton | 600 | 12–14" |
| 2 ton | 800 | 14" |
| 2.5 ton | 1,000 | 14–16" |
| 3 ton | 1,200 | 16–18" |
| 4 ton | 1,600 | 18–20" |
| 5 ton | 2,000 | 20–22" |
Branch runs to individual rooms are smaller — commonly 6" for ~100 CFM, 7" for ~150, 8" for ~200.
A central air conditioner moves about 400 CFM of airflow per ton — roughly 350 in humid climates and 450 in dry ones. A 3-ton system needs about 1,200 CFM.
A 3-ton system (about 1,200 CFM) uses a main supply trunk around 16–18 inches round, or an equal rectangular duct such as 8×18. Branch runs to individual rooms are much smaller, typically 6–8 inches.
Round ducts have the least friction for a given area and are the most efficient. Rectangular ducts fit inside walls and joists but need a larger cross-section to carry the same airflow.
Undersized ducts choke airflow, raise static pressure, get noisy and cut the system's capacity — a common reason a new, correctly sized AC still underperforms.
Airflow is estimated at about 400 CFM per ton; duct sizes are computed for the selected role's target velocity, with the rectangular equivalent from the equal-friction diameter formula. This is a planning estimate — a full ACCA Manual D design accounts for total effective length, fittings and available static pressure.
Standards: ANSI/ACCA 1 Manual D — Residential Duct Systems (ACCA) and the ASHRAE Handbook — Fundamentals (ASHRAE). See our full methodology.