Free HVAC Tool

HVAC Duct Sizing Chart & CFM Calculator

Enter room square footage (or CFM) to get the recommended round duct diameter, air velocity in FPM, and a noise rating. Supports supply branches, return ducts, and main trunks — with flex duct adjustment.

Built by SubcontractorHub — the software HVAC contractors use to quote, finance, and run every job.

Book a Demo

Calculate the Right HVAC Duct Size

Enter room size or a known CFM requirement to get an instant round duct diameter recommendation with velocity and noise check.

Required CFM

200

Min. duct size

8" round

Air velocity

573 FPM

Velocity rating

Good

Round duct options

DiameterMax CFMVelocity at 200 CFMNoise rating
8" round Recommended240573 FPMGood
9" round 320453 FPMGood
10" round 410367 FPMLow
⚠️

This is a simplified estimate. Proper duct sizing requires an ACCA Manual J load calculation and Manual D duct design by a licensed HVAC contractor or engineer. Duct length, fittings, static pressure, and system balancing all affect actual sizing requirements.

HVAC Duct Sizing Chart: CFM by Duct Size

Prefer to size ductwork off a chart instead of a calculator? These are the duct size and CFM figures HVAC contractors use for residential takeoffs — round and rectangular, rigid and flex. Every row is generated from the same capacity curve the calculator above uses, so the chart and the tool always agree. For the software side of the job, see our HVAC contractor platform and HVAC contractor software.

Round Duct Sizing Chart (CFM by Duct Size)

Rigid metal round duct capacity at a 0.1" w.g. per 100 ft friction rate. The flex column applies the standard 25% friction penalty — flex duct carries roughly 20% less air than rigid duct of the same diameter.

Duct diameterCFM rangeMax CFM (rigid)Max CFM (flex)Velocity at max
4" round25–50 CFM50 CFM40 CFM573 FPM
5" round40–80 CFM80 CFM65 CFM587 FPM
6" round60–120 CFM120 CFM95 CFM611 FPM
7" round90–175 CFM175 CFM140 CFM655 FPM
8" round120–240 CFM240 CFM190 CFM688 FPM
9" round160–320 CFM320 CFM255 CFM724 FPM
10" round205–410 CFM410 CFM330 CFM752 FPM
12" round300–600 CFM600 CFM480 CFM764 FPM
14" round415–830 CFM830 CFM665 CFM776 FPM
16" round550–1100 CFM1100 CFM880 CFM788 FPM
18" round700–1400 CFM1400 CFM1120 CFM792 FPM
20" round875–1750 CFM1750 CFM1400 CFM802 FPM

Rectangular Duct Size Chart (CFM by Duct Size)

Rectangular sheet-metal duct converted to its equivalent round diameter using the ASHRAE (Huebscher) equation, then rated against the same capacity curve as the round chart above.

Rectangular sizeEquivalent roundMax CFM
8×6"7.6" round210 CFM
10×6"8.4" round270 CFM
12×6"9.1" round335 CFM
14×6"9.8" round390 CFM
10×8"9.8" round390 CFM
12×8"10.7" round470 CFM
14×8"11.5" round550 CFM
16×8"12.2" round620 CFM
20×8"13.5" round770 CFM
12×12"13.1" round730 CFM
16×12"15.1" round980 CFM
20×12"16.8" round1220 CFM
16×20"19.5" round1665 CFM
20×20"21.9" round2075 CFM

Flex Duct CFM Chart (Flexible Duct Sizing)

Flexible duct has roughly 25% more friction loss than smooth rigid metal of the same diameter, so a flex run carries about 20% less air. This flex duct CFM chart applies that penalty to the capacity curve above — use it whenever the branch is flex rather than hard pipe. Every figure assumes the flex duct is pulled tight; sagging or compressed flex loses far more than 20%.

Flex duct sizeMax CFM (flex)Same size in rigidFlex size to match that rigid CFM
4" (4 inch) flex duct40 CFM50 CFM5" flex
5" (5 inch) flex duct65 CFM80 CFM6" flex
6" (6 inch) flex duct95 CFM120 CFM7" flex
7" (7 inch) flex duct140 CFM175 CFM8" flex
8" (8 inch) flex duct190 CFM240 CFM9" flex
9" (9 inch) flex duct255 CFM320 CFM10" flex
10" (10 inch) flex duct330 CFM410 CFM12" flex
12" (12 inch) flex duct480 CFM600 CFM14" flex
14" (14 inch) flex duct665 CFM830 CFM16" flex
16" (16 inch) flex duct880 CFM1100 CFM18" flex
18" (18 inch) flex duct1120 CFM1400 CFM20" flex
20" (20 inch) flex duct1400 CFM1750 CFMover 20″

How Many CFM for Each Round Duct Size?

The same capacity figures written out in plain language, for when you just need to know how many CFM a given duct size will handle. The first number is rigid metal pipe; the second is the same diameter run in flexible duct.

4 inch round ductcarries 25–50 CFM in rigid pipe (50 CFM maximum), or about 40 CFM as 4 inch flex duct.
5 inch round ductcarries 40–80 CFM in rigid pipe (80 CFM maximum), or about 65 CFM as 5 inch flex duct.
6 inch round ductcarries 60–120 CFM in rigid pipe (120 CFM maximum), or about 95 CFM as 6 inch flex duct.
7 inch round ductcarries 90–175 CFM in rigid pipe (175 CFM maximum), or about 140 CFM as 7 inch flex duct.
8 inch round ductcarries 120–240 CFM in rigid pipe (240 CFM maximum), or about 190 CFM as 8 inch flex duct.
9 inch round ductcarries 160–320 CFM in rigid pipe (320 CFM maximum), or about 255 CFM as 9 inch flex duct.
10 inch round ductcarries 205–410 CFM in rigid pipe (410 CFM maximum), or about 330 CFM as 10 inch flex duct.
12 inch round ductcarries 300–600 CFM in rigid pipe (600 CFM maximum), or about 480 CFM as 12 inch flex duct.
14 inch round ductcarries 415–830 CFM in rigid pipe (830 CFM maximum), or about 665 CFM as 14 inch flex duct.
16 inch round ductcarries 550–1100 CFM in rigid pipe (1100 CFM maximum), or about 880 CFM as 16 inch flex duct.
18 inch round ductcarries 700–1400 CFM in rigid pipe (1400 CFM maximum), or about 1120 CFM as 18 inch flex duct.
20 inch round ductcarries 875–1750 CFM in rigid pipe (1750 CFM maximum), or about 1400 CFM as 20 inch flex duct.

Duct CFM Calculation Formula

Every figure in these charts comes from two pieces of arithmetic. If you would rather calculate duct CFM by hand than read it off a chart, this is the formula:

Round duct area (sq ft) = π × (diameter in inches ÷ 24)²

CFM = air velocity (FPM) × duct area (sq ft)

Equivalent round for rectangular duct = 1.30 × (a × b)0.625 ÷ (a + b)0.25

Worked example — an 8 inch round duct at 700 FPM: area = π × (8 ÷ 24)² = π × 0.1111 = 0.349 sq ft. Then CFM = 700 × 0.349 = 244 CFM, which is why the chart rates 8 inch pipe at roughly 240 CFM. Dividing the diameter by 24 rather than 12 converts it to a radius in feet in a single step.

To run the duct CFM calculation formula backwards — sizing ductwork from a known airflow instead of rating ductwork you already have — solve for area first: area = CFM ÷ FPM, then diameter = 24 × √(area ÷ π). For 400 CFM at 700 FPM that is 0.571 sq ft, which works out to a 10.2 inch duct, so you would install 10 inch rigid pipe or 12 inch flex.

⚠️

Use this chart for quick field sizing and takeoffs. Final duct design should come from an ACCA Manual J load calculation and a Manual D duct design by a licensed HVAC contractor — total run length, fittings, and equipment static pressure all shift the real numbers.

How HVAC Duct Sizing Works

1

Start with the airflow requirement (CFM)

Every room or zone needs a specific amount of conditioned air. A rough residential rule of thumb is 1 CFM per square foot of floor area for cooling in a moderate climate, adjusted for ceiling height and climate severity. The more rigorous approach is a Manual J load calculation that accounts for insulation, window area, orientation, and local climate.

2

Select a duct size that handles the CFM at good velocity

Each duct size has a range of CFM it can carry while keeping air velocity in the comfortable zone (400–700 FPM for supply branches). Too small → high velocity, noisy airflow, and higher static pressure drop. Too large → low velocity, poor throw at registers, and potential stratification.

3

Adjust for flex duct friction

Flexible duct has approximately 25% more friction than smooth rigid metal duct at the same airflow. If you're using flex duct, size up one diameter compared to the rigid duct equivalent. Also keep flex duct stretched tight — kinked or compressed flex duct dramatically increases friction loss.

4

Verify with a full Manual D calculation

This calculator gives a starting point, not a final design. A complete ACCA Manual D calculation accounts for total duct length, number of elbows and fittings, system total external static pressure, and balancing across all branches. Always have a licensed HVAC contractor perform a Manual D design for new installations.

Common HVAC Duct Sizing Mistakes

Undersized main trunk

The main supply trunk must carry the total system airflow before it branches out. An undersized trunk causes high static pressure, forcing the blower to work harder, reducing efficiency, and shortening equipment life.

Using flex duct with same size as rigid

Flex duct requires a one-size-larger diameter compared to rigid metal for the same airflow. Using identical sizes for both materials results in an undersized flex run with noisy, high-velocity airflow.

Too many registers on one branch

Splitting one branch run to serve too many registers divides airflow too thin, leaving far rooms cold in summer. Each branch should be sized for its own load, not overloaded with additional tees downstream.

Ignoring return air capacity

An undersized return duct creates negative pressure in the living space, pulling unconditioned air through gaps and reducing system efficiency. Returns should be sized generously — at least equal in total area to the supply system.

Sharp bends in flex duct

A 90-degree bend in flexible duct can reduce effective diameter by 40% and increase friction by 50% or more. Route flex duct in gentle curves whenever possible. Sharp bends are one of the most common causes of poor airflow in residential retrofits.

Not accounting for equipment static pressure

Every air handler has a maximum external static pressure (ESP) it can operate against. Undersized ductwork raises static pressure, forcing the system into an operating range where airflow drops and efficiency falls. Match duct system design to the equipment's rated ESP.

From Load Numbers to a Signed Proposal

SubcontractorHub helps HVAC contractors build professional proposals, present good-better-best equipment tiers, and close jobs with financing and e-signature on the spot.

See How It Works
✓

HVAC proposals built in minutes

✓

Good-better-best equipment tiers

✓

Energy savings shown per tier

✓

Financing at the kitchen table

✓

Digital signature and same-day close

HVAC Duct Sizing FAQ

How do I calculate duct size?

Calculate required CFM first (roughly 1 CFM per sq ft for moderate climates). Then select a round duct size where the air velocity falls in the comfortable range: 400–700 FPM for supply branches, 400–800 FPM for returns, 700–900 FPM for main trunks.

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

A 144 sq ft room needs roughly 100–150 CFM. A 7-inch round duct (rated for 90–175 CFM) or a 6×8-inch rectangular equivalent is typical. In hot climates or rooms with large west-facing windows, size up to 8 inches.

What is CFM in HVAC?

CFM (Cubic Feet per Minute) is the volume of air a duct or system moves per minute. Residential systems typically move 350–450 CFM per ton of cooling. A 3-ton system moves about 1,200 CFM total, distributed across all supply registers.

What is the correct air velocity for HVAC ducts?

Supply branch ducts: 400–700 FPM. Return ducts: 400–800 FPM. Main supply trunks: 700–900 FPM. Above 900–1,000 FPM, ductwork generates noticeable noise. Below 400 FPM, air may not reach registers with adequate pressure.

What is Manual D duct sizing?

ACCA Manual D is the industry standard for residential duct system design. It calculates duct sizes based on friction rate, equivalent duct lengths, and airflow from a Manual J load calculation. Most codes now require Manual D for new construction.

How many CFM per ton of AC?

Residential systems are designed for 350–450 CFM per ton, with 400 CFM/ton being most common. A 3-ton system moves about 1,200 CFM. Under-ducted systems can't move enough air, causing the coil to freeze and reducing efficiency.

What is the difference between supply and return ducts?

Supply ducts carry conditioned air from the air handler to rooms. Return ducts carry room air back to be reconditioned. Returns are typically fewer, larger ducts; supplies are more numerous, smaller branches. A balanced system has approximately equal supply and return capacity.

Can I use flex duct instead of rigid?

Yes, but flex duct has 25% more friction than rigid metal. Size flex duct one diameter larger than the equivalent rigid duct recommendation. Keep flex duct stretched tight and avoid sharp bends — kinked flex duct dramatically reduces airflow.

How many CFM can an 8 inch round duct handle?

About 240 CFM as rigid metal pipe, with a comfortable working range of 120–240 CFM. The same 8 inch diameter in flex duct carries roughly 190 CFM. For comparison: 6 inch is 120 CFM rigid / 95 CFM flex, 10 inch is 410 / 330, and 12 inch is 600 / 480.

What is the formula to calculate duct CFM?

CFM = air velocity (FPM) × duct area (sq ft), where round duct area = π × (diameter ÷ 24)². An 8 inch duct at 700 FPM gives 0.349 sq ft × 700 = 244 CFM. Reverse it to size ductwork from airflow: area = CFM ÷ FPM, then diameter = 24 × √(area ÷ π).

HVAC Contractors: Close More Jobs With Better Proposals

SubcontractorHub gives HVAC contractors the tools to build professional proposals, present financing, and collect signatures on the spot.

Book a Free Demo

Resources

Related Guides & Tools

This calculator provides simplified estimates. Proper duct sizing requires an ACCA Manual J and Manual D calculation by a licensed HVAC contractor.

All calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements.