Duct Size Chart: CFM for Round, Rectangular & Flex Duct

Q: How do I calculate duct size from CFM?

To calculate duct size from CFM, divide CFM by target air velocity in FPM to get duct area in square feet (Area = CFM / Velocity), then convert to diameter or rectangular dimensions. Alternatively, standard ACCA Manual D friction charts size ducts at 0.1 in. w.g. friction loss per 100 feet.

Q: What size duct do I need for 1,200 CFM?

A 1,200 CFM airflow (standard for a 3-ton heat pump or AC system) requires a 16-inch round duct or a 20×10-inch rectangular duct at standard 0.1 in. w.g. friction rate, keeping residential trunk velocity under 900 FPM.

Q: What size duct do I need for a 12x12 room?

A standard 12×12 ft bedroom typically requires 100 to 150 CFM depending on climate, insulation, and window exposure. At 0.1 in. w.g. friction rate, this requires a 6-inch round duct (up to 109 CFM) or a 7-inch round duct (up to 163 CFM) for quiet residential operation.

Q: What size main trunk duct is needed for a heat pump?

Heat pumps typically require 400 CFM per ton of heating/cooling capacity. A 2-ton heat pump (800 CFM) requires a 14-inch round or 16×8-inch rectangular trunk, while a 3-ton heat pump (1,200 CFM) requires a 16-inch round or 20×10-inch rectangular trunk.

Q: How does return air duct sizing differ from supply duct sizing?

Return air ducts are sized larger than supply ducts to maintain lower air velocity (typically 600 to 700 FPM max, or a 0.05 to 0.08 friction rate). Lower return velocity prevents whistling grilles, reduces fan static pressure stress, and keeps living spaces quiet.

This field duct sizing chart provides reference CFM airflow capacities across standard round, rectangular, and flexible duct dimensions. All values are calculated using standard ACCA Manual D and ASHRAE friction loss at 0.1 in. w.g. per 100 ft (the standard residential design rate) for supply and return air ductwork.

Need a size for a specific CFM? Use the duct size calculator

Round Duct CFM Chart

Assumes rigid galvanized sheet metal at a standard residential friction rate of 0.1 in. w.g. per 100 ft. Velocity sanity guidelines: 600–700 FPM (branch runs), 700–900 FPM (main supply trunks).

Round duct airflow capacities from 4 inch to 24 inch diameter at 0.1 friction loss
Duct Diameter (in)	CFM @ 0.1 in.wg	Velocity (FPM)	Typical Use & Status
4" Round	37 CFM	426 FPM	Bathroom exhaust, small branch ✓ Branch (Quiet)
5" Round	67 CFM	492 FPM	Small bedroom branch, range run ✓ Branch (Quiet)
6" Round	109 CFM	553 FPM	Standard bedroom branch (100–120 sq ft) ✓ Branch (Optimal)
7" Round	163 CFM	611 FPM	Master bedroom or kitchen branch ✓ Branch (Optimal)
8" Round	232 CFM	666 FPM	Living room branch, small supply trunk ✓ Branch (Optimal)
9" Round	317 CFM	718 FPM	Large room branch, small zone trunk ✓ Trunk (Optimal)
10" Round	419 CFM	768 FPM	1-Ton AC main trunk, multi-room branch ✓ Trunk (Optimal)
12" Round	678 CFM	863 FPM	1.5-Ton to 2-Ton main supply trunk ✓ Trunk (Optimal)
14" Round	1,019 CFM	953 FPM	2.5-Ton main supply trunk line ⚠️ Notice: 953 FPM (>900 FPM limit)
16" Round	1,450 CFM	1039 FPM	3.5-Ton to 4-Ton trunk or commercial ⚠️ Notice: 1,039 FPM (Commercial / lower FR)
18" Round	1,980 CFM	1120 FPM	5-Ton residential trunk / light commercial ⚠️ Notice: 1,120 FPM (Commercial / lower FR)
20" Round	2,615 CFM	1199 FPM	Multi-zone commercial trunk line ⚠️ Notice: 1,199 FPM (Commercial)
22" Round	3,364 CFM	1274 FPM	Large commercial distribution ⚠️ Notice: 1,274 FPM (Commercial)
24" Round	4,233 CFM	1348 FPM	High-capacity commercial supply main ⚠️ Notice: 1,348 FPM (Commercial)

Quick Lookups: An 8 inch duct delivers ~232 CFM at 666 FPM (ideal for medium bedrooms), while a 14 round duct delivers ~1,019 CFM (standard 2.5-ton supply trunk) and a 16 inch duct delivers ~1,450 CFM (standard 3.5-ton trunk).

Source: Derived from standard ACCA Manual D and ASHRAE Fundamentals friction loss charts at 0.1 in. w.g. per 100 ft. These numbers are planning figures for initial sizing and do not substitute for a complete Manual J heat load calculation and Manual D duct system design.

Rectangular Duct CFM Chart

Airflow capacities for standard rectangular sheet metal duct sizes at 0.1 in. w.g. per 100 ft using Huebscher equivalent diameter equations. Common residential supply trunks are kept between 700 and 900 FPM.

Standard rectangular duct sizes airflow capacity and equivalent round diameter
Duct Size (W×H)	CFM @ 0.1 in.wg	Equiv. Round (in)	Velocity (FPM) & Status
8×6" Stock	200 CFM	7.6" dia	599 FPM ✓ Branch (Quiet)
10×6" Stock	264 CFM	8.4" dia	634 FPM ✓ Branch (Optimal)
12×6" Stock	330 CFM	9.1" dia	661 FPM ✓ Branch (Optimal)
14×6"	397 CFM	9.8" dia	681 FPM ✓ Branch (Optimal)
16×6"	465 CFM	10.4" dia	698 FPM ✓ Branch (Optimal)
10×8" Stock	393 CFM	9.8" dia	708 FPM ✓ Trunk (Optimal)
12×8" Stock	496 CFM	10.7" dia	743 FPM ✓ Trunk (Optimal)
14×8" Stock	600 CFM	11.5" dia	772 FPM ✓ Trunk (Optimal)
16×8" Stock	706 CFM	12.2" dia	795 FPM ✓ Trunk (Optimal)
18×8" Stock	814 CFM	12.9" dia	814 FPM ✓ Trunk (Optimal)
20×8" Stock	922 CFM	13.5" dia	830 FPM ✓ Trunk (Optimal)
12×10" Stock	673 CFM	12" dia	807 FPM ✓ Trunk (Optimal)
14×10" Stock	819 CFM	12.9" dia	843 FPM ✓ Trunk (Optimal)
16×10" Stock	969 CFM	13.7" dia	872 FPM ✓ Trunk (Optimal)
18×10" Stock	1,120 CFM	14.5" dia	896 FPM ✓ Trunk (Optimal)
20×10" Stock	1,274 CFM	15.2" dia	917 FPM ⚠️ Notice: 917 FPM (>900 FPM limit)
24×10" Stock	1,585 CFM	16.5" dia	951 FPM ⚠️ Notice: 951 FPM (>900 FPM limit)

Stocked Sizes: The most commonly stocked rectangular sheet metal duct sizes in residential HVAC distributor shops are 8×6", 10×6", 12×8", 14×8", 16×8" (~706 CFM), and 20×8" (~922 CFM).

Source: Sized according to standard ACCA Manual D equivalent rectangular duct tables. Note that for quiet residential systems moving over 1,200 CFM, consider reducing design friction rate to 0.08 in. w.g. or increasing trunk depth.

Flexible Duct CFM Chart

Airflow comparison between fully stretched flexible duct vs. typical field installed flexible duct at 0.1 in. w.g. per 100 ft. Velocity limit: keep strictly below 700 FPM.

Flexible duct CFM ratings for stretched vs typical installed conditions
Duct Diameter (in)	CFM (Fully Stretched)	CFM (Typical Installed)	Velocity (FPM) & Limit
4" Flex	33 CFM	30 CFM	344 FPM ✓ Quiet Branch Compliant
5" Flex	59 CFM	54 CFM	396 FPM ✓ Quiet Branch Compliant
6" Flex	96 CFM	88 CFM	448 FPM ✓ Quiet Branch Compliant
7" Flex	144 CFM	132 CFM	494 FPM ✓ Quiet Branch Compliant
8" Flex	204 CFM	188 CFM	539 FPM ✓ Quiet Branch Compliant
9" Flex	279 CFM	256 CFM	579 FPM ✓ Quiet Branch Compliant
10" Flex	369 CFM	338 CFM	620 FPM ✓ Quiet Branch Compliant
12" Flex	597 CFM	548 CFM	698 FPM ✓ Quiet Branch Compliant
14" Flex	897 CFM	823 CFM	770 FPM ⚠️ Exceeds 700 FPM max limit
16" Flex	1276 CFM	1172 CFM	839 FPM ⚠️ Exceeds 700 FPM max limit

Flex Derate Explanation: Flexible duct carries roughly 15% to 25% less CFM than rigid sheet metal of identical diameter due to internal spiral wire ribbing, standard 15% longitudinal field compression, and natural sag between support hangers.

Quick Lookups: A 12 flex duct delivers ~548 CFM installed, while a 14 flex delivers ~823 CFM but exceeds the recommended 700 FPM velocity threshold for flexible ductwork. For runs over 25 feet or airflow exceeding 600 CFM, rigid metal ductwork is strongly advised.

Printable Engineering Reference

Download Duct Sizing Chart CFM (PDF)

Download the complete 1-page printable reference sheet featuring all round, rectangular, flexible, and return air duct CFM tables branded with engineering limits.

Download Chart (PDF) Or open printer-friendly view →

Return Air Duct Sizing

Return air duct sizing requires special care because return grilles directly connect to occupied living areas without sound-dampening coil packages. While supply ducts typically operate between 700 and 900 FPM, return air ducts should maintain a lower design velocity between 500 and 650 FPM (or a conservative friction rate of 0.05 to 0.08 in. w.g./100 ft). Undersized return ducts starve the HVAC blower of required CFM, drastically increasing static pressure, freezing evaporator coils in summer, and generating loud return grille whistling.

Recommended return air duct sizes by AC and heat pump tonnage
System Size	Return CFM	Min. Round Return	Min. Rectangular Return	Design Velocity
1.0 Ton	400 CFM	12"	14×8" or 12×10"	509 FPM (Optimal (<600 FPM))
1.5 Ton	600 CFM	14"	18×8" or 14×10"	561 FPM (Optimal (<600 FPM))
2.0 Ton	800 CFM	14"–16"	20×10" or 24×8"	573 FPM (Optimal (<600 FPM))
2.5 Ton	1000 CFM	16"	24×10" or 20×12"	600 FPM (Standard (600 FPM))
3.0 Ton	1200 CFM	18"	26×10" or 22×12"	611 FPM (Standard (600–650 FPM))
3.5 Ton	1400 CFM	18"–20"	28×10" or 24×12"	642 FPM (Standard (600–650 FPM))
4.0 Ton	1600 CFM	20"	30×10" or 26×12"	660 FPM (Quiet Limit (660 FPM))
5.0 Ton	2000 CFM	22"–24"	34×10" or 30×12"	679 FPM (Quiet Limit (679 FPM))

* When designing multiple return drops or central hallway returns, ensure the sum of all return duct branch areas equals or exceeds the total cross-sectional area specified above.

How to Read This Chart

Duct sizing charts relate three core aerodynamic properties: Airflow (CFM), Duct Cross-Sectional Dimension, and Friction Rate (in. w.g. per 100 ft). This chart standardizes all values at a constant friction loss of 0.10 in. w.g. per 100 ft of equivalent duct length—the default standard specified in ACCA Manual D for residential systems with moderate equivalent duct runs.

To size a duct using this chart:

Identify your design CFM: Determine the heating and cooling airflow required for the zone or room (typically 400 CFM per ton of equipment capacity, or room-by-room CFM from your Manual J load calculation).
Select your duct material: Choose between rigid sheet metal (round or rectangular) or flexible ductwork. If using flex, look under the Typical Installed column to account for real-world wire resistance and installation sag.
Cross-reference velocity limits: Verify that the resulting velocity does not exceed 700 FPM for branch lines or 900 FPM for supply trunks. High air velocity creates wind rush and whistling through dampers and diffusers.

Have non-standard available static pressure, complex duct fittings, or custom friction rates? Calculate exact custom airflow and equivalent sizes using our interactive HVAC duct size calculator.

Frequently Asked Questions

Common questions on ductwork sizing, CFM airflow capacities, and engineering rules of thumb.

How do I calculate duct size from CFM?

To calculate duct size from CFM, divide CFM by target air velocity in FPM to get duct area in square feet (Area = CFM / Velocity), then convert to diameter or rectangular dimensions. Alternatively, standard ACCA Manual D friction charts size ducts at 0.1 in. w.g. friction loss per 100 feet.

What size duct do I need for 1,200 CFM?

A 1,200 CFM airflow (standard for a 3-ton heat pump or AC system) requires a 16-inch round duct or a 20×10-inch rectangular duct at standard 0.1 in. w.g. friction rate, keeping residential trunk velocity under 900 FPM.

What size duct do I need for a 12x12 room?

A standard 12×12 ft bedroom typically requires 100 to 150 CFM depending on climate, insulation, and window exposure. At 0.1 in. w.g. friction rate, this requires a 6-inch round duct (up to 109 CFM) or a 7-inch round duct (up to 163 CFM) for quiet residential operation.

What size main trunk duct is needed for a heat pump?

Heat pumps typically require 400 CFM per ton of heating/cooling capacity. A 2-ton heat pump (800 CFM) requires a 14-inch round or 16×8-inch rectangular trunk, while a 3-ton heat pump (1,200 CFM) requires a 16-inch round or 20×10-inch rectangular trunk.

What is the difference between flex and rigid metal duct capacity?

Flexible duct carries roughly 15% to 25% less CFM than rigid galvanized sheet metal of the exact same diameter. The inner helical wire ribs, natural installation compression, and minor sagging create higher friction resistance, requiring one size larger in flex runs.

How does return air duct sizing differ from supply duct sizing?

Return air ducts are sized larger than supply ducts to maintain lower air velocity (typically 600 to 700 FPM max, or a 0.05 to 0.08 friction rate). Lower return velocity prevents whistling grilles, reduces fan static pressure stress, and keeps living spaces quiet.