Duct size calculator: friction and velocity, both checked

Size on the friction rate and use velocity as the check, which is the order ACCA Manual D uses. The friction rate is the property that makes a system balance — every section sized at the same pressure drop per 100 ft means the drop along any path is proportional to its length — whereas velocity is a comfort limit that says nothing about whether the far bedroom gets its air. The exception is a section where noise is the whole problem, such as a short branch straight into a bedroom register, and that is what the velocity basis is for.

Because a rectangle has more wall touching the air than a circle enclosing the same area, and it is wall that makes friction. The Huebscher relation gives the round duct with the same pressure drop at the same airflow, and for a 20 × 8 it comes out at 13.48 in against the 14.27 in circle of equal area — flatten the duct further and the two diverge further still. A rectangular trunk laid out by matching the area of the round duct the calculator returned is undersized on the day it is installed, and it is the most repeated mistake in residential sheet metal.

It is a default, not a design — the setting a cardboard duct wheel is printed at and the rate most sizing tables are drawn for. The real figure is the blower's available static pressure, meaning its rating less the coil, the filter, the registers, the grilles and the balancing dampers, multiplied by 100 and divided by the system's total effective length. A short compact system computes well above 0.1 and a sprawling one well below it, and Manual D treats a result outside roughly 0.06 to 0.18 as a sign that something in the system needs to change rather than a number to size at.

No, and the gap is a factor of two at best. The friction equation here is for galvanized steel at an absolute roughness of 0.0003 ft; flex pulled drum-tight between supports runs at roughly twice that drop for the same diameter, and flex left with slack in a joist bay runs at several times it. A nominal 6 in flex is also nominal to its inner liner rather than to anything you can measure from outside, and a compressed one is not a 6 in duct in any sense this equation recognizes. Size on this page, then either upsize the flex or pull it properly.

4:1 is the usual line, and the penalty either side of it is continuous rather than a cliff. As the duct flattens it needs more sheet metal, more insulation and more hangers for the same airflow, and its friction rises, so the only thing it buys is the ceiling space it fits into. A lot of residential work runs to 5:1 and some commercial risers further; the limit on this page is editable for that reason. What ends the argument is arithmetic rather than taste: below a certain depth no width reaches the required equivalent diameter at all, and the page says so instead of returning a very wide number.

The next size up that exists, which the page picks for you — 16 in, since round duct is stocked every inch to 10 and in even inches after that. Rounding down to 14 in would put the same air through 23% less area, and the printed size table shows exactly what that costs: at 1,200 CFM a 14 in carries 1,123 ft/min at 0.136 in. w.c. per 100 ft against 859 ft/min at 0.070 in a 16 in. The whole neighborhood is printed rather than one answer because the choice between two adjacent sizes is usually about the joist bay, and it should be made with both consequences visible.

One section at a time, at the airflow that section carries. A duct layout is a series of these calculations — the trunk at full airflow, each takeoff at its room's airflow, the return at whatever it collects — plus a fitting-by-fitting effective-length count that decides the friction rate they all share. This page will not lay out a system, will not check that the branches balance, and is not a Manual D: it is the sizing arithmetic those methods rest on, with the table it read printed beside the answer for whoever signs it off.