# There is a new Duct System Sizes and Airflow Quick Chart that can be found in the PDF.

The average output of an heating, ventilating, and air conditioning unit is calculated by taking the size of the unit in tons and dividing it by 400.The total should be divided by the square footage of your house.The rate of loss of ducts affects the air flow from your system.

How do you calculate the duct size?The average output of an heating, ventilating, and air conditioning unit is calculated by taking the size of the unit in tons and dividing it by 400.The total should be divided by the square footage of your house.The rate of loss of ducts affects the air flow from your system.

The homes use 1 CFM for every square foot of space.In our example, a 100 square foot room will need 100 CFM of air volume.

Look at the plate on the furnace to figure out how much it produces.Multiply the number by the amount of air flowing through the furnace.Divide your number by 10.To calculate the size of your ducts, divide the length by the width.

Measure the diameter of the duct.To get the radius of the duct, divide the diameter by 2.To get the square of the radius, multiply it by itself.To get the area of the duct in square inches, you have to use a formula.

The square root is the total.The 4 ton unit is equivalent to 576 squared.The square root is 24.The return air duct and grill size is 24 by 24 inches.

Air flow is q (Cubic Feet per Minute, cfm) (m3/s)

It's important to know the size of your heating or cooling unit.The average output of an HVAC unit is 400.Divide by the total square footage of your home.You will get a multipliers for all your rooms.

The square root is the total.The 4 ton unit is equivalent to 576 squared.The square root is 24.The return air duct and grill size is 24 by 24 inches.

Remove the vent cover.Don't measure your existing cover.Measure the duct opening in your floor, wall or ceiling.This is the diameter of the duct opening.The vent cover size listed on the product listing is the same as the duct opening size.

Return ducts are larger than supply trunks.3.The closed loop system of the equipment makes supply air and return air equal.

Determine the length and width of the room where the flex duct will be located.Get the square footage of the room by dividing the length and width.Determine the amount of CFM that is required for the room.The online duct calculator can be used to calculate the needed flex duct size.