Duct size from CFM: round vs rectangular with recommended fpm ranges by use-case

In HVAC design, calculating the required airflow (CFM) for each room is a critical first step. However, that number is only half of the equation. The other, equally important half is designing a duct system that can actually deliver that specific amount of air efficiently and quietly. This is where duct sizing comes in—the process of converting a target CFM value into physical duct dimensions.

This guide will walk you through the fundamental principles and methods for sizing both round and rectangular ducts from a known CFM value. We'll explore the critical role of air velocity (FPM), compare the pros and cons of different duct shapes, and provide recommended velocity ranges for various applications. By mastering these concepts, you can ensure your HVAC system performs exactly as intended, delivering balanced comfort and peak efficiency.

The entire principle of duct sizing revolves around one simple, fundamental formula that connects airflow volume (CFM), air speed (velocity), and the physical size of the duct (area).

The Duct Sizing Formula

Area (sq ft) = CFM / Velocity (FPM)

• CFM (Cubic Feet per Minute): The volume of air you need to move. This value should come from your room-by-room load calculations (Manual J). You can learn more in our Guide to Calculating CFM Per Room.
• Velocity (Feet Per Minute, FPM): The speed at which the air moves through the duct. This is the variable you, the designer, choose based on the application to balance performance, noise, and friction.
• Area (Square Feet): The cross-sectional area of the duct that is required to achieve the target CFM at the chosen velocity. This is the value you are solving for.

The Importance of Velocity (FPM)

Choosing the right target velocity is arguably the most critical decision in duct sizing. It's a balancing act:

• Too High Velocity: Leads to excessive friction loss (requiring more fan power) and can generate undesirable noise (whistling or rushing sounds from vents).
• Too Low Velocity: Can result in oversized, costly ductwork. In long vertical runs, it may not be sufficient to carry heat effectively or, in some cases, entrain particles. For supply ducts, it can lead to poor air mixing in the room ("dumping").

Therefore, the FPM you choose will directly influence the size of your duct, the cost of the system, and the acoustic comfort of the space.

The optimal air velocity depends on the duct's location in the system and the type of space it serves. The table below provides common, industry-accepted FPM ranges.

For most residential systems, aiming for **700-900 FPM** in main trunks and **500-700 FPM** in branch runs is a solid and common design practice.

Once you've calculated the required cross-sectional area, the next step is to convert that area into physical duct dimensions. The process differs slightly for round and rectangular ducts.

Sizing Round Ducts

Round ducts are the most efficient shape for moving air as they have the least friction per unit of area. The calculation is straightforward.

1. Formula: Diameter (inches) = √(Area (sq ft) × 144 × 4 / π)
2. Example: Let's size a supply branch that needs to deliver 150 CFM. We'll target a velocity of 600 FPM.
   • Required Area = 150 CFM / 600 FPM = 0.25 sq ft
   • Diameter = √(0.25 × 144 × 4 / 3.14159) = √45.84 = 6.77 inches
3. Select Size: Since you can't buy a 6.77-inch duct, you must choose the nearest standard size. In this case, you would typically round up to a **7-inch round duct**.

Sizing Rectangular Ducts

Rectangular ducts are often used when space is limited, such as within wall cavities or bulkheads. While less efficient aerodynamically, they are practical. When sizing rectangular ducts, you must also consider the **Aspect Ratio**.

Aspect Ratio = Long Side / Short Side

For best performance, the aspect ratio should be kept as low as possible, ideally below 4:1. High aspect ratios increase friction, cost, and heat loss/gain.

1. Formula: Height (in) = (Area (sq ft) × 144) / Width (in)
2. Example: We need the same 0.25 sq ft area from the previous example, but it must fit within a 3.5-inch wall cavity. We'll use a standard 3.25-inch depth for the duct.
   • Required Area in sq inches = 0.25 sq ft × 144 = 36 sq in
   • Height (Width in this case) = 36 sq in / 3.25 in = 11.08 inches
3. Select Size: You would select a duct size of **3.25" x 12"**. The aspect ratio is 12 / 3.25 = 3.69, which is acceptable (below 4:1).

Our Ductwork Sizing Calculator is an excellent tool for quickly performing these conversions.

Duct sizing is a systematic process that directly translates an engineered airflow requirement (CFM) into a physical system component. The process is a chain of decisions: first determining the room's load to find the target CFM, then selecting an appropriate velocity (FPM) based on the application, and finally calculating the required area and converting it to practical round or rectangular dimensions.

By understanding the trade-offs between velocity, friction, noise, and cost, you can design a duct system that not only delivers the required airflow but does so efficiently and quietly. For further design, remember to account for friction loss using tools like our Duct Friction Loss Calculator and always consult ACCA Manual D for comprehensive residential duct design.