Question 1

What duct size do I need for 400 CFM?

Accepted Answer

About an 8-inch round duct or a 10×8 inch rectangular duct at typical residential design (≤ 900 FPM and 0.08 in. w.g. friction). For long runs (over 30 ft) or flex duct, upsize to 9 inches.

Question 2

What duct size for 1,200 CFM?

Accepted Answer

12-inch round, or rectangular equivalents like 14×12 or 16×10. This is the typical trunk size for a 3-ton system.

Question 3

Round duct vs rectangular — which is better?

Accepted Answer

Round duct is more efficient (less friction, less air leakage) and easier to seal. Rectangular is used where vertical clearance is tight, like floor joists or chases. For equivalent airflow, rectangular ducts have ~25% more surface area, more friction, and more leakage paths.

Question 4

Does flex duct change the size?

Accepted Answer

Yes. Flex duct internal friction is 2–3× higher than rigid metal at the same diameter, especially when sagged or kinked. Upsize one increment (e.g. 8″ rigid → 9″ flex), keep runs short, and pull tight at every support.

Question 5

What is friction rate in duct sizing?

Accepted Answer

Friction rate is the static pressure drop per 100 feet of duct, expressed in inches of water gauge (in. w.g.). Standard residential is 0.08–0.10 in. w.g./100 ft. Lower friction = larger duct (quieter, more efficient); higher friction = smaller duct (more noise, more blower load).

Question 6

What is the maximum air velocity for residential ducts?

Accepted Answer

Trunk ducts: 700–900 FPM. Branch ducts: 600 FPM. Above 900 FPM, ducts get noisy and registers whistle. Below 500 FPM, you risk poor mixing and stratification. Most residential designs target 700 FPM in trunks.

Question 7

Round-to-rectangular equivalent table?

Accepted Answer

Some common pairs: 6″ round ↔ 8×6 or 10×4, 8″ round ↔ 10×8 or 12×6, 10″ round ↔ 14×8 or 12×10, 12″ round ↔ 14×12 or 16×10. Rectangular sizes are based on hydraulic diameter equivalence — check a Ductulator for specific situations.

Question 8

Should the supply and return ducts be the same size?

Accepted Answer

Return ducts should be sized one increment larger than supply (or use multiple smaller returns) to keep return-side static pressure low. Undersized returns are the #1 cause of high static pressure complaints.

CFM (max)	Round (in)	Rectangular alt	Notes
50	4	8 × 4	Single small register branch
90	5	8 × 4	Bedroom branch
150	6	8 × 6	Living room branch
250	7	10 × 6	Combined two branches
400	8	10 × 8	Small trunk (1.5–2 ton)
600	9	12 × 8	2–2.5 ton trunk
850	10	14 × 8	2.5–3 ton trunk
1,100	12	16 × 10	3 ton trunk
1,700	14	18 × 12	3.5–4 ton trunk
2,400	16	22 × 12	4–5 ton trunk

HVAC Duct Sizing Calculator

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The duct sizing relationship

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Common duct sizing mistakes

Duct sizing FAQ