Find round duct size by CFM
Full chart ↓At 600 ft/min: 6-inch round ≈118 CFM; 8-inch ≈209; 10-inch ≈327; 12-inch ≈471. Values use clear inside area × velocity. Actual airflow also depends on duct resistance, fittings and the blower; this chart is a preliminary sizing check.
Example: 200 CFM at a 600 ft/min limit needs at least 7.82 inches of inside diameter. The next listed size is 8 inches, giving about 573 ft/min. The complete chart and sizing method below work without JavaScript.
Enter CFM and a chosen velocity limit for a preliminary round-duct area check.
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Round duct sizing chart: CFM at four velocities
Choose the diameter row and the velocity column. Values are calculated from the clear circular area, rounded to the nearest CFM; they are not fixed airflow ratings. The velocity columns are comparison points, not universal design recommendations.
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On a narrow screen, swipe the table horizontally to see every column. The first column stays visible.
| 4 in | 101.6 | 35 | 52 | 70 | 87 |
|---|---|---|---|---|---|
| 5 in | 127.0 | 55 | 82 | 109 | 136 |
| 6 in | 152.4 | 79 | 118 | 157 | 196 |
| 7 in | 177.8 | 107 | 160 | 214 | 267 |
| 8 in | 203.2 | 140 | 209 | 279 | 349 |
| 9 in | 228.6 | 177 | 265 | 353 | 442 |
| 10 in | 254.0 | 218 | 327 | 436 | 545 |
| 12 in | 304.8 | 314 | 471 | 628 | 785 |
| 14 in | 355.6 | 428 | 641 | 855 | 1,069 |
| 16 in | 406.4 | 559 | 838 | 1,117 | 1,396 |
| 18 in | 457.2 | 707 | 1,060 | 1,414 | 1,767 |
| 20 in | 508.0 | 873 | 1,309 | 1,745 | 2,182 |
| 22 in | 558.8 | 1,056 | 1,584 | 2,112 | 2,640 |
| 24 in | 609.6 | 1,257 | 1,885 | 2,513 | 3,142 |
| 26 in | 660.4 | 1,475 | 2,212 | 2,950 | 3,687 |
| 28 in | 711.2 | 1,710 | 2,566 | 3,421 | 4,276 |
| 30 in | 762.0 | 1,963 | 2,945 | 3,927 | 4,909 |
Original calculation: Q = A × V, with A = πD²/576 when D is in inches. Diameter conversion: 1 in = 25.4 mm. See ASHRAE duct-design principles for the separate pressure-loss analysis.
Duct Sizing Chart — all 17 reference rows. Read the stated conditions and sources.
CFM to duct size: quick lookup
Already know the required airflow? This inverse chart gives the smallest round inside diameter listed in the main table that stays at or below the selected velocity. The 600 and 800 ft/min columns are comparison limits, not recommended velocities for every application.
| Required airflow (CFM) | At or below 600 ft/min | At or below 800 ft/min |
|---|---|---|
| 50 | 4 in | 4 in |
| 75 | 5 in | 5 in |
| 100 | 6 in | 5 in |
| 150 | 7 in | 6 in |
| 200 | 8 in | 7 in |
| 300 | 10 in | 9 in |
| 400 | 12 in | 10 in |
| 500 | 14 in | 12 in |
| 600 | 14 in | 12 in |
| 800 | 16 in | 14 in |
| 1,000 | 18 in | 16 in |
| 1,200 | 20 in | 18 in |
| 1,500 | 22 in | 20 in |
| 2,000 | 26 in | 22 in |
| 3,000 | Above 30-in chart | 28 in |
Example: For 1,000 CFM, the area calculation needs at least 17.48 inches at 600 ft/min, so the next listed diameter is 18 inches. At 800 ft/min, it needs at least 15.14 inches, so the next listed diameter is 16 inches. Both still require the friction, fitting and available-pressure checks below.
Rectangular duct size chart
For rectangular duct, multiply the clear inside width × height to obtain area. The airflow below uses the same area × velocity method as the round chart. A lined duct needs its actual unobstructed airway dimensions.
| Inside size (in) | Area (ft²) | CFM at 600 ft/min | CFM at 800 ft/min |
|---|---|---|---|
| 8 × 6 | 0.333 | 200 | 267 |
| 10 × 6 | 0.417 | 250 | 333 |
| 10 × 8 | 0.556 | 333 | 444 |
| 12 × 8 | 0.667 | 400 | 533 |
| 12 × 10 | 0.833 | 500 | 667 |
| 14 × 10 | 0.972 | 583 | 778 |
| 16 × 10 | 1.111 | 667 | 889 |
| 18 × 10 | 1.250 | 750 | 1,000 |
| 20 × 10 | 1.389 | 833 | 1,111 |
| 20 × 12 | 1.667 | 1,000 | 1,333 |
| 24 × 12 | 2.000 | 1,200 | 1,600 |
Equal area does not mean equal friction. A long, narrow rectangle has a different wetted perimeter and pressure loss from a circular duct. Use the selected design method or a ductulator to find a friction-equivalent rectangular size; do not substitute the equal-area circle automatically. ASHRAE: duct resistance and equivalent sizes.
How to calculate duct size from CFM
Continuity connects the volume flow rate, the clear airway area and the average air velocity. Keep units consistent:
Q = A × V; therefore A = Q/V. For round duct, D = √[576Q/(πV)] in inches. For a rectangular duct, width × height = 144Q/V in square inches.
Worked example: A branch needs 200 CFM and the designer has selected a 600 ft/min limit. Required area is 200/600 = 0.3333 ft². The round inside diameter is √[576 × 200/(π × 600)] = 7.82 inches. An 8-inch clear opening has an area of 0.3491 ft² and a calculated velocity of 200/0.3491 = 573 ft/min.
A rectangular opening of 8 × 6 inches also provides 0.3333 ft², giving exactly 600 ft/min at 200 CFM. That confirms an area match only. The pressure-loss check can require either option to be larger.
For metric work, Q (m³/s) = A (m²) × V (m/s). Convert 1 CFM to 0.471947 L/s and 1 ft/min to 0.00508 m/s. Use one unit system throughout each equation.
Why CFM and diameter are only the first check
A blower delivers airflow at the operating point created by the complete system resistance. The available pressure must cover the duct paths and their fittings after accounting for the components included in the chosen blower-data boundary. Filters, coils, grilles and accessories can consume pressure before a designer assigns the remainder to ducts.
A friction-rate worksheet relates the duct pressure allowance to total effective length. For example, an available 0.20 in. w.g. across a 250 ft effective path gives 0.20 × 100/250 = 0.08 in. w.g. per 100 ft. This is a conditional arithmetic example, not a default friction rate for every home. Equivalent fitting lengths and pressure boundaries must be consistent.
Use ACCA Manual D for residential duct-system design. Room airflow, duct length, fittings, noise limits, return paths and balancing belong in that process. A diameter-only chart cannot establish the delivered airflow of an installed branch.
Flexible duct, metal duct and restricted openings
The geometry of an open 8-inch circle does not change with material, but its resistance can. A flex duct that is compressed, kinked or sharply bent has a different pressure-loss behavior from a properly extended duct. Its installed airflow cannot be inferred from the same nominal diameter alone.
Inspect the complete route: excess length, sag, tight turns, crushed sections and restrictive connections all matter. PNNL recommends full extension before connection and avoiding unnecessary loops. Correct compromised routing instead of assuming a larger blower setting will solve every restriction. PNNL: flex-duct length; PNNL: bends and compression.
Also distinguish the nominal duct size, clear inside dimension and grille face size. A grille’s free area is smaller than its overall face area, and an internal liner reduces the airway. Use the component manufacturer’s airflow and pressure-drop data for those parts.
A practical sequence for choosing a duct
- Establish the airflow. Use the room load and equipment design, not the duct diameter you happen to have available.
- Choose a velocity criterion. Account for the use of the space, sound goals and whether the duct is a trunk or branch.
- Check area. Use this chart or the finder to identify a preliminary inside size.
- Complete the friction check. Include the route, fittings, actual material and available pressure.
- Confirm installation and performance. Verify the return path, balancing provisions, sealing and measured airflow.
Supply and return ducts carry the system’s air through different paths, but neither has one universally correct velocity or size. Several branches can merge into a trunk; the upstream trunk must be evaluated at the combined design flow for that section.
For an existing comfort complaint, compare measured airflow and static pressure with the design and blower data. A chart may identify an area problem, but it cannot diagnose a dirty filter, closed damper, disconnected branch or undersized return by itself.
Duct Sizing Chart FAQs
How much CFM can a 6-inch duct carry?
A 6-inch clear round airway has 0.19635 ft² of area. That corresponds to about 79 CFM at 400 ft/min, 118 CFM at 600 ft/min, 157 CFM at 800 ft/min and 196 CFM at 1,000 ft/min. Actual delivered airflow requires the system pressure-loss check.
What size duct do I need for 200 CFM?
At a chosen 600 ft/min limit, the calculated minimum round inside diameter is 7.82 inches, so the next size listed here is 8 inches. At a different velocity or friction criterion, the selected size can change.
Is an 8-inch round duct equivalent to an 8 × 8 duct?
No. The 8-inch circle has about 50.27 in² of area; an 8 × 8 rectangle has 64 in². At the same average velocity the rectangle carries more volume. Friction equivalence also depends on shape and must be checked separately.
Can I use this chart for return ducts?
Yes for area-and-velocity calculations, provided the clear dimensions and airflow are correct. Select the return-system velocity and pressure-loss criteria separately, and use grille free-area and pressure-drop data where a grille is involved.
Can I size ducts only from air-conditioner tonnage?
Tonnage alone does not determine each branch size. Establish the equipment’s design airflow, allocate airflow to rooms and evaluate each path. A single CFM-per-ton assumption does not describe room loads, pressure loss or the required return system.
Why do different duct sizing charts give different CFM values?
They may use different velocities, friction rates, materials, lengths or installation assumptions. Check the chart conditions before comparing values. The main table here explicitly uses Q = area × velocity and does not embed a hidden friction-rate assumption.
Sources and Data Basis
Reference basis reviewed October 25, 2026. Follow the scope and conditions given with each table and lookup result.
- ACCA — Manual D Residential Duct DesignResidential duct sizing principles; blower operating point; pressure losses, effective length, fitting losses and velocity limits; flexible duct compression.
- ASHRAE 2025 Handbook — Chapter 21, Duct DesignAirflow, duct cross-sectional area, pressure losses and equal-friction design principles. Geometry chart is independently calculated Q=AV, not a reproduction of a friction sizing table.
- PNNL — No Kinks or Sharp Bends in Flex Duct InstallationKinks, bends, sag and compression increase duct-system pressure losses; full cross-sectional opening and appropriate routing.
- PNNL — No Excessive Coiled or Looped Flex DuctsFully extend flex duct; avoid excess length, loops and sag.