Rectangular Duct Aspect Ratio Calculator
Check a rectangular duct's aspect ratio.
Enter your values
ⓘ Live results
Results
- Aspect ratio
- 2.5:1
- Rating
- Good
- Equivalent round diameter
- 13.48 in
Quick answer
Aspect ratio is the long side divided by the short side; keeping it near 1:1 to 4:1 reduces metal use and friction penalties.
Aspect ratio = longer side / shorter side
How to use the Rectangular Duct Aspect Ratio Calculator
- 1Enter both inside dimensions of the rectangular duct.
- 2Divide the longer side by the shorter to get the aspect ratio.
- 3Keep ratios under about 4:1 to limit friction and sheet metal surface area.
Worked example
Using the values the calculator loads with:
Inputs
- Side A20 in
- Side B8 in
Results
- Aspect ratio2.5:1
- RatingGood
- Equivalent round diameter13.48 in
Frequently asked questions
Why avoid high aspect ratios?
Flat ducts have more surface area and friction per unit of airflow than squarer shapes, and are harder to insulate well.
What is an ideal ratio?
As close to 1:1 (square) as space allows, though 2:1 to 3:1 is common in real installations.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.