Flex Duct Sizing Calculator
Flex duct loses airflow when it is compressed. Enter the CFM, friction rate and compression to get the diameter.
How to use ↓Result
Duct Chart · Flex Duct Sizing Calculator · ductchart.com
Estimates only: they can differ from your chart or the manufacturer's data by 5% or more (flex by 2 times or more), so verify accuracy for your own application against the design documents, manufacturer data and local code.
How to use
- Enter the airflow and the friction rate.
- Pick the compression: fully stretched, 4% design, 15% typical, or your own number.
- Choose which published model to use. The one-line description updates.
- Press Calculate. The table shows all three models side by side.
Flex duct friction and compression
Flex duct is rougher than sheet metal, and every bit of compression or sag raises the pressure drop. This calculator starts with fully stretched flex (roughness 0.003 ft) and multiplies the friction by a compression factor, then solves for the diameter that gives your friction rate.
There is more than one published way to work out that factor, and they do not agree. The LBNL option comes from lab measurements on 6, 8 and 10 in. flex. It depends on the diameter and it is the default here. The ASHRAE option is a best-fit line to the Fundamentals 2001 chart (about ×1.4 at 4% compression and ×2.5 at 15%) and the ADC option follows the Air Diffusion Council table: no penalty to 4%, double at 15%, four times at 30%. Both give lower friction than the LBNL lab data, by 23 to 55% for 6 to 10 in. flex, so they size on the unsafe side. They are here so you can compare them with the numbers you may have seen quoted.
The LBNL lab data stop at 10 in. and at about 30% compression. For larger sizes the 10 in. value is simply held, so any result above 10 in. is flagged as extrapolated beyond the lab data. Above 30% compression no model is published at all.
The design option uses 4% compression, the limit makers give for fully extended flex. Under the LBNL model that is a friction multiplier of about ×2.0 at 6 in., ×1.9 at 8 in. and ×1.65 at 10 in. and up, so it is not a free allowance. Flex makers say to install it fully extended, support it at no more than 4 ft, and avoid sag.
Flex is stocked in whole inches from 3 to 10 in. and in even inches from 12 to 20 in., so the stock size here stops at 20 in. There is no 11 in. stock flex. For sheet metal or duct board use the duct size calculator.
Example: 400 CFM at 0.08 in. w.g. per 100 ft needs 10.8 in. of fully stretched flex. With the LBNL model, 4% compression raises that to 11.9 in. (friction ×1.65, stock 12 in.) and 15% compression to 13.7 in. (×3.43, stock 14 in.). That is why flex that is stretched tight and supported properly matters.
FAQ
Which flex model should I use?
LBNL is the default. It is based on lab measurements and gives the highest friction of the three, so it is the safer one to size from. ASHRAE and ADC are there for comparison: they under-predict the LBNL lab data. If the models give different stock sizes, take the larger.
What compression should I design for?
Fully stretched is the best case and the design option is 4%. The Typical option, 15%, is our own label for a mediocre install. Flex that is sagging or squashed can be worse.
Why does the multiplier change with size on LBNL?
The LBNL lab results were different for 6, 8 and 10 in. duct, so its factor changes with diameter. Above 10 in. the 10 in. value is held, which is an extrapolation. ASHRAE and ADC use one factor for every size.