01
Formula
Area = CFM ÷ velocity. Diameter = 12 × √(4 × area ÷ π).
Area is in square feet. Diameter is in inches. The duct size calculator does not add fitting losses.
Apply for financingDuct size
Duct size calculator for a round duct
This duct size calculator divides cubic feet per minute by a target velocity in feet per minute, then turns that area into a round duct diameter. It is a velocity method. It is not a full friction-rate or static-pressure design.
Round duct
10.24in
Area0.571 sq ft
Velocity700 ft/min
Airflow400 cfm
Planning estimate from the numbers you enter. Coverage, density, waste and price are assumptions, not a supplier quote or an engineering specification.
How it works
Area in square feet is airflow divided by velocity.
Diameter in inches equals 12 times the square root of four times the area divided by pi. Four hundred cubic feet per minute at 700 feet per minute needs 0.571 square feet of duct. The round diameter is 10.23 inches. Pick the next stock size up if 10.23 is not a size you can buy. A higher velocity makes a smaller duct and usually more noise and more friction. A lower velocity does the opposite. The HVAC load calculator estimates the BTU. This page sizes the airway for a CFM you already chose. Rectangular ducts are not drawn. Convert the area to a rectangle you can fit if the round duct will not.
02
Example
400 CFM at 700 feet per minute.
The area is 0.571 square feet and the round duct is 10.23 inches. Raise the velocity to 900 and the duct gets smaller. The noise and the fan may not thank you.
03
Tips
Velocity is a design choice, not a constant.
Trunk ducts and branch ducts often use different velocities. Type the velocity you are willing to hear and to pay for in fan power. Do not use the same 700 for a quiet bedroom branch and a main trunk unless that is really your rule.
04
Equipment
The fan and the coil are the equipment.
Dengaro arranges financing for HVAC equipment. Duct diameter is not a loan amount.
Important questions
Know what to expect.
Does this replace a ductulator?+
No. A ductulator also tracks friction per 100 feet. This duct size calculator only hits a velocity. Use it to see the diameter, then check friction if the run is long.
What velocity should I type?+
700 feet per minute is a moderate planning default for many branches, not a code. A noisy shop can accept more. A library cannot. Type the limit you were given.
How do I get a rectangle?+
Take the area in square feet and divide by one side in feet. The other side is that quotient. The round diameter is the headline because one number is easier to check.
Where does the CFM come from?+
From the room load and the temperature difference, or from a fan curve. The CFM calculator on this site converts air changes to CFM. This page starts after you have the CFM.
Financing built into the sale