HVAC Duct Calculator

Too large or too small HVAC ductwork sizing can cause problems similar to what happens when technicians install an improperly sized HVAC unit. To check for accurate measurements, many techs rely on HVAC duct sizing calculator free tools, such as a ductulator. 

Using the wrong size duct for the space can prematurely wear out HVAC components and will likely increase customers’ energy expenses. Incorrect duct size can also cause inadequate airflow to certain areas and produce unwelcome noise. None of those scenarios result in happy customers after they’ve paid big bucks for a new, more efficient HVAC system or upgraded ductwork. 

Free Online Ductulator Tool

A duct size calculator, commonly known as a ductulator, depends on factors like the size of the space you’re heating or cooling, air flow velocity, friction loss, and available static pressure of the HVAC system. Save time on the job and do less manual math by using our free, online ServiceTitan Ductulator to easily calculate the right size duct for your projects.

Below we walk through the various formulas you will need to compute, and enter into the duct calculator. 

Figure Square Footage of Spaces

A duct sizing chart relies foremost on the square footage of a home or office space—but, more importantly, the size of each individual room within the building.

To calculate the area of a rectangular or square room, simply multiply the length and width of the room. You can also refer to a building’s blueprint, zoning drawings on file with the local planning office, or a recent real estate listing for the space, if available.

So, if a room measures 10-by-10 feet, the total area equals 100 square feet. For rooms that aren’t perfectly square or rectangular, such as an L-shaped space, split the room into sections and total the area of each section. 

Determining Air Duct Sizing by Velocity of Air

Air velocity, or airflow, gets measured in cubic feet per minute (CFM) and is directly proportional to the size of ductwork. You must find the duct CFM of each room to figure out the size of air ducts to install. It’s important to do room-by-room calculations, otherwise temperatures will likely measure uneven throughout the house or office. 

To calculate the duct CFM for each room, you must first perform an HVAC load calculation for the whole house and for each room, using the Manual J method. 

Use the free ServiceTitan HVAC Load Calculator to figure the exact amount of BTUs per hour each room requires for sufficient heating and cooling, as well as the load capacity required for the entire house or building.

Size of HVAC Unit Required

You also must determine which size of HVAC equipment will work best to meet the energy demands for the space, based on your whole-home or whole-office HVAC load calculations. 

To calculate the required equipment size, divide the HVAC load for the entire building by 12,000. One ton equals 12,000 BTUs, so if a house or office needs 24,000 BTUs, it will take a 2-ton HVAC unit. If you get an uneven number, such as 2.33 for a 28,000 BTU load capacity, round up to a 2.5-ton unit. 

To use the duct CFM calculator, you must next calculate the equipment’s estimated airflow in CFM. Multiply the tonnage required (that you just calculated above) by 400 CFM, which is the average output of an HVAC unit. For a 2-ton HVAC unit, the equipment CFM totals 800.  

NOTE: The average airflow output in cooling mode is between 350 and 400 CFM. Heating season airflow requires approximately 65 percent of the airflow needed for cooling. Therefore, to ensure there’s adequate airflow for both cooling and heating, use the high-end threshold of 400 CFM when referencing a duct sizing chart CFM resource. 

Duct CFM Calculation Formula

Once you do the load calculations and figure out the equipment output required, apply this duct CFM calculation formula to determine each room’s demand:

Room CFM = (Room load/Whole house load) ✕ Equipment CFM 

As an example, say Room A needs 2,000 BTUs of heat gain based on HVAC room-by-room load calculations, and the home overall needs 24,000 BTUs, which requires a 2-ton furnace with a velocity of 800 CFM. 

24,000 BTUs ÷ 12,000 BTUs in 1 ton = 2 tons ✕ 400 CFM per ton = 800 CFM

Room A = (2,000 BTUs ÷ 24,000 BTUs) ✕ 800 CFM

Room A = 66.67 CFM

TIP: Approximately 1 CFM of air is required to heat or cool 1 to 1.25 square feet of floor area. It takes closer to 2 CFMs to cool rooms with a lot of windows or direct sunlight. 

Figure the Friction Loss Rate

Friction rate (FR) helps you decide the diameter and shape of ductwork you can use without negatively impacting optimal air flow. It’s calculated by using the available static pressure (ASP) divided by total effective length (TEL) and multiplied by 100 to show how much pressure drop the system can accommodate per 100 feet of effective length. You want a higher friction rate, because it means you can use smaller, more restrictive ductwork than on an HVAC project designed with a lower friction rate, which requires larger ducts. With a low friction rate, one faulty component can severely hamper air flow because there’s less room for error.

Refer to the duct CFM chart in the HVAC manufacturer’s specs to determine the external static pressure of the blower for that specific HVAC model. It’s typically displayed as a CFM chart for HVAC that breaks down different blower settings and total CFMs required for the house or building. 

The Total External Static Pressure (TESP) gets measured in inches of water column (wc or iws). As a rule of thumb, the majority of systems have a default friction rate of .05” wc, so you can use that average rate as your friction rate, calculate it using a ductulator chart, HVAC duct sizing software, or figure the friction rate yourself to get a more accurate measurement. 

From there, deduct the pressure drops created by any components you plan to add to the air distribution system, such as external coils, filters, grills, registers, and dampers. The Manual D method, which focuses on how to design duct systems, suggests using 0.03 iwc for a supply register, return grille, and balancing damper. Air filters typically list estimated pressure drop on the product packaging or the manufacturer’s website.

That deduction gives you the available static pressure (ASP), or static pressure budget, you’re working with when designing the duct system. You cannot exceed the ASP or the system will deliver improper airflow and cause equipment problems over time. 

ASP impacts HVAC ductwork sizing. The less static pressure available, the larger the ductwork required. If projected velocity seems too high for the system, select the next-largest duct size.

Total Effective Length of Ductwork

The total effective length (TEL) equals the measured length from the farthest supply outlet, through the equipment, and to the farthest return outlet—plus the equivalent lengths of all turns and fittings. Friction rate gets calculated based on the pressure drop per 100 feet. 

TEL takes into account the pressure drops that will happen from splits, turns, and other fittings in the HVAC ductwork plan. Instead of trying to calculate all of those individual instances of pressure loss, HVAC professionals measure the length of straight duct run that would create the same pressure drop, which is called effective length. Each fitting has an effective length that equates its pressure drop to an equivalent amount of straight duct. 

To configure TEL, add up the effective lengths of all fittings in the most restrictive run and add that number to the length of the straight sections between the return and supply in that run. Once you know the TEL, you’re ready to calculate friction rate, which an HVAC duct sizer tool uses to size all duct trunks and branches.

Friction Rate = (ASP X 100) ÷ TEL

Here’s an example the friction rate calculation:

Measured length of straight duct = 50 feet

Equivalent lengths of turns and fittings between the start and end of straight duct: 150 feet

50’ + 150’ = 200 feet TEL

External static pressure of air handler @ 1000 CFM = 0.5” wc

Subtract static drops for components = 0.03” wc for register, 0.03” wc for grille, and 0.15” wc for filter: 0.5 － 0.03 － 0.03 － 0.15 = 0.29” wc ASP

Friction Rate = (0.29 ✕ 100) ÷ 200 = 0.145’ wc

Other Details for Duct Sizing Calculator

There are a few other important factors to consider when using a free HVAC duct sizing calculator to determine HVAC duct dimensions and sizing, such as ducting material type. Do you plan on installing an HVAC rectangular duct or round duct? 

Keep in mind, duct material choice also affects airflow resistance and static pressure, therefore calculations for flexible duct sizing differ slightly from sheet metal ductwork. Flex duct CFM will measure less than the airflow in sheet metal and for coated fiberglass duct board. Rigid sheet metal provides the least airflow resistance. Flex duct CFM changes based on how it’s installed, with performance drastically reduced if not completely stretched out, or with sharp turns and twists. 

In the ServiceTitan Ductulator, select the type and shape of duct you plan to use to get the correct corresponding measurements in the duct size chart.

Want to grow your HVAC business? Learn more about what HVAC software can do for you by scheduling a demo today.

Disclaimer Statement

*Estimated in good faith, the duct size calculator is meant solely for generic, informative purposes. We do not guarantee the accuracy of this information. Please note that other external factors may affect or falsify the recommendations of this tool. For accurate values, consult a licensed heating and air conditioning professional or building engineer.