Duct sizing calculator

What size duct do I need for my AC system?

Residential HVAC sizing assumes 400 CFM of airflow per ton of cooling. A 3-ton AC unit moves about 1,200 CFM through the supply trunk and pulls 1,200 CFM back through the return. From there, duct diameter follows the equal-friction method: pick a friction rate (usually 0.08 inches of water column per 100 ft of duct for residential supply) and the duct size falls out of the math. Quick reference for round galvanized sheet metal trunks at 0.08 friction rate:

• 1.5 ton (600 CFM): 10 inch round trunk
• 2 ton (800 CFM): 12 inch round trunk
• 2.5 ton (1,000 CFM): 14 inch round trunk
• 3 ton (1,200 CFM): 14 to 16 inch round trunk
• 3.5 ton (1,400 CFM): 16 inch round trunk
• 4 ton (1,600 CFM): 18 inch round trunk
• 5 ton (2,000 CFM): 20 inch round trunk

Return ducts run one size larger than supply because return air is warmer and lower density. A 3-ton system with a 14 inch supply trunk usually wants a 16 inch return. Flex duct runs about 10 percent larger than the same airflow in sheet metal because the inner wall is rougher and creates more friction loss.

How the equal-friction duct sizing method works

Equal-friction is the duct sizing method ACCA Manual D teaches and the one nearly every residential HVAC contractor uses. You pick a target friction rate, usually somewhere between 0.05 and 0.10 inches of water column per 100 feet of duct, and you size every duct in the system so it stays at that friction rate. The advantage is simplicity: every branch and every fitting drops the same amount of static pressure per linear foot, which makes balancing the system at the registers a matter of damper trim rather than redesigning the duct layout.

The math behind it is the Darcy-Weisbach equation calibrated to standard air density and galvanized sheet metal roughness. The calculator above runs the full version. The shortcut most contractors use is a Trane ductulator slide rule or its online equivalent. Either way, the answer for 400 CFM at 0.08 friction rate is a 7 inch round branch, which is the workhorse size for individual rooms in a residential system.

Supply vs return duct sizing: why returns run bigger

A common contractor mistake is sizing the return duct the same as the supply. Returns should always be larger. Here is why: return air sits closer to the conditioned space temperature while supply air is colder (cooling) or hotter (heating), and warmer air at a given CFM has more volume because of lower density. ACCA Manual D recommends sizing returns at a friction rate of 0.05 to 0.06 in WC per 100 ft, versus 0.08 to 0.10 for supply. The result is a return that is one size larger than the matching supply trunk.

Undersized return ducts are the single biggest cause of high static pressure complaints in residential HVAC. A 3-ton system with a 14 inch supply but only a 12 inch return will routinely exceed 0.8 inches WC total external static, which damages compressors and shortens blower motor life. If you are pulling more than 0.5 in WC across just the return side, your return is too small.

Round vs rectangular duct: which one should you install?

Round duct moves more air per square inch of cross-section than rectangular because the perimeter-to-area ratio is better. A 14 inch round duct (154 sq in area) carries the same airflow as a 22 by 8 rectangular duct (176 sq in area). That means round costs less metal, has lower friction loss, and is faster to fabricate. Most HVAC contractors install round trunk lines wherever they can fit.

Rectangular makes sense in two cases: tight clearance in floor joists or wall cavities where a round duct will not fit, and projects with custom sheet metal shops that can fabricate rectangular faster than running long round sections. For homeowners renovating a tight ceiling space, a 20 by 8 rectangular trunk replaces a 14 inch round at the same CFM. Aspect ratios above 4 to 1 lose efficiency fast. Keep rectangular ducts close to square when you can.

Flex duct vs metal duct sizing: when to upsize

Insulated flex duct is cheaper, faster to install, and naturally insulated. It is also rougher inside than sheet metal, which means about 10 percent more friction loss at the same CFM. The fix is to upsize flex one nominal size compared with sheet metal. A 7 inch sheet metal branch becomes an 8 inch flex branch for the same room airflow.

Flex also kinks. A flex run with a sharp bend or sag can lose 50 percent of its airflow capacity at a single fitting. If you walk into an attic and see flex ducts drooping between trusses, that home is fighting static pressure problems. The fix is to use sheet metal trunks with short flex tails to the registers, supported every four feet, never bent past a 45 degree elbow without a turning vane.

How to read CFM duct sizing charts at a glance

Every CFM duct chart you find online is built from the same equal-friction math, but each one bakes in different assumptions. Read the fine print before trusting a chart. Three things to check: the friction rate (0.08 is standard residential, 0.10 is commercial), the duct material (sheet metal, flex, or duct board), and the standard air density assumption (most charts use sea level at 70 degrees Fahrenheit). If you are sizing for high altitude Denver or Albuquerque, real airflow is 12 to 15 percent lower than the chart predicts and you should upsize accordingly.

What velocity should air move through residential ducts?

Residential supply duct velocity should land between 600 and 900 feet per minute (FPM). Above 900 FPM you start hearing the duct: whistling at register grilles, low-frequency rumble in trunks, and turbulence at every fitting. Below 400 FPM the duct is oversized, which wastes metal and creates dead zones where air stratifies. Return ducts target 400 to 700 FPM because lower velocity reduces noise at the return grille where occupants notice it most.

Commercial ducts run faster: 900 to 1,500 FPM for medium-pressure systems and 2,500+ FPM for high-velocity rooftop units. Those systems use higher friction rates and rely on sound attenuators to keep the noise out of occupied spaces. In a house you cannot do that. Keep velocity in the 600 to 900 FPM band and your duct system stays quiet.

ACCA Manual D vs the contractor rule of thumb

Most HVAC contractors do not run a full Manual D when they replace a system. They use rules of thumb: 6 inch branches for bedrooms, 8 inch for the living room, 14 inch trunk for 3-ton, 16 inch trunk for 4-ton. That works for straight retrofits where the existing duct layout is already known to perform. For new construction, major renovations, or any home with comfort complaints, a real Manual D from software like Wrightsoft or Cool Calc is the only way to get the duct system right. This calculator hits the same numbers Manual D produces for individual runs. For the full system design including fitting losses and zone balancing, a contractor should run Manual D.