Duct Sizing by Friction Rate: What the Number Means

What the friction rate is

The friction rate is the pressure loss allowed in each 100 feet of duct, in inches of water column per 100 ft. A DOE Building America guideline (written by IBACOS, 2011) defines it as the pressure loss between two points separated by a set length. It comes from two numbers a designer already has to find:

• Available static pressure (ASP): what is left for the duct system after the pressure losses of the other components in the air path are subtracted from the blower's external static pressure (ASP = ESP − CPL in the ICC deck).
• Total effective length (TEL): the straight length of the longest duct path plus the equivalent length of every fitting on it, as that guideline describes it.

The formula, as printed in an ICC training deck on ACCA's process and in Energy Vanguard's walk-through, is:

FR = ASP × 100 ÷ TEL

The ICC deck's example is 0.23 in. w.c. over 375 ft, which gives 0.061. For our worked example we use made-up numbers: 0.30 in. w.c. available and 410 ft of effective length, so FR = 0.30 × 100 ÷ 410 = 0.0732, which we round to 0.073. These are example inputs, not a design. Real ASP and TEL come from the blower's data and a fitting-by-fitting length count.

Notice the direction. More available pressure or a shorter path gives a higher friction rate and smaller ducts. A long, fitting-heavy path gives a lower rate and bigger ducts. Energy Vanguard points out that this feels backwards until you read the number as the pressure drop you can afford instead of friction to be minimized.

From friction rate and airflow to a diameter

Once you have a friction rate, every duct section is sized with the same rate and its own airflow in cfm. The 2001 ASHRAE Fundamentals duct chapter, as described in a Berkeley Lab paper, uses the Darcy friction equation with the Altshul-Tsal friction factor. Charts and slide rules give the same kind of answer by eye. We did not check newer Handbook editions.

Our duct size calculator solves those equations directly. It assumes round galvanized sheet metal, an average roughness of 0.0003 ft (the 2017 Handbook value for galvanized duct, as quoted in a 2019 ASHRAE article), and standard air at 0.075 lb/ft³ (AMCA), with the viscosity of air at 70 °F worked out from Sutherland's formula as NASA gives it (NASA Glenn). That article notes the galvanized value rests on older tests that simulated joints, so real ducts can be rougher. Altitude, temperature and flex duct are not covered. A paper duct calculator can read a few percent differently, because charts are drawn and read by eye.

Velocity falls out of the same inputs: velocity in fpm is airflow in cfm divided by the duct's cross-section in square feet.

Worked example: 400 cfm at 0.073

Take one section that must carry 400 cfm at the 0.073 rate computed above. The exact diameter from the equations is 10.34 in., and the air moves at 686 fpm. Ducts come in whole inches, so you round up and see what each size does:

The 10 in. duct is smaller than the exact answer, so it runs above the 0.073 you designed for (0.0858). The next size up, 11 in., is the smallest that is not undersized. Check which sizes your supplier stocks before you count on any one of them.

Why not just use 0.10

Energy Vanguard says a lot of people assume 0.1 instead of calculating the rate, and that this undersizes ducts when the real figure is lower. The ICC deck likewise warns against designers who always use 0.10 and says the rate must be calculated for each system.

Here is what that does to the same 400 cfm section:

Designing at 0.10 shrinks this duct by 0.65 in. (10.34 down to 9.69), which is about 12% less cross-section, and pushes the air about 95 fpm faster. Spread over the whole job it compounds. If every foot of the 410 ft effective length ran at 0.10, the system would want 0.10 × 410 ÷ 100 = 0.41 in. w.c. against the 0.30 available in our example. That is more pressure than the system has to spend. This is arithmetic on example numbers, not a measurement of any real system.

The velocity check

The friction rate is one of two checks. The other is velocity. Energy Vanguard reports that Manual D's Table N3-1 sets maximum velocities of 900 fpm for supplies and 700 fpm for returns. ACCA's Manual D page confirms that its Tables N3-1 and N3-2 cover maximum air velocities, but the numbers themselves are in the sold manual, which we have not reproduced or read. Treat 900 and 700 as reported values, not code requirements.

When the friction-rate size is too fast, you size by velocity instead, which means a larger duct. Run the same 0.073 rate at five airflows and the check changes the return size at the two larger airflows:

At 800 cfm the exact diameter runs 820 fpm, fine for a supply under 900 but over 700 for a return, so the return goes up to 15 in. (652 fpm). At 1,200 cfm the exact diameter runs 909 fpm, just over 900, but rounding up to 16 in. already brings the supply to 859 fpm; the return is set by velocity at 18 in. (679 fpm). The calculator applies the same check. Sizes are rounded up to the whole inch as our convention.

One difference to know about: Energy Vanguard's own 400 cfm example at 0.073 reads about 725 fpm on a paper duct calculator and uses a 12 in. return, because that shop stocks even sizes above 10 in. Our equations give 686 fpm and 11 in.

Bigger ducts mean less resistance, so a short run can pass more air than its room needs. Energy Vanguard's answer is balancing dampers, and the DOE Building America guideline says Manual D bases the rate on the longest path and uses it for every run, which slightly oversizes shorter ones and is why its standard of care calls for balancing dampers in each run.

Where a calculation stops and a design starts

Everything above sizes one section of round duct. It does not count fittings, pick registers, read the blower table or balance the system. Those belong to ACCA's Manual D, which the ICC deck lists as the standard referenced in the 2015 editions of the model residential and mechanical codes (your jurisdiction may have adopted a newer edition). Airflow for each room comes from the room's share of a Manual J load; our guide on BTU per square foot covers why that step is a load calculation and not a ratio.

Whether a Manual D design is required for a permit depends on the code your jurisdiction has adopted. This is general information, not engineering or code advice: ask your local building department. For rectangular duct, you can enter one side and the calculator returns the other side of a rectangle with the same friction as the round size, using Huebscher's equivalent-diameter formula, De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25 (Engineering ToolBox), which goes back to R.G. Huebscher's 1948 research on the flow resistance of different duct shapes (Snips); the rectangle's own area and velocity are different, so don't read it as the same duct.

Next step

Work out the friction rate for your own job from the blower's available static pressure and the total effective length, then enter it with each section's airflow into the duct size calculator. Use the result as a check on a design, and take the real design from Manual D and your local code official.