Round Duct Conversion Guide
Round ducts are common because they are simple, strong, and efficient. Many projects still require rectangular ducts. Ceiling depth, wall cavities, equipment outlets, or architectural limits can force a shape change. This calculator helps you compare a round duct with a rectangular duct before you cut metal or order parts.
Why Shape Conversion Matters
A duct is not only a hollow shape. Its area, perimeter, friction behavior, and air speed all affect performance. A rectangular duct with the same area as a round duct may not move air with the same resistance. Long narrow ducts create more wall contact. More wall contact can raise friction. That is why this tool offers area, hydraulic diameter, and equal friction approaches.
Method Choice
Use equal area when space planning is the main task. It keeps the internal opening close to the round duct area. Use hydraulic diameter when you want a simple perimeter based comparison. It is helpful for fluid flow checks. Use equal friction when you want a result closer to duct design practice. The equal friction equation estimates a rectangular size with a similar round equivalent diameter.
Using Width or Aspect Ratio
You can enter a fixed rectangular width when one side is already limited. This is useful under beams or beside equipment. You can also enter an aspect ratio. The ratio method builds both sides from the chosen relationship. A ratio near one gives a square duct. Larger ratios create flatter ducts. Very flat ducts should be reviewed carefully because they may become noisy or costly.
Airflow and Velocity
Airflow input is optional. When entered, the calculator estimates velocity in both ducts. This helps you see whether the selected rectangular size changes air speed. High velocity may increase sound. Low velocity may waste space. Always confirm the final design with local standards, fittings, transitions, and pressure loss checks.
Best Practice
Treat the result as a planning value. Round dimensions, available gauge, insulation, turning vanes, and fabrication rules can change the final size. Keep aspect ratios reasonable when possible. Review clearances before selecting a final duct. A balanced shape often installs better, leaks less, and performs more predictably. Check transitions carefully to protect airflow during final installation.