Cfm Duct Chart

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• CFM chart (PDF/XLSX)
• Rectangular equivalents (PDF)
• Sizing calculator (XLSX)
• Airflow worksheet (PDF/DOCX/XLSX)

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Round duct CFM chart

Friction rates are in inches of water column per 100 feet of straight duct. Values are calculated for smooth galvanized steel round duct at standard air density using the empirical friction relation Δp ≈ 0.109 × CFM^1.9 ÷ D^5.02 — they agree closely with common duct calculators (ductulators).

Rectangular duct equivalents

Equivalent diameter uses the Huebscher formula, De = 1.30 (a·b)^0.625 ÷ (a + b)^0.25, which gives the round duct with the same friction loss and airflow. Flat, wide ducts carry less air than their area suggests because more of the air is close to the walls.

Worked example

A bedroom needs 120 CFM and a living room 400 CFM, with a design friction rate of 0.1. The calculator gives an exact diameter of 6.23 in for the bedroom — so a 6-inch run, which carries about 109 CFM — and 9.83 in for the living room, so a 10-inch duct (about 419 CFM) at roughly 733 ft/min. If the ceiling space only allows an 8-inch-high trunk, the equivalent rectangular size is about 11 × 8 in.

Choosing a friction rate

The friction rate is the pressure loss allowed per 100 ft of duct. In residential design it comes from the blower’s available static pressure after filters, coils, registers and fittings are accounted for, divided by the total effective length of the longest run including fitting equivalent lengths. Many simple systems end up between about 0.06 and 0.1 in. w.c./100 ft. Lower friction rates mean bigger, quieter ducts; higher rates allow smaller ducts but need more fan pressure.

Velocity and noise

These are common rules of thumb for quiet residential systems; design guides and equipment manufacturers give specific limits.

Typical room airflow

Room airflow comes from a heating and cooling load calculation (such as Manual J), not from floor area alone. As a very rough cross-check, many cooling systems move around 350–450 CFM per ton of capacity, and the total is shared between rooms in proportion to each room’s load. A small bedroom might need 50–120 CFM and a large open living area several hundred. Use the calculator to turn each room’s CFM into a duct size, then check that the trunk carries the sum of its branches.

Sizing trunks and branches

1. List every supply run with its CFM.
2. Size each branch with the calculator at your design friction rate.
3. Add up the CFM of all branches fed by each trunk section.
4. Size each trunk section for its total, reducing the trunk as branches leave it.
5. Check velocities, especially near registers and in the main trunk.
6. Size returns for the full system airflow at a low friction rate to keep them quiet.

Flexible duct

Flexible duct has a rougher, corrugated lining and loses more pressure than smooth metal duct of the same diameter — and much more if it is compressed, sagging or kinked. Many designers pick a larger flex size than the metal-duct chart suggests, keep runs as short and straight as possible and support them properly. The manufacturer’s friction data should be used for a formal design.

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