🧰What size duct does this run need?

✓ Tested & verified Updated: Aug 9, 2026 How we calculate this

Duct size calculator

Air the run has to carry (CFM)

What this run is

12 inch round duct

The maths asks for 10.2 in, and 12 in is the next size sold — which puts the air at 509 FPM.

Cross-section needed: 82 sq in at 700 FPM
Rectangular equivalent: 14 × 8 in
Flex duct instead of metal: go up one size — 12 in becomes the minimum, not the answer

Flex duct is not the same as metal at the same diameter — the ribbed inner wall costs real airflow. Go up one size when the run is flex, and pull it taut.

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How do you work out duct size from CFM?

Divide the airflow by the velocity you want, and that is the cross-section. At 700 feet per minute — the usual figure for a supply branch — a 400 CFM run needs 0.57 square feet, or 82 square inches. That works out at 10.2 inches across, so the next size sold is 12 inch. A 10-inch duct would carry it too, at 733 FPM, which is still inside the quiet band — this tool rounds up because the cost of a size too big is money and the cost of a size too small is noise you cannot fix afterwards.

Velocity is the number that decides the size, and it is a comfort choice, not a physics constant. Push air faster and the duct gets smaller and cheaper, and at some point you can hear it in the room. Around 700 FPM a branch is quiet; at 900 FPM a main trunk in the basement is fine because nobody is sleeping next to it; above about 1,000 FPM, registers start to hiss. Bedrooms deserve 500.

Flex duct is a size behind metal. The corrugated inner liner has several times the friction of smooth galvanised, and a flex run that sags between joists is worse again. Where the maths says 8 inch and the material is flex, fit 10 — and stretch it tight, because a flex duct compressed to 70% of its length can lose half its airflow.

Round duct, at the three velocities that matter. Read down the column that matches the run.

Duct	500 FPM (quiet)	700 FPM (branch)	900 FPM (trunk)
4 in	44 CFM	61 CFM	79 CFM
5 in	68 CFM	95 CFM	123 CFM
6 in	98 CFM	137 CFM	177 CFM
7 in	134 CFM	187 CFM	240 CFM
8 in	175 CFM	244 CFM	314 CFM
10 in	273 CFM	382 CFM	491 CFM
12 in	393 CFM	550 CFM	707 CFM
14 in	535 CFM	748 CFM	962 CFM
16 in	698 CFM	977 CFM	1,257 CFM

The jump from 8 to 10 inch is the one that surprises people: two inches of diameter carries more than half again as much air, because area goes with the square of the radius. It is also why squeezing a run down one size to clear a beam costs so much more than it looks like it should.

Frequently asked questions

What size duct do I need for 400 CFM?

A 10-inch round duct at a normal supply velocity of 700 FPM. At 900 FPM — a main trunk — you could get away with 9 inch, and for a quiet bedroom run at 500 FPM you would want 12 inch. The CFM alone does not fix the size; the velocity you choose does.

How many CFM will a 6 inch duct carry?

About 140 CFM at 700 FPM, which is a typical bedroom register, or 180 CFM if you push it to 900. Six inch is the smallest size worth running to a room — 4 and 5 inch are for individual registers and small returns, not for a run that has to feed a whole bedroom.

What velocity should duct air travel at?

500 to 700 FPM in branch runs, 700 to 900 in the main trunk, and 600 or lower for returns, which are noisier than supplies for the same speed. Residential systems that hiss are almost always running over 1,000 FPM somewhere — usually a branch that was sized to fit the joist bay rather than the airflow.

Is flex duct the same size as metal duct?

No. For the same diameter, flex carries noticeably less air because of the ribbed liner, and a sagging or compressed run is far worse. The practical rule is to go up one nominal size when using flex and to pull every run taut and support it every four feet.

How do I convert round duct to rectangular?

Match the cross-sectional area and then check the aspect ratio. The tool shows the rectangular equivalent at 8 inches deep, which is the common joist-bay depth. Keep the ratio under about 4:1 — a very flat duct has far more wall surface per unit of air and the friction climbs sharply.

Does duct length change the size I need?

It changes the pressure the fan has to overcome, not the cross-section this calculation gives. A long run with several elbows may need the next size up, or a fan with more static pressure, but the starting point is always the airflow and the target velocity. Elbows are the expensive part — each 90° turn costs roughly as much as adding ten feet of straight duct.

How we calculate this

Cross-sectional area is airflow divided by velocity: area in square feet = CFM ÷ FPM, converted to square inches by multiplying by 144. Round duct diameter follows from the area of a circle, d = 2√(A/π), and is then rounded up to the next size round duct is sold in. The velocity applied to each run type follows standard residential duct design practice — 500–700 FPM for branches, 700–900 for main trunks and up to 600 for returns — as used in ACCA Manual D. The rectangular equivalent shown matches cross-sectional area at 8 inches deep, which is a common joist depth; for a precise conversion, equal-friction rectangular equivalents are slightly larger in area than the round duct they replace. This is a velocity-based sizing method, which is the normal way to size an individual run; a whole system also needs a friction-rate calculation across the longest path, and long runs with several elbows may need the next size up.

Worked example

With Air the run has to carry (CFM) 400, What this run is Supply branch — 700 FPM, this page works out 12 inch round duct. The maths asks for 10.2 in, and 12 in is the next size sold — which puts the air at 509 FPM.

Cross-section needed82 sq in at 700 FPM
Rectangular equivalent14 × 8 in
Flex duct instead of metalgo up one size — 12 in becomes the minimum, not the answer

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