Introduction
The Duct Size Calculator helps you find the right duct size for your HVAC system. Picking the correct duct size is one of the most important steps in heating and cooling work. If a duct is too small, air cannot flow properly, and your system has to work harder. If a duct is too big, you waste money and space. This tool works out the numbers for you. Just enter your airflow needs in CFM (cubic feet per minute), and the calculator will tell you the right duct dimensions. For round ducts or rectangular ducts, this calculator gives reliable results that match industry standards. It is built for HVAC technicians, contractors, and anyone working on duct design for residential or commercial buildings.
How to Use Our Duct Size Calculator
Enter your airflow requirements and duct specifications below, and this calculator will give you the correct round duct size, matching rectangular duct options, air velocity, friction rate, and total pressure drop. The tool has four tabs: Duct Sizing, Equivalent Sizes, Pressure Drop, and a Reference guide.
Calculation Direction: Choose whether you want to start with a CFM value and find the right duct size ("CFM → Duct Size"), or start with a known duct size and find out how much air it can handle ("Duct Size → CFM").
Sizing Method: Pick how you want the calculator to size your duct. "Size by Friction Rate" uses a target friction loss per 100 feet of duct. "Size by Velocity" uses a target air speed in feet per minute. The field you don't pick will be calculated for you.
Duct Material: Select the type of duct you are using: Metal, Ductboard, or Flex. Each material has a different roughness value, which changes how much friction the air encounters inside the duct.
Airflow Rate (CFM): Enter the total cubic feet per minute of air that needs to move through the duct. You can type a math expression like 200+300+500 and press Enter to add the values together. This field becomes an output when you use "Duct Size → CFM" mode.
Friction Loss Rate: Enter your target friction loss in inches of water gauge per 100 feet of duct. A common value for residential systems is 0.08 to 0.10. This field is active only when you choose "Size by Friction Rate."
Air Velocity: Enter your target air speed in feet per minute (FPM). For homes, 600 to 900 FPM on supply ducts is typical. This field is active only when you choose "Size by Velocity."
Total Equivalent Length (TEL): Enter the total length of the duct run in feet, including extra length added for fittings like elbows, tees, and transitions. The calculator uses this to figure out the total pressure drop across the full run.
Round Diameter / Rectangular Size (Reverse Mode): When using "Duct Size → CFM" mode, enter either a round duct diameter in inches or a rectangular duct width and height. The calculator will then tell you the CFM capacity, velocity, and friction rate for that duct.
Equivalent Sizes Tab: Enter a round duct diameter and a maximum aspect ratio to see a table of all standard rectangular duct sizes that provide the same airflow capacity as that round duct.
Pressure Drop Tab: Enter the airflow in CFM, duct diameter in inches, duct length in feet, and material type to calculate the total pressure drop and friction rate for a specific duct section. A chart shows how pressure drop changes across different duct diameters.
Reference Tab: View recommended air velocities for different building types, standard round duct sizes, material roughness values, key HVAC formulas, and aspect ratio guidelines used throughout the calculator.
What Is Duct Sizing and Why Does It Matter?
Duct sizing is the process of choosing the right width and shape for the air ducts in an HVAC system. Ducts are the tubes and channels that carry heated or cooled air from your furnace or air conditioner to every room in a building. If the ducts are too small, the system has to work harder to push air through, which wastes energy and creates noise. If the ducts are too large, air moves too slowly, the system costs more to build, and it may not deliver air where it's needed. Proper duct sizing keeps your system quiet, efficient, and balanced. When planning the overall HVAC system capacity for a space, our AC Tonnage Calculator can help you determine the right cooling load, which directly influences how much airflow your ducts need to carry.
Key Factors in Duct Sizing
Three main things determine the correct duct size: airflow, friction loss, and air velocity. Airflow is measured in cubic feet per minute (CFM) and tells you how much air a room or zone needs. Friction loss is how much resistance the air meets as it moves through the duct, measured in inches of water gauge per 100 feet of duct. Velocity is the speed of the air inside the duct, measured in feet per minute (FPM). A good duct design balances all three so the system delivers the right amount of air without being too loud or wasting energy. To learn more about flow dynamics involving the Reynolds number, a key parameter in the friction calculations this tool uses, see our Reynolds Number Calculator.
Round vs. Rectangular Ducts
Ducts come in two common shapes: round and rectangular. Round ducts handle airflow most efficiently because they have less surface area touching the air, which means less friction. Rectangular ducts are easier to fit into tight spaces like walls and ceilings, but they create more friction and use more material. When switching between shapes, HVAC designers use a formula called the rectangular equivalent diameter to find a rectangular duct that moves the same amount of air as a given round duct. ASHRAE, the leading authority on HVAC standards, recommends keeping the aspect ratio of rectangular ducts at 4:1 or less. The aspect ratio is the wider side divided by the narrower side. A square duct (1:1 ratio) is the most efficient rectangular option.
Duct Material Matters
The material your duct is made from directly affects friction and performance. Galvanized sheet metal is the most common choice in commercial buildings because it has a very smooth interior, creating the least friction. Ductboard, made from rigid fiberglass, has a rougher inside surface and creates about 1.4 times more friction than metal, but it provides built-in insulation. Flexible duct (flex) has the roughest interior and can produce up to twice the friction of metal duct. Flex duct should be kept as short and straight as possible to avoid excessive pressure drop.
Understanding Friction Rate and Pressure Drop
Friction rate tells you how much pressure the air loses for every 100 feet of duct it travels through. A common target for residential systems is about 0.08 to 0.10 inches of water gauge per 100 feet. Total pressure drop depends on the friction rate and the total equivalent length (TEL) of your duct run. TEL includes the actual length of straight duct plus extra length added for fittings like elbows, tees, and transitions, since each fitting creates additional resistance. If the total pressure drop is too high, the blower fan won't be able to deliver enough air to the rooms farthest from the unit.
Velocity and Noise
Air velocity inside a duct directly affects noise levels. For homes, supply duct velocity should generally stay between 600 and 900 FPM to keep things quiet. Offices and retail spaces can tolerate 1,000 to 2,000 FPM, while industrial settings may run up to 3,000 FPM. Choosing a duct that's one size larger than the minimum can noticeably reduce noise in living spaces without a big increase in cost.
How Professionals Size Ducts
HVAC professionals typically use one of two methods. The equal friction method sizes every duct in the system to the same friction rate per 100 feet, which keeps things simple and balanced. The velocity method sizes ducts based on a target air speed, which is useful when noise control is the top priority. Both methods rely on the Darcy-Weisbach equation and the Colebrook-White formula to calculate friction factors based on duct size, air speed, and material roughness. These are the same engineering formulas used in the ASHRAE Handbook of Fundamentals.