How do I calculate duct size from CFM?

Divide CFM by desired velocity to get area in square feet. For round ducts: Diameter (inches) = √(Area × 4 × 144 ÷ π). For example, 400 CFM at 700 FPM needs about 0.57 sq ft area, which equals a 10-inch round duct.

What velocity should I use for residential ductwork?

For residential systems: Main supply ducts 700-900 FPM, branch supply ducts 600-700 FPM, return air ducts 500-700 FPM. Lower velocities reduce noise but require larger ducts. Higher velocities save space but increase noise and friction loss.

Round duct vs rectangular duct - which is better?

Round ducts are more efficient (less friction loss per CFM), easier to install, and quieter. Rectangular ducts fit better in tight spaces like between joists. Use round when possible; rectangular when space is limited. Maintain aspect ratio under 4:1 for rectangular ducts.

How do I size flex duct compared to sheet metal?

Flex duct has 50-100% higher friction loss than sheet metal due to the corrugated interior. Increase flex duct size by 1-2 inches compared to sheet metal, or derate CFM capacity by 20-30%. Keep flex duct runs as short and straight as possible.

What is equivalent rectangular duct size?

Equivalent rectangular duct delivers the same airflow as a round duct with equal friction loss. Use the formula: De = 1.3 × (W × H)^0.625 ÷ (W + H)^0.25. A 10-inch round duct equals approximately 8×8 or 6×12 rectangular duct.

How do I calculate friction loss in ductwork?

Friction loss depends on velocity, duct size, and material. Use the formula: f = 0.109136 × (CFM^1.9 ÷ D^5.02) for sheet metal. Target 0.08-0.1 inches WC per 100 feet. Higher friction = more fan energy required.

What size duct do I need for a 3-ton AC?

A 3-ton AC needs about 1,200 CFM total airflow (400 CFM per ton). Main supply duct at 800 FPM needs 16-18 inch diameter or equivalent rectangular. Return duct at 600 FPM needs 18-20 inch diameter. Always verify with Manual D calculations.

How do I size return air ducts?

Return ducts should handle 100% of supply airflow. Size for 500-700 FPM velocity (lower than supply) to reduce noise near return grilles. A single return serving 1,200 CFM at 600 FPM needs a 20-inch round or 14×18 rectangular duct.

Duct Sizing Calculator

Our Duct Sizing Calculator helps HVAC professionals, contractors, and DIYers determine the optimal duct dimensions for any airflow requirement. Using industry-standard ASHRAE methods, it calculates round duct diameter, equivalent rectangular dimensions, velocity, and friction loss. Properly sized ductwork ensures efficient airflow, reduces noise, and minimizes energy consumption.

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400 CFM

Calculate

Airflow (CFM)

CFM

Target Velocity

FPM

Residential: 600-900 FPM

Duct Type

Material

Duct Size

straighten

10 inch

Round duct diameter

Air Velocity

700 FPM

Quiet

Equivalent Rectangular

8×8 6×12 10×6

Friction Loss

trending_down

Per 100 ft

0.08 in WC

Total

0.02 in WC

recommend

Recommendation

10" duct is well-sized for 400 CFM at 700 FPM. Ideal for residential.

Input Values

Calculate

Duct Size from CFM

CFM from Duct Size

Velocity Check

Airflow (CFM)

CFM

Target Velocity

FPM

Duct Type

Round

Rectangular

Oval

Duct Material

Sheet Metal (Galvanized)

Flexible Duct

Fiberglass Duct Board

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Enter your values and calculate to receive AI-powered insights about your results.

air CFM to Duct Size

100 CFM 5-6"

200 CFM 7-8"

400 CFM 10"

600 CFM 12"

800 CFM 14"

1200 CFM 16-18"

speed Velocity Guide

Main Supply 700-900 FPM

Branch Supply 600-700 FPM

Return Air 500-700 FPM

Flex Duct ≤600 FPM

lightbulb Quick Tips

1 Round ducts are 27% more efficient than rectangular
2 Flex duct needs +1-2" larger than sheet metal
3 Keep aspect ratio under 4:1 for rectangular

How to Use This Calculator

Select Calculation Mode

Choose to calculate duct size from CFM, CFM from duct size, or check velocity

Enter Airflow or Size

Input CFM required or existing duct dimensions

Set Target Velocity

Enter desired velocity (600-900 FPM for residential)

Choose Duct Type

Select round, rectangular, or oval duct shape

Review Results

See recommended size, velocity, and friction loss

The Formula

Duct size is determined by the required airflow (CFM) and desired air velocity. The cross-sectional area equals CFM divided by velocity. For round ducts, diameter is calculated from the area. Rectangular ducts use equivalent diameter for the same airflow capacity.

Area = CFM ÷ Velocity, Diameter = √(4 × Area ÷ π)

lightbulb Variables Explained

CFM Cubic Feet per Minute (airflow rate)
Velocity Air velocity in feet per minute (FPM)
Area Cross-sectional area in square feet
D Duct diameter in inches
W × H Rectangular duct width × height

tips_and_updates Pro Tips

Keep velocity under 900 FPM for main supply ducts to minimize noise in residential systems

Return air ducts should be sized for lower velocity (500-700 FPM) than supply ducts

Flex duct has higher friction loss - increase size by 1-2 inches compared to sheet metal

Use rectangular ducts in tight spaces but maintain aspect ratio under 4:1 for efficiency

Branch ducts (to individual rooms) typically use 600-900 FPM velocity

Total friction loss should stay under 0.1 inches WC per 100 feet for efficient systems

Proper duct sizing is critical for HVAC system performance. Undersized ducts restrict airflow, reduce efficiency, and increase noise. Oversized ducts waste materials and can cause air stratification.

Residential main supply: 700-900 FPM. Branch ducts: 600-700 FPM. Return air: 500-700 FPM. Commercial systems can use higher velocities but with increased noise and friction.

Round ducts have 27% less surface area than equivalent rectangular ducts, resulting in lower friction loss and material cost. Use rectangular when space is limited, maintaining aspect ratio under 4:1.

Frequently Asked Questions

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