Duct sizing.

How this works

The relationship between duct diameter, airflow, and friction loss in standard residential HVAC follows a power-law curve. At the standard 0.08″ wc per 100 ft friction class:

CFM ≈ k × D^2.7

   where:
     D = duct diameter (inches)
     k ≈ 0.95 for 0.08" wc per 100 ft (standard residential)
     k ≈ 1.20 for 0.10" wc per 100 ft (commercial)
     k ≈ 1.50 for 0.15" wc per 100 ft (high-velocity)

This is empirical fit to the ASHRAE friction chart values (Manual D, Table 3); the exponent 2.7 comes from the dependence of friction loss on velocity squared and area. It's accurate to within a few percent for residential round galvanized duct.

Round vs rectangular: equivalent diameter

For the same airflow capacity, the equivalent rectangular duct dimension follows the ASHRAE equation:

D_eq = 1.30 × (a × b)^0.625 / (a + b)^0.25

   where a, b = rectangular duct dimensions

This calculator inverts the equation to find rectangular sizes equivalent to your round size, with the constraint that the aspect ratio shouldn't exceed about 4:1 (above that, the rectangular duct loses significant capacity to wall friction).

Velocity considerations

Beyond friction loss, you want velocity in a reasonable range:

• Residential supply trunk: 700-900 ft/min
• Residential branch: 600-700 ft/min
• Return air: 500-700 ft/min (quieter; lower pressure budget)
• Commercial trunk: 1200-1800 ft/min
• Commercial branch: 800-1200 ft/min

Above these ranges, ductwork becomes noisy and inefficient. Below them, ducts are oversized and expensive.

Common pitfalls

• This is a starting-point sizing. Real duct design (ACCA Manual D) accounts for trunk-branch hierarchies, fittings, equivalent length, and balancing. Use this calculator to ballpark and double-check.
• 1 ton of AC ≠ 1 ton of refrigerant. A ton of cooling capacity = 12,000 BTU/h = roughly 400 CFM of airflow at design conditions. So a 3-ton AC needs ~1200 CFM total system airflow.
• Returns are usually undersized. Most installations have too-small return ducts, leading to high static pressure, equipment short-cycling, and reduced efficiency. Returns should be sized for 500-700 ft/min velocity — usually one size larger than the supply trunk.
• Flex duct vs rigid is a real performance difference. A 6" flex duct stretched and supported correctly handles ~80% of a rigid 6". Compressed or sagging flex can drop to 50%. Always specify supports and limit length.
• Static pressure budget matters more than CFM alone. A typical residential blower delivers ~0.5" wc total external static. Filter + coil + supply trunk + return = that budget. If your sized duct system needs 0.7" wc, the equipment won't deliver rated CFM.

Sources

• Standard duct sizing: ACCA Manual D — Residential Duct Systems.
• Friction loss tables: ASHRAE Fundamentals Handbook, Chapter 21 (Duct Design).
• Equivalent diameter formula: ASHRAE rectangular-to-round equivalent, derived from constant friction loss at constant CFM.

Disclaimer. This is a simplified sizing calculator for ballpark and verification work. Production residential design should follow ACCA Manual J (loads), Manual S (equipment selection), and Manual D (duct design). Commercial design follows ASHRAE methods.

See also