HVAC Duct Sizes
Structural & HVAC · 11 specs
Direct Answer
Standard residential and commercial HVAC duct dimensions are specified by round spiral diameter (4 to 20 inches) and equivalent rectangular cross-sections based on airflow capacity (CFM) at 0.10 inches water column friction loss per 100 feet.
Source: SMACNA HVAC Duct Construction Standards / ACCA Manual D · Last reviewed Aug 28, 2026
Specs
| Governing Standards | SMACNA / ACCA Manual D Residential Duct Systems |
| Standard Sizing Metric | Cubic Feet per Minute (CFM) at 0.10" w.g. / 100 ft friction rate (Residential) / 0.08" (Quiet) |
| 4" Round Duct | Capacity: ~30 CFM | Small bathrooms, range hood make-up air |
| 5" Round Duct | Capacity: ~50 CFM | Small powder rooms, walk-in closets |
| 6" Round Duct (Standard Branch) | Capacity: ~100 CFM | Standard 10×12 ft bedrooms |
| 7" Round Duct | Capacity: ~150 CFM | Master bedrooms, medium living areas |
| 8" Round Duct | Capacity: ~200 CFM (0.5 Ton A/C) | Large master suites, kitchens |
| 10" Round Duct | Capacity: ~350 CFM (~1.0 Ton A/C) | Open living rooms, main branches |
| 12" Round Duct | Capacity: ~550 CFM (~1.5 Ton A/C) | Main trunk lines |
| 14" Round Duct | Capacity: ~800 CFM (2.0 Ton A/C) | Main central supply trunk |
| Airflow Rule of Thumb | 1 Ton of Air Conditioning = 400 CFM of airflow |
Round Duct Diameter vs CFM Capacity & Equivalent Rectangular Sizes
| Round Duct Diameter (in) | Cross-Section Area (sq in) | Airflow Capacity (CFM @ 0.10" wg) | Airflow Capacity (CFM @ 0.08" wg) | Equivalent Rectangular Duct (W × H) | A/C Tonnage Support |
|---|---|---|---|---|---|
| 4" | 12.6 sq in | 30 CFM | 25 CFM | 6" × 4" | 0.08 Ton |
| 5" | 19.6 sq in | 55 CFM | 45 CFM | 8" × 4" | 0.14 Ton |
| 6" | 28.3 sq in | 100 CFM | 85 CFM | 10" × 4" or 8" × 6" | 0.25 Ton |
| 7" | 38.5 sq in | 150 CFM | 130 CFM | 12" × 4" or 10" × 6" | 0.38 Ton |
| 8" | 50.3 sq in | 200 CFM | 175 CFM | 14" × 4" or 10" × 8" | 0.50 Ton |
| 9" | 63.6 sq in | 275 CFM | 240 CFM | 16" × 4" or 12" × 8" | 0.70 Ton |
| 10" | 78.5 sq in | 375 CFM | 325 CFM | 16" × 6" or 12" × 10" | 0.95 Ton (~1.0 Ton) |
| 12" | 113.1 sq in | 600 CFM | 520 CFM | 18" × 8" or 14" × 12" | 1.50 Tons |
| 14" | 153.9 sq in | 900 CFM | 780 CFM | 20" × 10" or 16" × 14" | 2.25 Tons |
| 16" | 201.1 sq in | 1,300 CFM | 1,120 CFM | 24" × 12" or 18" × 16" | 3.25 Tons |
Why Rectangular Ducts Require Larger Cross-Sectional Area
Circular ducts provide the maximum hydraulic radius and lowest boundary-layer skin friction for moving air:
- Huebscher Equivalent Equation: A 10-inch round duct (78.5 sq in) carries the same CFM as a 12 × 10-inch rectangular duct (120 sq in) — a 52% increase in physical area required due to corner turbulence and surface drag.
- Velocity Noise Thresholds: In residential supply branches, air velocity must remain under 700 to 900 FPM (feet per minute); exceeding 1,000 FPM produces audible whooshing turbulence through registers.