How duct sizing works
Every foot of duct costs the blower some static pressure. The friction rate is how much you are willing to spend per 100 ft of duct, in inches of water column. Pick a rate, and for any airflow there is exactly one round diameter that lands on it. This calculator solves the Darcy-Weisbach equation with the Colebrook friction factor for galvanized steel and standard air, which is the same physics behind a slide-rule ductulator and the published round-duct charts.
Round is the reference. A rectangular duct is matched by equivalent diameter, not by equal area, because a flat duct has more wall rubbing on the same air: De = 1.30 (a·b)^0.625 ÷ (a+b)^0.25 (ASHRAE). Tell the tool the height you can fit and it returns the width that gives the same friction loss as the round answer, then checks the face velocity in that rectangle.
The velocity verdict is the part a chart cannot give you. A trunk at 1,100 FPM will carry the air and it will also be heard in the living room. Residential design targets: trunks 700–900 FPM, branches 500–700, returns 500–700.
Round duct capacity chart (galvanized, standard air)
Generated by the same function the calculator runs; CFM at two common friction rates and the velocity at 0.10.
| Round duct | CFM at 0.08 in. wc/100 ft | CFM at 0.10 in. wc/100 ft | Velocity at 0.10 |
|---|---|---|---|
| 4 in. | 33 | 37 | 425 FPM |
| 5 in. | 60 | 68 | 497 FPM |
| 6 in. | 98 | 111 | 564 FPM |
| 7 in. | 148 | 168 | 627 FPM |
| 8 in. | 212 | 240 | 687 FPM |
| 9 in. | 291 | 329 | 744 FPM |
| 10 in. | 386 | 436 | 799 FPM |
| 12 in. | 628 | 709 | 902 FPM |
| 14 in. | 947 | 1,068 | 999 FPM |
| 16 in. | 1,351 | 1,524 | 1,091 FPM |
| 18 in. | 1,847 | 2,083 | 1,178 FPM |
| 20 in. | 2,442 | 2,753 | 1,262 FPM |
| 22 in. | 3,144 | 3,544 | 1,342 FPM |
| 24 in. | 3,959 | 4,461 | 1,420 FPM |
Quick sizes by tonnage (0.08 in. wc, supply trunk)
| System | Airflow | Round trunk | Velocity | Rectangular, 10 in. tall |
|---|---|---|---|---|
| 1.5 ton | 600 CFM | 12 in. | 764 FPM | 12 × 10 in. |
| 2 ton | 800 CFM | 14 in. | 748 FPM | 16 × 10 in. |
| 2.5 ton | 1,000 CFM | 15 in. | 815 FPM | 18 × 10 in. |
| 3 ton | 1,200 CFM | 16 in. | 859 FPM | 22 × 10 in. |
| 3.5 ton | 1,400 CFM | 17 in. | 888 FPM | 24 × 10 in. |
| 4 ton | 1,600 CFM | 18 in. | 905 FPM | 26 × 10 in. |
| 5 ton | 2,000 CFM | 19 in. | 1,016 FPM | 32 × 10 in. |
Trunk sizes only. Returns go one size up; branch runs to registers are sized from the room's own CFM.
Quick answers
What size duct do I need for a 3-ton AC unit?
At 400 CFM per ton a 3-ton system moves about 1,200 CFM. At a 0.08 in. wc friction rate that calls for a 16 in. round supply trunk (about 860 FPM), or a rectangular equivalent such as 18 × 12 or 20 × 10. Returns run one size larger, 18 in. round, to keep velocity near 700 FPM.
What friction rate should I use?
0.08 in. wc per 100 ft is the common residential design point for supply trunks; 0.10 suits short, simple runs. Returns and long or flex-heavy systems are often designed at 0.06 or lower. Strictly, the friction rate comes from an ACCA Manual D calculation of your available static pressure divided by the total effective length.
How fast should air move in a duct?
Residential supply trunks are designed for 700–900 FPM; branch runs 500–700 FPM; returns 500–700 FPM. Above 900–1,000 FPM you get noise at takeoffs and registers plus higher static pressure. Below 500 FPM a trunk is larger than it needs to be.
Is a rectangular duct with the same area as a round duct equivalent?
No. A rectangle has more wall for the same area, so more friction. The ASHRAE equivalent-diameter equation, De = 1.30 (a·b)^0.625 ÷ (a+b)^0.25, gives the round size with the same friction loss. An 18 × 12 rectangle equals a 16.0 in. round even though its area matches a 16.6 in. round.
Does this work for flex duct?
The chart assumes galvanized sheet metal, fully extended. Flex duct pulled tight carries about the same; flex with sag or compression can lose half its capacity. Size flex one step larger than the metal answer, and pull it tight.
Sources & standards
Read the method
CFM Per Ton: The 400 Rule
400 CFM/ton standard air — when to run 350 in humid climates and 450 in dry ones.
Read the guide →What Size Duct for a 3-Ton Unit?
Trunk and return sizes at 1,200 CFM, round-to-rectangular equivalents, and velocity limits.
Read the guide →How Many BTU Per Square Foot?
The 20 BTU/sq ft cooling baseline, climate-zone heating table, and square-footage-to-tons chart.
Read the guide →