HVAC Duct Size Calculator
This tool uses a known airflow to help you choose a practical duct size. It shows a round size, a rectangular option, air velocity, and pressure loss. Start with the design CFM from a room load calculation. Do not guess the airflow from duct size alone.
1Enter the design airflow
Use the design CFM from a room load calculation. Do not use equipment tonnage by itself.
Include the added length caused by elbows, boots, dampers, and other fittings.
2Choose the duct
Squeezed or sagging flex duct can carry much less air than a straight duct of the same listed size.
Closest standard round duct
12-inch round
The formula gives 10.3 inches before the tool rounds up.
Round velocity
509 FPM
Rectangular option
14 × 7 in
588 FPM
Round-duct pressure loss
0.037 in. w.g.
This is friction for the rounded-up duct size. Include fittings in effective length.
This tool sizes one duct section. A full Manual D design must also cover registers, return paths, blower static pressure, filter and coil losses, balancing, leaks, and every fitting.
Inputs stay in this browser. No account or quote request is required.
Quick planning answer
Home duct branches often use a friction rate near 0.06 to 0.10 inch of water column per 100 feet. The right rate depends on available static pressure and total effective length. At 400 CFM and a 0.08 friction rate, the equation calls for a little more than 10 inches. The tool rounds up to a common 12-inch duct.
Find the CFM before choosing duct size
A room-by-room Manual J calculation finds the heating and cooling load for each room. Those loads help set the supply CFM. Manual D then uses blower data, filter and coil pressure losses, fittings, and the longest duct path to find the friction rate.
The order matters. A rule like 'one six-inch duct per bedroom' can give too little air to a sunny corner room and too much to an inside room. The register boot and grille must also be large enough for the final airflow.
Effective length includes the fittings
A duct may measure 25 feet but act like 80 feet after you count a sharp elbow, takeoff, boot, and damper. Use the fitting lengths from a design guide instead of adding a guessed percentage. The result shows how much pressure the full effective length uses.
- Pull flex duct tight and support it well. Deep sags quickly add resistance.
- Check return air separately. Include filter face velocity and return paths when doors are closed.
- Wide, flat rectangular ducts use more metal. They can flex, make noise, or leak if they are not reinforced.
- After installation, measure external static pressure and compare it with the blower table.
Air velocity can point to comfort problems
High air velocity can cause noise at fittings and grilles. Very low velocity is not always better. A larger duct must fit, stay sealed, and still push air far enough from the register. This tool warns you when either the round or rectangular option is above a cautious planning limit of 900 FPM.
Measure airflow in an existing system when possible. Leaks, crushed flex duct, a dirty filter, closed dampers, or a wet or dirty coil can reduce the CFM that reaches a room.
Questions people ask before buying
What size duct do I need for 400 CFM?
At a common friction rate of 0.08 in. w.g. per 100 ft, a smooth round duct is near 10 inches. Duct material, effective length, available static pressure, air velocity, and fittings can change the size.
What friction rate should I use for residential ducts?
Many examples use 0.06 to 0.10 in. w.g. per 100 ft. To find the right rate, divide the blower's available static pressure by the longest path's effective length. Then multiply by 100.
Can I size flex duct like metal duct?
Flex duct has more real-world resistance, especially when it is squeezed or sagging. This tool adds a planning allowance. Final design should use the maker's pressure table for the way the duct will be stretched and supported.
Does a duct calculator replace Manual D?
No. This tool sizes one duct section after you know the CFM and friction rate. Manual D covers the full supply and return system, including fittings, paths, pressure, balancing, registers, and grilles.
Method, sources and limits
For round ducts, the tool uses the common equal-friction formula: FR = 0.109136 × CFM^1.9 / diameter^5.02. It adds 10% to the planned diameter for flex duct and rounds up to a standard size. For the displayed pressure loss, it puts that rounded diameter back into the formula, then multiplies the recalculated friction rate by effective length and divides by 100.
For rectangular ducts, the tool uses the Huebscher equivalent-diameter formula and preserves the selected aspect ratio within its dimensional limits. It keeps air velocity at or below 1,200 FPM. Effective length must already include the fittings.
- Residential duct system design, Manual D (ACCA)
- HVAC: How to Size and Design Ducts (CED Engineering)
- HVAC ducts in conditioned space (U.S. Department of Energy Building America)