This calculator lets you determine the cross-sectional and surface areas of air ducts. Use it for rectangular or round ducts, depending on your HVAC layout. Input dimensions in inches, centimeters, or millimeters, and get the area in square inches, cm², ft², or m². Surface area is also computed to help estimate material requirements or insulation coverage.
Air Duct Area Calculator
This calculator determines two types of area for air ducts: Cross-sectional Area (the internal airflow opening) and Surface Area (the exterior material area). It supports both rectangular and round duct shapes.
Duct Shape
Duct Width
Duct Height
Duct Diameter
Duct Length
Cross-sectional Area
Surface Area
How to Use the Tool
- Select the duct shape: rectangular or round.
- Enter the width and height (for rectangular) or diameter (for round).
- Enter the duct length.
- Choose units for each input and desired output formats.
- Click “Calculate Area” to see both cross-sectional and surface areas.
Formula Used
For rectangular ducts:
Cross-sectional Area = Width × Height
Surface Area = 2 × (Width + Height) × Length
For round ducts:
Cross-sectional Area = π × (Diameter / 2)²
Surface Area = π × Diameter × Length
Unit Conversions
- 1 inch = 2.54 cm
- 1 inch = 25.4 mm
- 1 foot = 12 inches
- 1 meter = 39.37 inches
- 1 ft² = 144 in²
- 1 m² = 1550.0031 in²
Reference: NIST Metric Conversion Card SP 365
Common Duct Sizes And Areas
| Shape | Width (in) | Height (in) | Length (in) | Cross-sectional Area (in²) | Surface Area (ft²) | Diameter (in) |
| Rectangular | 6 | 6 | 36 | 36 | 6 | |
| Rectangular | 12 | 6 | 48 | 72 | 12 | |
| Rectangular | 20 | 10 | 60 | 200 | 25 | |
| Rectangular | 24 | 18 | 72 | 432 | 42 | |
| Rectangular | 36 | 24 | 120 | 864 | 100 | |
| Round | 36 | 12.57 | 3.14 | 4 | ||
| Round | 48 | 50.27 | 8.38 | 8 | ||
| Round | 60 | 113.1 | 15.71 | 12 | ||
| Round | 72 | 254.47 | 28.27 | 18 | ||
| Round | 120 | 452.39 | 62.83 | 24 |
Frequently Asked Questions (FAQ)
1. What’s the difference between cross-sectional and surface area?
Cross-sectional area is the internal opening that allows airflow. Surface area is the total outer surface of the duct, used for estimating insulation or materials.
2. Which shape is better—round or rectangular?
Round ducts are more efficient for airflow with less resistance. Rectangular ducts are easier to fit in tight spaces. The best choice depends on the installation environment.
3. Do I need to include insulation thickness in calculations?
No. This calculator only uses internal dimensions. If you’re planning insulation, calculate it separately based on added thickness.
4. What units should I use?
Use any supported unit—inch (in), centimeter (cm), millimeter (mm), foot (ft), or meter (m). The tool converts internally for accurate results.
5. Can I calculate for flexible ducting?
Yes, but only if you input the fully extended dimensions. This tool assumes rigid shapes and doesn’t account for bends or compression.
6. How accurate are the results?
Results are accurate for standard geometry. Real-world values may vary slightly due to duct shape deviations or manufacturing tolerances.
7. Can I switch between metric and imperial?
Yes. Each input and output field allows selection of metric or imperial units. Results update accordingly.
8. What duct sizes are commonly used?
Residential ducts often range from 4–12 inches round or 6×6 to 20×10 inches rectangular. Commercial and industrial ducts can be much larger.
9. Why calculate surface area?
Surface area helps estimate material cost, insulation coverage, and heat loss potential in ductwork systems.
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