Duct Sizing Chart PDFs
Three printable duct sizing charts by CFM, one page each, plus two more that work the other direction. No email, no account, no download gate. Every number is calculated from the ASHRAE friction equations, and the assumptions are printed on the face of each sheet so you can check them against whatever you already use.
CFM Duct Sizing Chart
Supply duct, galvanized sheet metal, 0.1 in. w.g. per 100 ft. Round and rectangular sizes with velocity and actual friction.
Return Duct Sizing Chart
Return air duct, held to 700 fpm so the return stays quiet. Comes out one size larger than supply for the same CFM.
Flexible Duct Sizing Chart
Flex duct, fully extended. Runs one to two sizes larger than sheet metal for the same air.
Two more, starting from the size instead
The three above start from a room's airflow and give you a duct size. These two go the other way, for when the duct or the grille is already in the ceiling and the question is what it can actually carry. Same friction rate, same disclaimer, same one page each.
Flex Duct CFM Chart
Max CFM for every flex duct size from 4 to 20 inch at 0.1 in. w.g. per 100 ft, with the same diameter in sheet metal in the next column so you can see what the swap costs.
Return Grille CFM Chart
Return grilles and supply registers by nominal size at three face velocities, with the free area printed so you can substitute the real figure off a manufacturer sheet.
What is on each sizing sheet
CFM, the round duct size, the actual velocity in fpm, the actual friction in in. w.g. per 100 ft, and two rectangular options. The two rectangular columns are different answers rather than the same answer twice: the first is the closest to square, which is quieter and uses less metal, and the second is the flattest size that still moves the air, for when joist depth decides it for you.
The assumptions sit above the table on every sheet, and the disclaimer and the site address sit at the bottom of every page, so a sheet that gets printed and passed around still says where it came from and what it assumes.
Blank forms, same idea
The other thing worth having a pad of in the truck is a blank quote and a blank work order. Those are on their own pages, each one a single page PDF with the fields explained and an example filled in: the HVAC estimate template, the service call invoice template, and the change order form. Same terms as these sheets: no email, no account, direct download.
These charts are free and always will be. If you also need to turn the job into a signable quote and an invoice, that is the paid tool this site pays for itself with. See what it costs.
Download questions
Do I have to give an email address to download these?
No. The links go straight to the files. There is no form, no account, and no newsletter attached to them.
Can I print these and hand them out?
Yes. Print them, keep them in the truck, put one on the shop wall, send them to whoever. They are laid out to print on Letter paper at one page each.
What friction rate are these charts based on?
0.1 in. w.g. per 100 ft, the everyday residential convention, using the equal friction method. The return chart adds a 700 fpm velocity limit on top of that. The grille sheet is the exception: a grille is sized on face velocity and free area, not on friction, so that one has no friction rate at all. Every assumption is printed on the PDF itself, above the table.
Why do these disagree with the chart I already have?
Usually because the other one assumes a different friction rate or a different duct roughness and does not say which. That is why the assumptions are printed on the face of these rather than left out. Check them against yours and the difference will normally explain itself.