Recommended Duct Size
8" Round
Velocity
573 fpm
Friction Loss
0.021 in.wg
Noise
Moderate
8" Round
573 fpm
0.021 in.wg
Moderate
Inputs
Result
Area = 200/700 = 0.286 sq ft = 41.1 sq in. Diameter = 2√(41.1/π) = 7.23". Rounds up to 8" standard. Actual velocity with 8" duct = 573 fpm (moderate noise).
Inputs
Result
Area = 800/700 = 1.143 sq ft = 164.6 sq in. Diameter = 2√(164.6/π) = 14.5". Rounds to 16". Rectangular at 1.5:1 = 18"×12". Equiv. length = 50 + (3×20) = 110 ft.
For 200 CFM at a max velocity of 700 fpm, you need about a 7" round duct (actual area: 38.5 sq in). At 600 fpm max velocity, step up to an 8" duct. Rectangular equivalent at 1.5:1 ratio would be approximately 9" x 6".
| CFM Required | Round Duct (700 fpm) | Round Duct (600 fpm) | Rectangular (1.5:1) |
|---|---|---|---|
| 100 CFM | 5" | 6" | 7" × 5" |
| 200 CFM | 7" | 8" | 9" × 6" |
| 400 CFM | 10" | 11" | 12" × 8" |
| 600 CFM | 12" | 14" | 15" × 10" |
| 800 CFM | 14" | 16" | 18" × 12" |
Match the cross-sectional area. A 10" round duct has ~78.5 sq in area. For a 1.5:1 rectangular duct, solve A x B = 78.5 and A/B = 1.5, giving roughly 11" x 7". Use the same area to maintain airflow capacity.
| Round Duct | Area (in²) | Rect. 1.5:1 | Rect. 2:1 |
|---|---|---|---|
| 6" | 28.3 | 7" × 5" | 8" × 4" |
| 8" | 50.3 | 9" × 6" | 10" × 5" |
| 10" | 78.5 | 11" × 7" | 13" × 6" |
| 12" | 113.1 | 13" × 9" | 15" × 8" |
| 14" | 153.9 | 15" × 10" | 18" × 9" |
Main trunk ducts: 700-900 fpm. Branch ducts: 500-700 fpm. Supply registers: 300-500 fpm. Exceeding 700 fpm in branches causes noticeable air noise. Flex duct should stay under 600 fpm due to higher friction.
| Duct Section | Velocity Range (fpm) | Max Recommended | Noise Level |
|---|---|---|---|
| Main trunk | 700–900 | 1,000 fpm | Low–moderate |
| Branch duct | 500–700 | 700 fpm | Low |
| Supply register | 300–500 | 500 fpm | Minimal |
| Return grille | 300–500 | 600 fpm | Minimal |
| Flex duct | 400–600 | 600 fpm | Moderate |
Yes. Flex duct has about 50% more friction than sheet metal due to its corrugated interior. Many HVAC professionals recommend going one size larger (e.g., 8" instead of 7") when using flex duct to compensate for the increased resistance.
| Material | Friction Factor | Recommended Size-Up | Best Use |
|---|---|---|---|
| Sheet metal | 1.0× (baseline) | None | Trunk lines, permanent runs |
| Fiberglass-lined | 1.1× | None | Noise-sensitive areas |
| Flex duct | 1.5× | +1 size (e.g., 7"→8") | Short branch runs only |
Each standard 90-degree elbow adds about 15-25 feet of equivalent duct length (roughly 20 feet average). This increases total friction loss. A 30-foot run with 2 elbows acts like a 70-foot straight run for pressure calculations.
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Last Updated: Mar 9, 2026
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