Rectangular Duct Aspect Ratio Calculator
Check a rectangular duct's aspect ratio.
Enter your values
Enter the side a in in.
Enter the side b in in.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Check a rectangular duct's aspect ratio. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Side A, Side B |
| Main output | Aspect ratio |
| Formula | Aspect ratio = longer side / shorter side |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Side A(in)
- Enter the side a in in.
- Side B(in)
- Enter the side b in in.
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.
What do I need to enter into the Rectangular Duct Aspect Ratio Calculator?
Just 2 values: side a, side b. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Rectangular Duct Aspect Ratio Calculator work out the 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. It applies the formula Aspect ratio = longer side / shorter side and shows the working so you can check each step by hand.
Is the Rectangular Duct Aspect Ratio Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.