Duct sizing.
Duct sizing: Calculate round duct diameter and rectangular equivalent for a given CFM at standard friction class (0.08 in.
For a given CFM and friction class, returns recommended round duct diameter and rectangular-equivalent sizes. Plus velocity check and 1-ton-of-AC = 400-CFM rule applied.
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.
Common questions
What CFM airflow rating is required for a 1200 sq ft house?
As a rule of thumb, cooling needs about 400 CFM of airflow per ton, and a 1,200 sq ft home usually needs a 2 to 2.5-ton system — roughly 800 to 1,000 CFM total. A Manual J load calculation is the proper method.
How do you calculate duct size using CFM and friction rates?
Size ducts from the required CFM against a target friction rate — typically 0.1 inches of water column per 100 feet of duct — using a duct slide rule (ductulator) or friction chart.
How many CFM of ventilation does a bathroom exhaust fan need?
The common rule of thumb is about 1 CFM of exhaust per square foot of bathroom floor area (so an 8×10 bathroom wants roughly an 80 CFM fan), with 50 CFM the usual minimum.
How do you convert round duct diameters to equivalent rectangular duct sizes?
Use an equivalent-diameter table or Huebscher's equation to map a round duct to a rectangular one that carries the same airflow at the same friction (pressure drop), rather than matching raw cross-sectional area.
What is a comfortable air velocity inside residential branch ducts?
To keep whistling and noise down, air velocity in residential supply branch ducts is usually kept between about 700 and 900 feet per minute (FPM).
How many CFM of airflow are required per ton of air conditioning?
The standard design figure is about 400 CFM of airflow per ton (per 12,000 BTU/hr) of cooling capacity, with roughly 350–450 CFM acceptable depending on climate.
What happens to static pressure if you downsize a main HVAC trunk duct?
Restricting the trunk chokes the airflow, so static pressure inside the system rises. That strains the blower motor, reduces airflow to the rooms and cuts efficiency.
What size round flexible duct is needed to carry 200 CFM?
At the standard 0.1 in wc friction rate, an 8-inch flexible duct carries only about 120 to 160 CFM, so moving 200 CFM usually calls for a 9-inch flex run (or roughly 8-inch smooth rigid metal, which flows more).
How do you calculate the air exchange rate for a room?
Multiply the airflow in CFM by 60 to get cubic feet per hour, then divide by the room's volume in cubic feet. The result is the air changes per hour (ACH).
What is the maximum recommended velocity inside a kitchen range hood duct?
Kitchen exhaust ducts run higher velocities — about 1,500 to 1,800 FPM — to keep grease suspended in the airstream instead of settling on the duct walls.
How much CFM airflow does a 6-inch round rigid metal duct carry?
At the standard 0.1 in wc design friction rate, a smooth 6-inch round metal duct carries roughly 100 CFM.
What is the structural difference between supply ducts and return ducts?
Supply ducts push conditioned air from the air handler out to the rooms. Return ducts pull stale room air back to the unit to be filtered and re-conditioned.
How do you convert FPM velocity directly into CFM volume?
Multiply the air velocity in feet per minute by the duct's cross-sectional area in square feet: CFM = velocity (FPM) × area (sq ft).
What size rectangular duct is needed to handle 1,000 CFM?
At the standard 0.1 in wc friction rate, a 1,000 CFM main trunk needs roughly a 12×14-inch or 10×18-inch rectangular duct.
What causes high static pressure in a flexible duct layout?
Sagging runs, sharp 90-degree turns and over-compressing the flexible inner liner all add turbulence and dramatically raise internal static pressure.
How many tons of cooling is 12,000 BTU?
Exactly one ton. One ton of refrigeration is defined as removing 12,000 BTU of heat per hour.
What size duct do I need for a 3 ton AC unit trunk line?
A 3-ton unit moves about 1,200 CFM (3 × 400), which needs roughly a 14-inch round main duct or a 12×18-inch rectangular trunk.
What is the friction loss rate inside standard commercial duct design?
Commercial supply ducts are often sized a little tighter than residential, targeting a friction loss of about 0.08 to 0.15 inches of water column per 100 feet.
How do you size a combustion air duct for gas furnaces?
Fuel-gas code generally requires a minimum free opening of about 1 square inch per 1,000 BTU/hr of the total input rating of the gas appliances in a confined space. Follow the specific two-opening or known-air-infiltration method in the code.
What is the standard aspect ratio limit for rectangular ducts?
To limit added friction and noise, a rectangular duct's aspect ratio (width to height) is ideally kept under about 4:1.
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.