The right sized ductwork ensures your HVAC system will work efficiently. If the ducts are too small, the system will work harder and use more energy; oversized ducts will reduce air velocity, and you might not know air is venting into rooms. HVAC ductwork sizing involves using a complex formula. To simplify things, here is a general overview of how ductwork size is calculated:

Square Footage

Measure the size of your house and each room. Have an HVAC technician get an accurate measurement, or use a tape measure to measure the length and width of every room. Multiply both numbers to get the square footage and add up the final value for each space.

For odd-shaped rooms, break them down into rectangular shaped parts to calculate measurements.

Friction Loss Rate

Knowing the friction loss rate helps contractors determine the static pressure of air throughout the ductwork. This has a big impact on air flow. Friction loss rate is impacted by the length of the duct and number of coils, filters, registers, grills, damper, and turns. Measuring friction loss rate is complex and should be done by an experienced contractor.

Cubic Feet per Minute (CFM)

Cubic feet per minute is calculated by multiplying the size of the HVAC unit (in tons) by 400. Divide the answer by your home’s square footage to find out total CFM. A duct sizing calculator uses CFM and other factors to determine what you need. Contractors often use special calculating tools or software to arrive at a final conclusion (otherwise, one would have to be extremely good at math!).

Duct Size vs. CFM

Round ducts are available in flexible and metal pipe forms. In the FREE guide below are a few duct size estimates based on duct size, in inches (flexible/metal pipe).

Sky Heating’s HVAC Ductwork Sizing Guide 

Download the Free Guide Here!

Schedule HVAC Ductwork Sizing with Sky

Sky Heating and Air Conditioning provides HVAC and ductwork service throughout the Portland, OR, area and Southwest Washington. Our licensed technicians can properly size and install your ductwork to fit your home and meet your needs, as well as increase efficiency and reliability. For more information on ductwork size, and any other questions you might have, reach out to us online, call (541) 275-2918, or schedule an appointment online.