How this duct size calculator works
This is a free online duct sizer that sizes a duct from airflow in your browser — no download, no app store, no Excel template. Unlike the old slide-wheel sizers, it shows you velocity and friction loss together, applies a flex correction, and converts round to rectangular and back, which paper wheels and many phone apps either skip or hide behind a paywall.
Sizing a duct comes down to one trade-off: a smaller duct is cheaper and easier to route but moves air faster, which means more friction (static pressure your blower has to overcome) and more noise. There are two standard ways to settle it, and this duct calculator lets you switch between them:
- Equal-friction (by friction rate): you fix a friction rate — commonly about 0.08–0.10 in. w.g. per 100 ft for residential work — and the round diameter is solved so the duct loses exactly that much pressure per 100 ft. Sizing every run to the same rate keeps the system roughly balanced.
- Target velocity: you cap the air speed instead. Branch runs are usually kept around 700–900 fpm to stay quiet; trunks can run faster; returns are kept slower. The diameter follows from
d = 13.54 × √(CFM / V).
Whichever you choose, the tool reports the other one too, so you can immediately see if a friction-sized duct is running uncomfortably fast or a velocity-sized duct is wasting blower capacity.
The formulas (all public physics)
Because everything is computed live, you can type any value — the chart tool uses these same equations for its lookup tables.
Velocity, static pressure and why both matter
Velocity is how fast the air moves; friction loss (a static-pressure drop) is what that speed costs your blower. A duct can satisfy a friction target yet still be too fast and whistle, or hit a velocity cap yet have so little friction that it is needlessly large and expensive. This air-velocity and static-pressure calculator shows the velocity in fpm and m/s and the friction in in. w.g. and Pa per 100 ft for the size it picks, so you size on real numbers. Typical guidance: keep branch velocity around 700–900 fpm, returns lower, and main trunks a little higher.
Round, rectangular, oval and flex
The same airflow can run in different shapes. This calculator gives you the round diameter first, then the matching rectangular sizes (at the aspect ratio you choose), a flat-oval option, and the flex-duct size. Rectangular and oval sizes are matched by equivalent diameter, not equal area — a rectangular duct of the same cross-sectional area as a round one actually carries less air because its higher perimeter adds friction. The square-to-round and round-to-rectangular duct converter on the second tab does this conversion on its own.
Flex duct deserves a warning. Its corrugated bore is far rougher than smooth metal, so for the same CFM and friction it must be larger — and that is only true when the flex is pulled fully taut. Sagging or compressed flexible duct has dramatically higher resistance and is a classic cause of weak airflow. Choose the flex material option to size it with a realistic roughness correction.
Worked examples
Example 1 — 400 CFM branch by friction
At 0.08 in. w.g./100 ft in galvanized steel, 400 CFM solves to about a 10 in round duct at roughly 730 fpm — an equivalent rectangular size near 12 × 8 in. In flex you would step up a size to keep the same friction.
Example 2 — 1200 CFM trunk by velocity
1200 CFM is about a 3-ton air handler at 400 CFM/ton. Sized to a 900 fpm target it lands near a 15.6 in round; sized to 0.10 in. w.g./100 ft it lands near 14.6 in at about 1030 fpm. Try ?cfm=1200&velocity=900&mode=return to see the lower-velocity return-air sizing.
Example 3 — round to rectangular
A 10 in round duct is equivalent to roughly an 11 × 8 in or 14 × 6 in rectangular duct (equivalent diameter De ≈ 10 in). The convert tab works it out for any size.
Duct sizing FAQ
What size duct do I need for 400 CFM?
At a common residential design friction rate of about 0.08 in. w.g. per 100 ft, 400 CFM needs roughly a 10 in round duct, running near 730 fpm. Size to a 900 fpm velocity instead and you get about a 9 in round. The right answer depends on whether you size by friction or velocity, which is exactly what this duct size calculator lets you switch between. For flexible duct, step up one size because its rough bore raises friction.
What size duct for 1200 CFM?
At about 0.10 in. w.g. per 100 ft, 1200 CFM (roughly a 3-ton system at 400 CFM per ton) needs about a 14–15 in round duct or an equivalent rectangular size such as 18 × 10 in, with a velocity near 1000 fpm — acceptable for a trunk. Enter 1200 above and toggle friction or velocity to compare; switch to return-air mode to keep velocity lower on the return side.
Should I size by friction rate or by velocity?
Equal-friction design picks a friction rate (often 0.08–0.10 in. w.g. per 100 ft for homes) and sizes every duct to it, which keeps the system balanced. Velocity sizing caps the speed to limit noise — roughly 700–900 fpm for branches and a bit higher for trunks, lower still on returns. This tool shows both at once so you can see if a friction-sized duct is too fast (noisy) or a velocity-sized duct wastes static pressure.
How do I convert a round duct to a rectangular duct?
Use the equivalent-diameter relationship De = 1.30 × (a·b)^0.625 / (a+b)^0.25, where a and b are the rectangular sides. A rectangular duct carries the same airflow at the same friction as the round duct whose diameter equals De — not the round duct of the same area. For example a 12 × 8 in rectangular duct is equivalent to about a 10.7 in round. The convert tab does this both ways, including square-to-round and round-to-oval.
Does flex duct need to be bigger than metal?
Yes. Flexible duct has a much rougher inner wall than smooth galvanized steel, so for the same CFM and friction it needs a larger diameter — and that assumes the flex is pulled fully taut. Compressed or sagging flex has dramatically higher resistance and can choke airflow. This calculator applies a flex roughness correction so the flex size it reports is realistic, and warns that un-stretched flex performs far worse.
Is this duct sizer free, and does it need a download?
It is completely free and needs no download, app store, account or Excel sheet. Many polished duct sizing apps are paid downloads; this one runs entirely in your browser, works offline once loaded, and keeps every calculation on your device. You can also copy a shareable link that reopens the calculator with your exact CFM, friction and shape filled in.