Joseph is an HVAC technician and a hobbyist blogger. He’s been working as an HVAC technician for almost 13 years, and he started blogging just...Read more
Ductwork is an important part of any HVAC system, and it is important to know how to calculate duct size correctly in order to ensure proper airflow and efficient operation. There are a few different methods that can be used to calculate duct size, but the most common is the use of the ductulator. This tool allows you to input the dimensions of the space that will be receiving air from the ductwork, as well as the CFM (cubic feet per minute) rating of the fan or blower that will be providing air flow.
The ductulator will then provide you with a recommended size for your ductwork.
- Determine the air flow rate in cubic feet per minute (CFM)
- This is the amount of air that will be flowing through the ductwork
- Measure the longest length and width of the area where the ductwork will be installed
- Calculate the area of the space that will receive ductwork
- This is done by multiplying the length times width
- Select a rectangular duct size from a chart that corresponds to your CFM and area calculations
Q: How Do I Calculate the Size of a Rectangular Duct
In order to calculate the size of a rectangular duct, you will need to know the dimensions of the room in which it will be installed, as well as the desired airflow. Once you have these measurements, you can use one of several online calculators (such as this one from HVACDesign) to determine the appropriate duct size. Keep in mind that larger ducts may be more expensive and difficult to install, so it is important to choose a size that is neither too small nor too large for your needs.
Once You Have These Measurements, You Can Use the Following Formula: Width X Height X Length = Volume in Cubic Feet
You can use this formula for any item in your home, from a bookcase to a coffee table. Just make sure you measure in feet and not inches!
If you need to calculate the volume of something that isn’t rectangular, like a sphere or pyramid, there are different formulas you can use.
You can find those formulas online or in a math textbook. To find the volume of a rectangular object, you need to know its width, height and length. Once you have these measurements, you can use the following formula:
width x height x length = volume in cubic feet. For example, let’s say you want to calculate the volume of a bookshelf that is 3 feet wide, 6 feet tall and 2 feet deep. The calculation would look like this: 3 x 6 x 2 = 36 cubic feet.
This formula will work for any rectangular object in your home, from a coffee table to a dresser. Just make sure you measure in feet and not inches! If you’re not sure how many inches are in a foot, there are 12 inches in one foot.
There are also different formulas you can use to find the volume of objects that aren’t rectangular, like spheres and pyramids. These formulas can be found online or in most math textbooks.
Ductwork sizing, calculation and design for efficiency – HVAC Basics + full worked example
To get started, measure the length and width of the room you’re trying to heat or cool. Once you have those numbers, multiply them by 4 to calculate the square footage of your space. For example, if your room is 10 feet long and 12 feet wide, 10 x 12 = 120.
So, your room’s square footage would be 120 square feet. Now that you know the square footage of your room, you need to find out how much airflow (in cubic feet per minute) your HVAC system produces. To do this, divide the total BTUs (British Thermal Units) of your system by 500.
For example, if your system produces 60,000 BTUs, then 60,000 / 500 = 120 CFM (cubic feet per minute). Once you know both the square footage of your space and how much airflow your HVAC system produces, simply divide the former by the latter to calculate duct size. In our example above with a 120-square-foot room and a120-CFM HVAC system: 120 / 120 = 1 inch diameter ductwork.