HVAC Duct Size & CFM Calculator

Understanding CFM and Duct Sizing

CFM, cubic feet per minute, is the volume of air an HVAC system moves per minute. Getting CFM and duct size right matters as much as choosing the correct AC tonnage. An undersized duct creates excess pressure, noise, and turbulence. An oversized duct moves air too slowly, reducing its ability to mix with room air and deliver effective cooling. The goal is a duct system where every register delivers the right CFM at a comfortable velocity.

How to Use This Calculator

1. Select your AC or air handler size. This determines the total CFM the system must move: 400 CFM per ton of cooling capacity is the standard residential benchmark.
2. Enter the number of supply registers. The total CFM is divided equally between them. For a 3-zone system from a 1.5 ton unit, each branch needs 200 CFM.
3. Select the duct type. Round duct is most efficient; rectangular is used where space is limited; flexible duct adds approximately 15% to friction loss.
4. Select the target velocity. Low velocity (500 FPM) for bedrooms; standard (700 FPM) for most residential; high (900 FPM) for short runs or commercial use.

CFM Per Ton Reference

High humidity climates use lower CFM/ton (350 to 375) to extend the air-contact time with the coil for better dehumidification.

AC Size	Total CFM (standard 400/ton)	At 350 CFM/ton (high humidity)	At 450 CFM/ton (low humidity)
0.75 ton	300 CFM	263 CFM	338 CFM
1 ton	400 CFM	350 CFM	450 CFM
1.5 ton	600 CFM	525 CFM	675 CFM
2 ton	800 CFM	700 CFM	900 CFM
2.5 ton	1,000 CFM	875 CFM	1,125 CFM
3 ton	1,200 CFM	1,050 CFM	1,350 CFM

Round Duct Diameter Quick Reference

D = (CFM / (velocity × 0.7854))^0.5 × 12 inches.

CFM	At 500 FPM	At 700 FPM	At 900 FPM
100 CFM	6.8 in / 173 mm	5.7 in / 145 mm	5.0 in / 127 mm
200 CFM	9.6 in / 244 mm	8.1 in / 206 mm	7.1 in / 180 mm
400 CFM	13.5 in / 343 mm	11.4 in / 290 mm	10.1 in / 257 mm
600 CFM	16.6 in / 422 mm	14.0 in / 356 mm	12.3 in / 312 mm
800 CFM	19.1 in / 485 mm	16.1 in / 409 mm	14.3 in / 363 mm
1,000 CFM	21.4 in / 543 mm	18.0 in / 457 mm	16.0 in / 406 mm

Supply vs Return Duct Sizing

Return ducts carry room air back to the AC unit and operate at lower velocity than supply ducts. A common design practice: size return ducts for 400 to 600 FPM using the same total CFM as the supply system. Return ducts are therefore typically one to two sizes larger than their corresponding supply duct. A system with 600 CFM supply at 700 FPM (14-inch round) would need a 16 to 18 inch return duct at 400 to 500 FPM.

In mini-split split AC systems (common in Asia, Europe, and many residential applications), ducting is typically not used, the indoor unit blows directly into the room. This calculator primarily applies to ducted central air systems and is most relevant for North American, Australian, and Middle Eastern whole-home HVAC systems.

Worked Example: Sizing Ducts for a 2 Ton System

A 2 ton central AC feeds 4 supply registers across a small apartment. Target velocity is standard residential (700 FPM). Using round duct:

• Total CFM: 2 tons times 400 CFM per ton equals 800 CFM.
• CFM per register: 800 divided by 4 equals 200 CFM each.
• Main trunk duct: 800 CFM at 700 FPM needs a 16.1 inch (409 mm) round duct.
• Branch duct to each register: 200 CFM at 700 FPM needs an 8.1 inch (206 mm) round duct.
• Return duct: size at 500 FPM for 800 CFM, requiring a 19.1 inch (485 mm) round duct, about one and a half sizes larger than the supply trunk.

If flexible duct is used instead of rigid metal, increase each diameter by roughly one size to compensate for the higher friction of the corrugated interior surface.

Common Duct Sizing Mistakes

• Undersizing branch ducts. The main trunk duct is often sized correctly but the branches to individual rooms are too narrow, resulting in weak airflow at far registers.
• Ignoring flex duct friction. Flexible duct has 15 to 20% more friction than rigid duct. Sizing the same as rigid duct will result in reduced airflow at the register.
• Not accounting for elbows. Each 90-degree elbow adds approximately 10 ft of equivalent duct length in friction resistance. A run with four elbows adds 40 ft of equivalent friction.
• Forgetting return air paths. Every supply register needs a return air path. Without it, positive pressure builds in the room, forcing cooled air out through gaps and warm air in through others.

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