Calculator Inputs
Formula Used
The calculator converts duct area into a round duct diameter by rearranging the circle area formula.
Area = π × Diameter² ÷ 4
Diameter = √(4 × Area ÷ π)
When airflow is supplied, velocity is calculated with this formula:
Velocity = CFM ÷ Area in square feet
When a target velocity is supplied, airflow capacity is calculated with this formula:
CFM = Velocity × Area in square feet
The design allowance changes the starting area before the diameter is calculated. A positive value increases the duct size. A negative value reduces it.
How to Use This Calculator
- Select the area source. Use direct area, rectangular dimensions, or airflow with velocity.
- Enter the required values in the form fields.
- Add a design allowance when extra capacity is needed.
- Choose a rounding method for practical duct sizing.
- Press the calculate button to view results above the form.
- Use CSV or PDF download buttons to save the report.
Example Data Table
| Square Inch Area | Exact Round Diameter | Common Selected Diameter | Selected Area |
|---|---|---|---|
| 28.27 sq in | 6.00 in | 6 in | 28.27 sq in |
| 50.27 sq in | 8.00 in | 8 in | 50.27 sq in |
| 78.54 sq in | 10.00 in | 10 in | 78.54 sq in |
| 113.10 sq in | 12.00 in | 12 in | 113.10 sq in |
| 176.71 sq in | 15.00 in | 15 in | 176.71 sq in |
Square Inch to Round Duct Guide
Why Area Matters
Round duct sizing starts with area. Area controls how much air can pass through the duct. A small area raises air speed. A larger area lowers air speed. This is important for comfort, noise, and system balance. Many duct layouts begin with a rectangular opening or a known square inch value. This calculator turns that value into an equivalent round diameter.
Practical Diameter Selection
The exact answer may not match a real duct size. For that reason, the calculator includes rounding choices. You can round to the nearest increment. You can also round upward for safer airflow. The standard diameter option helps when you need a common sheet metal size. Always compare the selected area with the design area before choosing the final duct.
Airflow and Velocity Checks
Airflow is measured in CFM. Velocity is measured in feet per minute. The same duct area can perform differently at different airflow rates. High velocity may create noise and pressure loss. Low velocity may need more space. The calculator shows velocity when CFM is entered. It can also estimate capacity at your target velocity.
Design Allowance
A design allowance gives extra control. Use a positive allowance when fittings, lining, filters, or future load changes need more area. Use a small negative allowance only when you understand the system limit. The allowance changes the design area before diameter is calculated. This makes the result more flexible for planning.
Best Use Cases
This tool is helpful for HVAC layout checks, duct replacement, shop drawings, and rough design reviews. It is not a full duct loss model. Final sizing should consider friction rate, fittings, available static pressure, equipment data, and local code rules. Use the exported report as a clear record of the calculation.
FAQs
What does this calculator convert?
It converts a square inch duct area into an equivalent round duct diameter. It also shows selected area, radius, circumference, velocity, and airflow capacity when related values are entered.
What formula is used?
It uses the circle area formula. The diameter equals the square root of four times area divided by pi. This gives the round duct diameter for the entered area.
Can I enter rectangular duct dimensions?
Yes. Choose the rectangle option. Enter width and height in inches. The calculator multiplies them to get square inch area, then converts that area to round diameter.
What is design allowance?
Design allowance increases or decreases the calculated area before conversion. It is useful when you want extra capacity, lining clearance, or a planning safety margin.
Why is selected diameter different from exact diameter?
The exact diameter may include decimals that are not practical. The selected diameter follows your rounding option or the next common standard duct size.
How is velocity calculated?
Velocity equals airflow divided by duct area in square feet. Enter CFM to see velocity for both the design area and the selected round diameter.
Can I download my result?
Yes. After calculation, use the CSV button for spreadsheet data. Use the PDF button for a clean result report that can be saved or shared.
Is this a final duct design tool?
No. It is a sizing and conversion aid. Final duct design should also check friction loss, fittings, static pressure, equipment capacity, and local requirements.