How rectangular sizing works: equivalent diameter
A rectangular duct doesn't need its own separate friction physics. Instead, every rectangle has an "equivalent diameter", the diameter of a round duct that produces the same friction rate at the same CFM, given by the Huebscher formula: De = 1.30 · (a·b)0.625 / (a+b)0.25, where a and b are the two sides. Once De is known, the rest of the problem is exactly the round-duct problem already solved on this site: plug De and the CFM into the same Darcy-Weisbach and Swamee-Jain equations to get the actual friction rate. The one thing that does NOT carry over unchanged is velocity, a rectangle's actual cross-sectional area (a times b) is generally not the same as the equivalent round duct's area, so velocity here is always calculated from the rectangle's own area, not borrowed from the round equivalent.
Sizing from a known side
The calculator's "Size a duct" mode takes the side that's usually fixed first in a real installation, most often duct height, set by joist depth or a dropped-ceiling cavity, plus a CFM and friction rate target. It finds the continuous equivalent diameter that hits that friction rate, solves for the matching open side, and rounds that side up to the next whole inch (a practical fabrication increment), then reports the actual friction rate and velocity that whole-inch rectangle produces. "Check a duct" mode runs the same math in reverse: give it both sides of a duct you already have, or are considering, and it reports the friction rate, velocity, and equivalent diameter that duct actually delivers at a given CFM.
Why use rectangular instead of round
Round duct is the more efficient shape, less wall surface per unit of cross-sectional area means less friction loss and less material for the same airflow. Rectangular duct gets chosen anyway, almost always because of space: a joist bay, a wall cavity, or a dropped ceiling often has enough width but not enough height for the round duct that same airflow would need. Flattening a duct into a rectangle trades some efficiency for fitting the available space. That trade gets worse as the rectangle gets flatter for the same area, which is why aspect ratio matters on top of raw sizing.
Aspect ratio is the ratio between a rectangle's long and short side. A near-square duct (low aspect ratio) is close to round in efficiency; a very flat duct (high aspect ratio) has much more wall surface for the same area, which raises both friction loss and noise, and gets harder to fabricate and seal well. A common convention in duct design is to keep the aspect ratio at 4:1 or under wherever possible. That's a design guideline worth following, not a hard limit this calculator enforces, an available cavity sometimes forces a flatter duct than that, and when it does, expect more friction and more noise than the equivalent-diameter number alone suggests.
Velocity limits still apply
The same velocity guidance used for round duct carries over here: a common design ceiling is about 900 feet per minute for supply duct and 700 feet per minute for return duct, mainly to control noise. Because rectangular velocity is calculated from the rectangle's actual area rather than the equivalent round duct's area, it's worth checking separately from the friction rate, a rectangle sized only to hit a friction-rate target can still land above a comfortable velocity ceiling, especially at flatter aspect ratios.
How these numbers are derived
Equivalent diameter comes from the Huebscher formula above, a public-domain result from 1948 that this calculator computes directly rather than reading off a table. That De then feeds the same Darcy-Weisbach and Swamee-Jain equations used on the round duct chart, at the same installed galvanized roughness of ε = 0.15 mm and the same standard air assumptions (ρ = 1.204 kg/m³, μ = 1.825×10⁻⁵ Pa·s). Velocity is computed separately, directly from CFM divided by the rectangle's own cross-sectional area. Nothing here is copied from a scanned chart or a manufacturer's table, both the conversion and the friction physics are computed live and shown openly. Flexible duct behaves differently again, its liner roughness is much higher, so it gets its own chart; see the flexible duct CFM chart for that.
Frequently asked questions
How do I convert round duct to rectangular?
Use the Huebscher equivalent diameter formula: De = 1.30 * (a*b)^0.625 / (a+b)^0.25, where a and b are the rectangle's sides. Solved the other way, pick one side and this calculator finds the other side that produces a given equivalent diameter, and therefore the same friction rate and airflow as that round duct.
What size rectangular duct do I need for 400 CFM?
It depends on the constrained side. With an 8 inch height (a common joist-bay limit) at the 0.10 in.wc/100 ft default, 400 CFM needs roughly an 8x11 inch duct (the calculator above computes the exact figure live). A 10x8 rectangle is close, carrying about 399 CFM at that same rate; check the table for more combinations.
Is rectangular or round duct better for the same airflow?
Round is more efficient: for the same cross-sectional area a circle has less wall surface than a rectangle, so it loses less pressure per foot and needs less material. Rectangular duct is chosen when height is constrained, most often by joist bays or a dropped ceiling, not because it performs better.
What is equivalent duct diameter?
It is the diameter of a round duct that has the same friction rate as a given rectangular duct at the same CFM. It lets a rectangular duct be sized and checked using the same friction charts and equations built for round duct, instead of solving rectangular duct hydraulics directly.
Does duct aspect ratio matter?
Yes. A flatter duct (a high ratio between its long and short side) has more wall surface for the same cross-sectional area, which raises both friction and noise. A common convention is to keep the aspect ratio at 4:1 or under; it is a design guideline, not a hard computed limit in this calculator.