What the Heck is a Friction Rate?

Friction rate is a value located on a ductulator. We use it during the design phase of a duct system to predict the operating static pressure of a duct system. The challenge with friction rate is that it is often misapplied by folks using a rule of thumb (ROT).

In order to apply friction rate properly, we need to understand its intent and its limitations.

Friction rate is expressed in inches of water column, which happens to correspond to the pressure scale often used to measure static pressure in duct systems. That means they are the same thing, right? No, but they are related.

The ductulator also provides another piece of information about friction rate, which is f/100; this means the friction rate shown on the ductulator is based on 100’ of straight pipe in the size selected. The challenge here is that our ducts are not all 100’ long, and they are not straight. That's what brings us to the term “effective length” (EL) given for fittings; a fitting with an EL of 25’ has an equal resistance to 25’ of straight pipe. Fittings add up quickly in a system, which means that our total EL (TEL) of a duct system is beyond 100’.

That all brings us to the ROT many duct designers use in the field, which is setting a ductulator on a 0.1” friction rate to choose the size of a duct needed for a given CFM of airflow. That ROT becomes problematic because the actual length of the duct is not considered, which often leads to undersized ducts and low system airflow caused by excessive resistance (high static pressure).

A little math is required to figure out what friction rate the ductulator really needs to be set on to get an accurate picture of what the measured static pressure will be after the duct system is complete. You can see that math in the equation below:

Friction Rate = (Target Static Pressure x 100) / TEL

Example: Let's say we have a desired duct static pressure of 0.10” and a TEL of 200′. We multiply 0.10 by 100 (0.10 x 100 = 10). Then, we divide 10 by the TEL of 200 (10 / 200 = 0.05). So, we can tell that we need a friction rate of 0.05” on the ductulator to select the correct size duct.

So, one little math equation fixes this issue? Whoa there, cowboy, it's not quite that easy.


The equation is only one piece of the puzzle. Unfortunately, there are rarely any easy ways to get accurate answers, which means that using ROTs like a 0.1” friction rate during duct design can lead to poor results.

I recommend becoming comfortable with ACCA Manual D. You can download and use the Manual D speed-sheet from ACCA (https://www.acca.org/standards/speedsheets) for better results.

Don’t be an ROT’er.


—Eric Kaiser

One response to “What the Heck is a Friction Rate?”

  1. Hey Eric,

    What happens when we fall outside of the “ACCA wedge” of .06-.12 (or whatever it is) for sizing ducts. Does airflow become unpredictable at that point? Thanks for the article!

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Related Tech Tips

What is Flux?
In HVAC, you will hear the word “flux” used in two totally different parts of the trade. You may have heard of magnetic flux before. Magnetic flux is the lines of force that emanate from a magnet. You often hear about magnetic flux lines in discussions of motor theory. Transformers also use electromagnetic flux to […]
Read more
Tunnel Vision and How to Avoid It
How many times has the following situation happened to you? You're on your way to that final service call. While you're listening to the customer explain their complaints over the phone, there's this precise moment where you've thought: “I know what it is already. This will be a quick one.” Sometimes, intuition proves to be […]
Read more
Defrost Time & Temperature
The most common method to defrost appropriately in refrigeration involves both time for initiation and a combination of time and temperature for defrost termination (ending defrost). But why can't we just use temperature or time alone? (You may wonder.) Imagine a common freezer with a designed box temperature of -10°F and a coil temperature of […]
Read more

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from