CFM Duct Chart: How Much Air Each Round Duct Size Carries
Updated · By the Roof to Footing editors
A CFM duct chart shows how many cubic feet of air per minute (CFM) a duct can carry at a given friction rate. For round galvanized metal duct at a friction rate of 0.10 inch of water per 100 feet, a 6-inch duct carries about 110 CFM, an 8-inch about 230 CFM and a 10-inch about 420 CFM. Pick the friction rate below and the chart updates, or enter the airflow you need to find the smallest duct size.
Round duct CFM chart
400 CFM at 0.10 needs at least a 10-inch round metal duct (733 feet per minute).
| Diameter | CFM | Velocity (FPM) |
|---|---|---|
| 4 in. | 35 | 426 |
| 5 in. | 65 | 492 |
| 6 in. | 110 | 553 |
| 7 in. | 160 | 611 |
| 8 in. | 230 | 666 |
| 9 in. | 320 | 718 |
| 10 in. | 420 | 768 |
| 12 in. | 680 | 863 |
| 14 in. | 1,020 | 953 |
| 16 in. | 1,450 | 1,039 |
| 18 in. | 1,980 | 1,120 |
| 20 in. | 2,620 | 1,199 |
How the chart is calculated
The numbers come from the friction loss equation for round galvanized duct published in the ASHRAE Handbook of Fundamentals:
friction loss = 0.109136 × CFM^1.9 ÷ diameter^5.02
where friction loss is in inches of water per 100 feet of straight duct and diameter is in inches. Solving it for CFM at a chosen friction rate gives the chart. Velocity is the CFM divided by the duct's cross-section area in square feet.
What friction rate to use
The friction rate is a design choice: how much pressure the blower spends pushing air through each 100 feet of duct. Residential systems are often designed between about 0.05 and 0.10. A lower rate means bigger ducts, quieter air and less work for the blower; a higher rate means smaller ducts. A designer following ACCA Manual D sets the rate from the blower's available pressure and the length of the longest run.
Rules of thumb (and their limits)
- About 400 CFM per ton of cooling is the typical airflow target for central air and heat pumps. A 2-ton system wants about 800 CFM total. See 2 ton heat pump.
- Room airflow depends on the room's heating and cooling load, not just its size. A sunny room with big windows needs more air than a north-facing bedroom of the same size.
- Velocity: keeping branch ducts in a residential system under roughly 600 to 900 feet per minute keeps noise down at the registers.
Flex duct carries less
The chart is for smooth, straight metal duct. Flexible duct has more friction, and much more if it's compressed, kinked or sagging. Fully stretched flex comes closest to the chart; flex installed with sags and tight bends can carry far less air than the same size metal. If you use flex, pull it tight, support it every few feet and use a size larger when in doubt.
Fittings add length
Every elbow, takeoff and boot adds resistance, usually counted as "equivalent length" of straight duct. A 20-foot run with three elbows can behave like 60 feet or more of straight duct. Count them when you size a run.
Exhaust ducts
The same chart applies to round exhaust ducts for bath fans and range hoods. A range hood rated 400 CFM needs a properly sized duct and roof or wall cap to actually move that much air. See roof caps for vents.