Duct Size Calculator

Give it the airflow and the speed you want to move it at, and it gives you the duct. Round diameter, or one side of a rectangle and the other side worked out for you. It sizes duct only: it is not a static-pressure calculation, a full system design or a code check.

Airflow the duct must carry

The air going through this duct, in CFM: cubic feet of air per minute. If this is a branch, use the branch airflow, not the whole system.

Air speed you want (feet per minute)

How fast you want the air to move. Take the figure from your design or from what you normally do. Faster air needs a smaller duct but makes more noise. Slower air is quieter but needs a bigger duct and more room in the ceiling. No figure is supplied here, because main ducts, branches and returns are each run at different speeds.

One side of a rectangular duct

Inches. Usually the side the space decides for you (a joist bay, a bulkhead depth). Leave it blank if the run is round.

Result

Fill in a few more boxes and the answer will show here.

Fill in the calculator first. Then this button saves the whole sheet.

Loading the calculator.

This is a general measurement calculator. Confirm dimensions, product coverage and job requirements before ordering or installing materials.

Sizing duct by the velocity method

A duct is sized by the air it has to carry and the speed you are willing to move it at. Divide the airflow in cubic feet per minute by the velocity in feet per minute and you get the cross-section the duct needs in square feet. Multiply by 144 and you are in square inches, which is how duct actually gets talked about on site.

Velocity is the trade-off, and it is yours to set. Move the same air faster and the duct gets smaller, cheaper and louder. Slow it down and the duct grows, gets quieter, and takes more metal and more ceiling. Trunks, branches and returns are all run at different speeds and local practice varies, which is why this tool asks for the figure instead of choosing one for you.

Round and rectangular ducts are not the same just because the opening is the same size. A rectangle has more wall rubbing against the same air, so it behaves like a smaller pipe than its area suggests. That is what the "round duct this rectangle behaves like" figure is for, and this tool works it out with the standard equivalent-round formula. Compare fittings and friction charts against that figure, not against the diameter the area suggests.

The common mistake is flattening a duct to fit a joist bay and assuming area is all that matters. A 4 by 20 has the same cross-section as an 8 by 10 and a noticeably worse equivalent round. Give this tool the side your space forces and it will tell you what the other one has to be. Zeus keeps a price book that doubles as inventory, so the duct and fittings you settle on become priced lines instead of a size scribbled on a bulkhead.

Built for HVAC work: see the HVAC job app.

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