HVAC Ductwork Sizing (with Calculator): How to Size Air Ducts Easily

Our Ductwork Calculator, which some call a ductulator, gives accurate results for duct sizing diameter when the right information is plugged into it.

That’s what this page is about – knowing the correct data to input into the Ductwork Calculator for air flow CFM, friction loss (which is loss due to friction) and the velocity the air needs to move.

Let’s walk through this process, because it is fairly complex, and there are several places where a significant error in the information put into the calculator will result in duct sizing that isn’t right for your HVAC system.

The Pick HVAC Ductwork Calculator

Here is the calculator for homeowners with significant HVAC experience and for HVAC pros who understand what these boxes are asking for. As explained below, you must know the CFM needed for your entire home and for each room you are sizing ductwork to serve.

Below the calculator, each step of the process is explained in detail, so you can be sure that you’ve got the right duct sizing information for a home that is properly heated and air conditioned

Planning to call a pro? The information here will make you a savvy homeowner that can discuss duct sizing with an HVAC professional, and you’ll have a good sense of whether their calculations are accurate. If you decide to get professional guidance and ductwork estimates, use the Free Local Quotes option or the toll-free number on this page for convenient, no-obligation advice and cost estimates.

Here is the Ductwork Sizing Calculator:

Ductwork Sizing Calculator

Duct Size Chart

Do your own calculations and compare them to our chart – or use the chart for quick, accurate calculations for duct size based on CFM needed for a room. 

One size is given for round ducts, its diameter. You have two or three options for rectangular ducts that give you the same or nearly the same square footage per duct. 

Duct Airflow (CFM)Round DuctRectangular Duct
Note: Maximum Friction Rate: 0.1 inWg/100ft

Ductwork Sizing Calculator – Step by Step

CFM, Maximum Friction Rate and Maximum Velocity – what are these? How are they determined?

Getting these data right means you’ll get the ductwork size right. This is vitally important, something we’ll belabor a bit, because ducts that are too small or narrow will wreck efficiency and make the system work too hard, leading to early mechanical failure.

Your home won’t be comfortable either. Ductwork that is too large won’t have enough pressure, and the velocity of air won’t carry it to the furthest reaches of the home or building. This will result in significant temperature imbalances that nobody will like.

Step 1: CFM – System Size & Blower Size

How many cubic feet per minute, CFM, does your system’s blower need to move to provide sufficient air flow and circulation in your home and to each room?

The total number is determined by the unit size of the AC, heat pump or air conditioner in tons/BTUs.

Sizing an HVAC System

The best method for determining system size is to use a Manual J load calculation for the entire home or building. It uses a long list of inputs such as square footage, layout, insulation, window quality, climate and much more to determine proper HVAC system size for the space.

A quick way to get a close estimate is to use a Central AC BTU Calculator like this one. It uses the most critical information about your home or building to determine the load requirement – how big the HVAC must be to do the job. You might want to open the AC BTU Calculator in a separate window for convenience, so you can easily return to this Ductwork Sizing page.

The general guideline is that you need 300 to 350 CFM per ton (12,000 BTUs) for proper heating and closer to 400 CFM per ton of AC. Humid summer air is heavier than dry winter air, so the blower needs more power to push and pull it around your ductwork.

As a result, it is better to use the 400 CFM number to be sure the blower you select will do the job all year.

Key calculation & example: Divide the total number of BTUs needed by 12,000, since 1 ton of air conditioning is equal to 12,000 BTUs. That means that for each “ton” of AC, it can move 12K BTUs per hour out of (AC) or into (heating) your space.

Let’s say you need 36,000 BTUs of air conditioning. Divided by 12,000 is 3 – you need a 3-ton system.

Now, multiply 3 x 400 CFM to get 1200. You would need a blower that can move 1200 CFM.

Pro Tip: Round up – If you’re more than halfway to the next larger size, round up to it. In our example, if you needed 33,500 BTU, or 2.75 tons, round up to 3 tons and multiply by 400 CFM per ton for the same result – 1,200 CFM.

What we know so far: You have determined the total number of CFM you need to move through the ductwork to ensure balanced, efficient heating and air conditioning that doesn’t put a strain on the system or on the ductwork. Later, you’ll learn how to determine necessary CFM and ductwork sizing for each room.

Step 2: Maximum Friction Rate

This is also called the Maximum Friction Loss Rate – it measures how much air flow loss is caused by friction within the ductwork.

We recommend that you leave this Box right where it is, at 0.1 if you are something of a novice when it comes to the physics of air flow in ductwork. Novice nothing – the fact is that most HVAC technicians use software to determine maximum friction rate, expensive software that most homeowners don’t possess.

However, if you have a grasp of available static pressure (ASP) and measuring the total effective length (TEL) of ductwork, and the necessary equipment to make accurate measurements, then you can figure Max. Friction Rate (FR) precisely. Go for it. The equation is:

Otherwise, you are safest going with the default 0.1 in Wg/100 feet. It represents the greatest amount of friction any system should experience. Most technicians work with a default of 0.05, also written as 0.05 inches wg (water gauge) or wc (water column).

If you are experienced and you do the math – and your maximum friction rate exceeds 0.1, your solution is to employ a larger blower to overcome the higher-than-normal friction and achieve the proper amount of airflow to move heated and cooled air through supply ducts and back to the HVAC unit through return ducts.

FYI: Choosing a different duct material might also help.

Each type of ductwork creates friction loss as the air moves through it. Flat sheet metal ductwork is the smoothest – causes the least friction loss. Coated fiberglass duct board, which isn’t common, is next. Flex duct creates the most friction loss, and importantly, when it isn’t stretched tight, it might create an unacceptable amount of friction.

What you need to know is the total friction loss rate. If the total is higher than the maximum of 0.1” wc, that level of static pressure in the ductwork indicates the need for a larger blower to move the air.

Good Q: What’s Wg/100ft? Wg stands for “inch water gauge.” It is often termed “iwc” or “wc” for inch in water column. These represent a technical measurement of how pressure in a system draws water in a tube, like suction.

Friction Rate and Ductwork Length

As you research sizing ductwork, the term total effective length will loom large for this reason: Friction rate is calculated based on pressure drops per 100 feet. That is why the measurement in our ductwork calculator is “in wg/100ft” or per 100 feet.

You’ll see TEL, the acronym for total effective length.

What is TEL?

Determining total effective length, or TEL, is a science in itself:

Ducts: For straight runs, duct TEL or effective length is the same as length. This includes the main duct lines and the lines split off from it.

Fittings: TEL isn’t just the length of the duct runs. That’s where it starts, but it also includes the pressure drops / friction increase caused by the coils, vents, dampers and fittings like elbows, T’s and Y’s used to put ductwork together. Fittings are the most crucial accessory affecting TEL.

Each fitting is rated with a number representing the amount of friction it causes in feet of ductwork equivalency.  Factors are the shape of the fitting, such as a 90-degree or 45-degree bend, but also how many pieces it is. For example, a 2-piece 45-degree elbow causes the same amount of friction as 15 feet of ductwork, or 50% more friction than a 3-piece elbow of the same angle, which has a TEL rating of 10.

The sum of the TEL of the fittings is almost always higher – much higher – than the TEL of the ductwork runs. For example, a recent ductwork design that came to our attention shows a TEL of about 50 for the actual ductwork runs, combined supply and return. The equivalent duct TEL of the fittings is over 350!

Why we don’t use TEL, or total effective length, in our Ductwork Calculator

Because the layout of most ductwork systems fit into the “normal” range, and our calculator factors for that already.

If you know for sure that your total effective length is beyond standard norms, then you have two options:

Choose a more powerful blower – for example, if your calculation based on the number of tons multiplied by 400 yields 1,200 CFM, consider choosing a blower capable of 1,500 CFM.

Lower the acceptable maximum friction rate by about 25%, which, as the calculator will show, requires ducts with larger diameter / total capacity.

Step 3: Maximum Velocity

We’re going to recommend the same thing here – that you leave the maximum velocity in FPM (feet per minute) at 1500. Higher velocity would create a volume of air flow that would cause “windy” noises in your ductwork and vents and will damage them over time. If a given duct diameter produces a velocity above 1500 FPM, then using ductwork with larger diameter is another way to offset it.

Plus, if your ductwork isn’t properly sealed, the higher velocity would only multiply the amount of air flowing out of gaps and seams in the ducts into untreated spaces like the attic or basement.

Summary of Ductwork Sizing Factors

Here again are the important pieces of the sizing puzzle:

Home Square Footage – Find this information on a blueprint, closing documents or by measuring it yourself using length x width for each room or zone.

HVAC Unit Size – Every HVAC system should be tailored to the home it will serve. A load calculation such as a Manual J is the most precise way to determine this, but our AC BTU Calculator gives a very close estimate. Our Heating BTU Calculator is just as useful.

Blower Size – As explained above, the CFM of the blower is determined by measuring needed BTUs by 12,000 and multiplying the number you get by 400 to find CFM. For example, 48,000 BTUs divided by 12,000 = 4, and 4 x 400 = 1,600 CFM blower.

CFM Room by Room – It’s important to know what diameter  or capacity ducts are needed for each room, and knowing the CFM of air flow it needs is the place to start.

Use the HVAC CFM Calculator to quickly determine needed CFM based on the square footage of the space.

Total Effective Length – Our calculator factors this automatically based on standard TELs. It is vitally important to max. friction rate, aka friction loss.

Maximum Friction Rate – This rating is determined by TEL and the amount of friction caused by duct material, length of duct runs and the friction equivalent for various types of fittings used to construct the ductwork.

Maximum Velocity – The air flow speed the ductwork can safely handle without causing too much pressure, excessive noise or potential air loss and waste.

Our Recommendation

This is complex and technical information that many HVAC technicians struggle to grasp – and wouldn’t accurately calculate without specialized ductwork sizing software.

While DIY ductwork sizing gives you a good estimate of what you need, it makes sense to have an experienced HVAC contractor give you a second opinion. Our free estimate tool can help you can at least 3 local quotes in a minute.

Ductwork sizing is very important to indoor comfort and to the durability, performance and efficiency of the HVAC equipment – as important as sizing the equipment. Since ductwork is expensive and should last 20-50 years, sizing it right is essential.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

1 thought on “HVAC Ductwork Sizing (with Calculator): How to Size Air Ducts Easily”

  1. Is there a minimum cfm per cubic foot requirement in ACCA ? I have a room 156 sq. Ft. which has a 6 inch flex supply duct, and an 8 inch flex return. My understanding is 1 cfm per 1 cu. Ft .


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