HVAC Duct Size Calculator
Calculate the right duct size from airflow (CFM) using target velocity or friction rate. Get round and rectangular dimensions, actual velocity and sizing guidance instantly in your browser.
Duct sizing calculator
Advertisement
What Size Duct Do I Need?
Duct size depends on the airflow the duct must carry and the target air velocity or friction rate. Start with CFM. Divide by the target velocity in FPM to get the required cross-sectional area in square feet. The round diameter comes from that area. The calculator then shows the next standard nominal size, because a decimal such as 10.24 in is not a product you order.
At 700 FPM, 400 CFM needs an exact round diameter of 10.24 in and a 12 in standard duct. Change the velocity or the friction rate and the size changes with it. Final whole-system design is more involved than one straight run.
How to Use the Duct Size Calculator
- Enter CFM.
- Choose supply, return, or another application.
- Choose target velocity or friction rate.
- Select round or rectangular.
- Read the recommended standard size and actual velocity.
How Duct Sizing Works
Required duct area. Area in square feet = CFM ÷ target velocity in FPM. A lower velocity needs more area, which means a larger, usually quieter duct. A higher velocity allows a smaller duct and can raise noise and pressure loss. No single velocity is correct for every HVAC installation.
Round diameter. Diameter in inches = 12 × √(4 × area ÷ π). The same relationship is D ≈ √(183.35 × CFM ÷ FPM).
Rectangular area. Area in square feet = width × height ÷ 144. If one side is fixed, the other side is the required area divided by that side, then rounded up to an even inch.
Equivalent round diameter. De = 1.30 × (width × height)^0.625 ÷ (width + height)^0.25. This compares friction. It is not the same thing as identical velocity. Equal area is not equal friction, and a very flat rectangle is worse.
Velocity. Actual velocity = CFM ÷ the area of the duct you would install. After you step up to a standard size, velocity drops below the target.
Friction rate. Friction mode solves the round diameter with the Darcy-Weisbach equation and the Swamee-Jain friction factor. Roughness is 0.15 mm for galvanized steel, 0.05 mm for aluminum, 0.9 mm for duct board, and 3.0 mm for fully extended flexible duct. Air density uses the temperature and elevation you enter, with 70°F and sea level as the defaults. The result is inches of water per 100 ft of straight duct.
Standard-size rounding. The exact diameter stays visible. The recommended size is the next listed nominal diameter at or above that exact number, from 4 in through 36 in. A 10.24 in calculation recommends 12 in. It does not replace 10.24 with 12 in in the exact line.
This calculator is a planning tool for one duct run. It is not a Manual D design and it is not a code-compliance certificate. Final sizing still depends on blower data, available static pressure, fittings, equivalent length, leakage, balancing, noise, and the installed system.
CFM to Duct Size Chart
Every row uses a 700 FPM target on round galvanized steel at 70°F and sea level. Calculated diameter is the exact round size. Recommended standard size is the next listed nominal size. Actual velocity is the speed inside that larger duct, so it is under 700 whenever the size steps up. Common use is a planning description, not a code class. Friction in the standard duct is the Darcy-Weisbach estimate for that size, not a universal design rate.
| Airflow | Common use | Calculated diameter | Standard size | Actual velocity | Estimated friction |
|---|---|---|---|---|---|
| 100 CFM | Small register branch | 5.12 in | 6 in | 509 FPM | 0.086 in. w.c. / 100 ft |
| 200 CFM | Single-room supply | 7.24 in | 8 in | 573 FPM | 0.074 in. w.c. / 100 ft |
| 300 CFM | Single-room supply | 8.86 in | 9 in | 679 FPM | 0.088 in. w.c. / 100 ft |
| 400 CFM | Large room or short trunk | 10.24 in | 12 in | 509 FPM | 0.036 in. w.c. / 100 ft |
| 500 CFM | Large room or short trunk | 11.44 in | 12 in | 637 FPM | 0.055 in. w.c. / 100 ft |
| 600 CFM | Supply trunk | 12.54 in | 14 in | 561 FPM | 0.036 in. w.c. / 100 ft |
| 800 CFM | Supply trunk | 14.48 in | 16 in | 573 FPM | 0.031 in. w.c. / 100 ft |
| 1,000 CFM | Main trunk | 16.18 in | 18 in | 566 FPM | 0.027 in. w.c. / 100 ft |
| 1,200 CFM | Main trunk | 17.73 in | 18 in | 679 FPM | 0.037 in. w.c. / 100 ft |
| 1,600 CFM | Large trunk or light commercial main | 20.47 in | 22 in | 606 FPM | 0.024 in. w.c. / 100 ft |
| 2,000 CFM | Large trunk or light commercial main | 22.89 in | 24 in | 637 FPM | 0.023 in. w.c. / 100 ft |
Common Duct Size Examples
400 CFM
Input. 400 CFM, 700 FPM target, round galvanized steel, 70°F at sea level
Target assumption. 700 FPM on smooth galvanized round duct. Not a code limit.
Exact calculated size. 10.24 in. Next standard size. 12 in. Actual velocity. 509 FPM.
Interpretation. The exact round size is 10.24 in, and the next duct you can commonly buy is 12 in. The air slows once you step up, which is why the actual velocity is below the 700 FPM target.
500 CFM
Input. 500 CFM, 700 FPM target, round galvanized steel, 70°F at sea level
Target assumption. 700 FPM on smooth galvanized round duct. Not a code limit.
Exact calculated size. 11.44 in. Next standard size. 12 in. Actual velocity. 637 FPM.
Interpretation. 500 CFM at this velocity still lands on a 12 in standard round duct. The exact size is larger than the 400 CFM case, so the 12 in duct runs faster.
800 CFM
Input. 800 CFM, 700 FPM target, round galvanized steel, 70°F at sea level
Target assumption. 700 FPM on smooth galvanized round duct. Not a code limit.
Exact calculated size. 14.48 in. Next standard size. 16 in. Actual velocity. 573 FPM.
Interpretation. 800 CFM is a common 2-ton planning airflow at 400 CFM per ton. At 700 FPM the standard round size steps up to 16 in.
1,000 CFM
Input. 1,000 CFM, 700 FPM target, round galvanized steel, 70°F at sea level
Target assumption. 700 FPM on smooth galvanized round duct. Not a code limit.
Exact calculated size. 16.18 in. Next standard size. 18 in. Actual velocity. 566 FPM.
Interpretation. 1,000 CFM is the 2.5-ton planning airflow at 400 CFM per ton, and it is also a common trunk number. The standard size is the next listed diameter at or above the exact result.
1,200 CFM
Input. 1,200 CFM, 700 FPM target, round galvanized steel, 70°F at sea level
Target assumption. 700 FPM on smooth galvanized round duct. Not a code limit.
Exact calculated size. 17.73 in. Next standard size. 18 in. Actual velocity. 679 FPM.
Interpretation. 1,200 CFM is the 3-ton planning airflow at 400 CFM per ton. The exact size is just under 18 in, so 18 in is the listed standard size and the velocity ends up under the 700 FPM target.
Duct Size by AC Tonnage
AC tonnage is not itself a duct size. The airflow has to be chosen first. These rows use 400 CFM per cooling ton from the Duct CFM Calculator, then size a round galvanized duct at 700 FPM. A 2 ton plan is about 800 CFM, 2.5 ton about 1,000 CFM, 3 ton about 1,200 CFM, 4 ton about 1,600 CFM, and 5 ton about 2,000 CFM. Blower tables can differ.
| Cooling tons | Planning CFM | Exact diameter | Standard size |
|---|---|---|---|
| 2 ton | 800 CFM | 14.48 in | 16 in |
| 2.5 ton | 1,000 CFM | 16.18 in | 18 in |
| 3 ton | 1,200 CFM | 17.73 in | 18 in |
| 4 ton | 1,600 CFM | 20.47 in | 22 in |
| 5 ton | 2,000 CFM | 22.89 in | 24 in |
Round vs Rectangular Duct
Round duct is often the simpler shape to size. Less internal surface means less friction for the same airflow job, and the friction model is built on a round section.
Rectangular duct is useful where a joist bay, chase, or ceiling will not accept a round pipe. A 12 × 8 section can fit where a 12 in round cannot.
Equivalent diameter is not the same thing as identical velocity. A rectangle can have the same friction behavior as a round duct and still move the air at a different speed, because the areas are not equal. Aspect ratio matters: past about 4:1, the flat shape costs more pressure than a squarer duct.
Supply vs Return Duct Sizing
A supply branch is often planned slower than a supply trunk. The branch is close to the room, so velocity shows up as register noise. A trunk can run faster because it is usually farther from the grille and it is collecting or feeding several branches.
Return ducts are often planned slower than the supply that serves the same rooms. Return air also has to get through a grille and, on many systems, a filter. Sizing the supply and ignoring the return leaves the blower short of air even when the supply math looks fine.
The application list is a set of planning starts: about 600 FPM for a supply branch, 900 FPM for a supply trunk, 500 FPM for a return branch, and 700 FPM for a return trunk. Commercial low-pressure ductwork is often planned faster. Verify the number against the equipment and the noise limit. These are not code maximums.
Flex Duct Sizing
Flexible duct has a higher friction rate than galvanized steel at the same inside size because the liner is rougher and the installation is rarely a perfect cylinder. This calculator does not add a flat percentage. It changes the absolute roughness in the same Darcy-Weisbach model: 0.15 mm for galvanized steel and 3.0 mm for fully extended flex.
At 400 CFM and 0.08 in. w.c. per 100 ft, fully extended flex sizes larger than galvanized steel in this same model. The flex FAQ states both diameters. Compressed, sagging, or sharply bent flex is rougher than 3.0 mm. Use the duct manufacturer’s data for that condition, pull the flex tight, and support it.
Common Mistakes
- Choosing a duct size from square footage alone, with no CFM.
- Using CFM without saying whether the target is velocity or a friction rate.
- Sizing the supply and ignoring the return.
- Treating flex as if it had the same friction as smooth metal.
- Keeping an undersized branch because it happens to fit the joist bay.
- Ignoring fittings and equivalent length. A straight-run loss is not the whole path.
- Treating 400 CFM per ton, or any rule of thumb, as a full Manual D design.
- Confusing AC tonnage with the airflow the blower actually delivers.
Related HVAC Calculators
- Calculate HVAC airflow from AC tonnage or from a known duct and velocity with the Duct CFM Calculator.
- Estimate room BTU before you turn a cooling load into duct airflow.
- Calculate furnace BTU for heating output. Keep heating BTU out of a cooling-airflow estimate.
- Size a heat pump when the capacity question is tons, not duct diameter.
- More HVAC and home calculators are grouped with the calculator tools.
Frequently Asked Questions
What size duct do I need for a given CFM?
You need the airflow and a velocity or friction-rate target. Area in square feet is CFM ÷ FPM for a velocity target. The round diameter comes from that area, and the calculator then shows the next standard size separately. A friction-rate target solves the diameter with a roughness model instead.
How do I calculate duct size from CFM?
Divide CFM by the target velocity in feet per minute to get area. For a round duct, diameter in inches = 12 × √(4 × area ÷ π). For a rectangle, pick one side and divide the required area by that side. Then step up to a size you can actually buy.
What size duct do I need for 400 CFM?
At a 700 FPM planning velocity on round galvanized duct, 400 CFM calculates to 10.24 in exact. The next standard size is 12 in, and the air in that larger duct moves at about 509 FPM. A higher allowed velocity produces a smaller duct.
What size duct do I need for 500 CFM?
At 700 FPM, 500 CFM calculates to 11.44 in exact and a 12 in standard round duct, with about 637 FPM inside that standard size.
What size duct is needed for 1,000 CFM?
At 700 FPM, 1,000 CFM calculates to 16.18 in exact and a 18 in standard round duct. The velocity inside the standard duct is about 566 FPM because the manufactured size is larger than the exact result.
What size duct is needed for a 3-ton AC?
A 3-ton planning figure at 400 CFM per ton is 1,200 CFM. At 700 FPM that airflow calculates to 17.73 in exact and a 18 in standard round duct. Equipment blower data can call for a different CFM.
What velocity should I use for a residential duct?
There is no single residential velocity in the code that this page treats as a limit. Common planning starts are about 600 FPM for a supply branch, 900 FPM for a supply trunk, 500 FPM for a return branch, and 700 FPM for a return trunk. Lower is usually quieter. Higher can shrink the duct and raise noise and pressure loss.
What is duct friction rate?
Friction rate is the pressure used to push air along the duct, in inches of water column per 100 feet. A lower rate generally means a larger duct. This calculator estimates it with Darcy-Weisbach for a straight run. Elbows and boots add effective length that a single rate does not include.
What is the difference between round and rectangular duct?
Round duct usually has less internal surface for the same airflow job. Rectangular duct fits joist bays and chases where a round pipe will not. Equivalent diameter compares friction. It does not mean the air speed matches a round duct of that diameter.
Does flex duct need to be larger than rigid duct?
Often yes, if you are holding the same friction rate. At 400 CFM and 0.08 in. w.c. per 100 ft, this model sizes galvanized steel at 10.21 in and fully extended flex at 11.48 in, because flex is assigned 3.0 mm roughness instead of 0.15 mm. That is not a universal percentage. Sagging flex needs manufacturer data, not this straight-run estimate.
How do I check an existing duct size?
Open Check an existing duct, enter the CFM you expect and the inside diameter or the width and height. The result is velocity, area, an equivalent round diameter for a rectangle, and a planning-range verdict for the application you picked. It also shows how much CFM that size would carry at the application’s planning velocity.
Is this a replacement for Manual D?
No. Manual D accounts for available static pressure, the longest effective length, every fitting, leakage, and room-by-room CFM. This page sizes or checks one straight run and labels the assumptions.