Calculator Inputs
Formula used
The calculator uses two common conversion methods. Equal area keeps the open cross section the same.
Round area = π × D² ÷ 4
Unknown rectangular side = Round area ÷ Known rectangular side
Equal friction uses the standard rectangular equivalent diameter relationship.
De = 1.30 × (A × B)^0.625 ÷ (A + B)^0.25
Here, D is round diameter. A and B are rectangular sides. The calculator solves the unknown side until De matches the round diameter.
How to use this calculator
- Enter the existing round duct diameter.
- Select the dimension unit used by your drawing.
- Choose equal area, equal friction, or compare both.
- Select whether the fixed rectangular side is width or height.
- Enter airflow if you want velocity checks.
- Enter a rounding increment for shop-friendly dimensions.
- Press the convert button and review the result above the form.
Example data table
| Round duct | Fixed side | Method | Approximate rectangular size | Best use |
|---|---|---|---|---|
| 10 in | 12 in width | Equal area | 12 in × 6.55 in | Space planning |
| 12 in | 14 in width | Equal friction | 14 in × 9.67 in | Pressure control |
| 16 in | 20 in width | Equal area | 20 in × 10.05 in | Low profile runs |
Round to Rectangular Duct Sizing Guide
Why duct shape changes matter
Round duct is efficient, quiet, and easy to seal. Rectangular duct is useful when ceiling height, wall cavities, beams, or equipment clearances limit the available space. A conversion should not be guessed. Small shape changes can affect area, velocity, pressure loss, noise, and balancing. This calculator helps compare both sizing goals before a transition is fabricated.
Equal area sizing
Equal area conversion keeps the same open area as the round duct. It is simple and helpful for rough layout work. The method divides the round duct area by one fixed rectangular side. The answer is the missing side. Equal area can be useful when airflow speed is the main concern. However, it does not always match friction loss. A flat rectangular duct may have more surface contact. That extra perimeter can increase resistance.
Equal friction sizing
Equal friction sizing uses equivalent diameter. It compares a rectangular duct with a round duct that has similar flow resistance. This is often better for design checks. The calculator uses the rectangular equivalent diameter formula. It then solves the missing side by iteration. The result may have more area than the equal area size. That happens because rectangular shapes usually need more area to carry air with similar friction.
Aspect ratio checks
Aspect ratio compares the long side with the short side. A 16 by 8 duct has a two to one ratio. Lower ratios are usually easier to build, seal, and balance. Very flat duct can create noise and pressure problems. Many projects limit aspect ratios to four to one or less. This tool flags the selected ratio against your chosen limit.
Velocity and airflow review
Airflow input is optional. When entered, the calculator converts flow into metric velocity and feet per minute. This helps spot ducts that may be too small. High velocity can raise sound levels. Low velocity can reduce throw or make balancing harder. The right target depends on system type, branch use, and local design practice.
Rounding for real fabrication
Calculated dimensions can include long decimals. Shops often use practical increments, such as one eighth inch, half inch, five millimeters, or ten millimeters. Rounding makes the result easier to order and install. Rounding also changes area and equivalent diameter. That is why the result shows raw size and rounded size separately. Use the final dimensions as a design aid. Confirm them with project standards, fittings, insulation space, and code requirements.
Design cautions
Always review the transition length, nearby elbows, branch takeoffs, and damper positions. A good size can still perform poorly when fittings are cramped. Leave service space for sealing and insulation. Check hanger clearance before ordering metal. For critical systems, compare the result with a duct friction chart and the project airflow schedule. This final check reduces surprises during balancing and commissioning. It also supports smoother startup.
Frequently Asked Questions
What does this calculator convert?
It converts a round duct diameter into a rectangular duct size. You enter one fixed rectangular side. The calculator finds the missing side using equal area, equal friction, or both methods.
Which method should I choose?
Use equal area for quick space checks. Use equal friction when pressure loss matters. Comparing both methods is often best because it shows the tradeoff between compact dimensions and airflow resistance.
Why are equal area and equal friction results different?
Rectangular ducts have different wall contact and perimeter behavior than round ducts. Equal area preserves open area. Equal friction tries to preserve resistance. A flatter duct may need more area to match friction.
What is equivalent round diameter?
Equivalent round diameter is the round duct diameter that behaves similarly to a rectangular duct for friction comparison. It helps designers compare shapes that do not share the same geometry.
What is aspect ratio?
Aspect ratio is the longer rectangular side divided by the shorter side. A lower ratio is usually easier to fabricate and seal. A high ratio can increase pressure loss and noise risk.
Can I use millimeters or meters?
Yes. Select the unit before calculating. The calculator keeps dimensions consistent. It also converts area for velocity calculations when airflow is entered.
Why does rounding change the final result?
Rounding adjusts dimensions to a buildable increment. This changes area, perimeter, aspect ratio, and equivalent diameter slightly. The tool displays raw and rounded sizes for comparison.
Is airflow required?
No. Airflow is optional. Enter it when you want velocity checks. Leave it at zero when you only need rectangular dimensions.
Does this replace duct design software?
No. It is a sizing and comparison aid. Final duct design should consider fittings, leakage, insulation, balancing dampers, fan capacity, sound limits, and applicable standards.
What fixed side should I enter?
Enter the side controlled by your available space. For a low ceiling, width may be flexible while height is fixed. For a narrow shaft, width may be fixed instead.
Can I use the result for transitions?
Yes, it can help size transitions. Keep transitions gradual when possible. Sudden shape changes can create turbulence, noise, and added pressure loss.