Find the return air duct size for a 1.5 to 5 ton AC from its tonnage or actual CFM, in round or rectangular inches, at typical return velocities.

Tons assume 400 CFM per ton, a common rule of thumb. Choose Enter airflow to use your blower’s actual CFM.

Typical maximum residential return velocities (ACCA Manual D). Lower velocity means a bigger, quieter duct.

Assumes equal airflow in every duct.

Optional settings

Return Air Duct Size Formula

The return air duct size calculator estimates internal duct area from actual airflow and a selected mean duct velocity, or solves airflow and velocity from internal duct dimensions. At the same airflow, a lower mean velocity requires more area. Velocity and area alone do not determine the system pressure drop, grille performance or noise.

Core Equations

The calculator uses actual airflow, or 400 CFM per US ton of refrigeration when you pick a system size. The examples and chart below also assume 400 CFM per ton:

Q = 400T

Calculate the required internal duct cross-sectional area from airflow and mean duct velocity:

A = (Q) / (V)

Then convert that area into equal-area square or round internal duct dimensions:

S = √(144A)
D = 2√((144A) / (π))

Under the illustrative 400 CFM per US ton assumption only, the expressions for equal-area internal dimensions become:

S = √((57600T) / (V))
D = 2√((57600T) / (π V))
  • T = cooling capacity in US tons of refrigeration, not mass in short tons or metric tonnes
  • Q = airflow in cubic feet per minute (CFM)
  • V = selected mean velocity through the internal duct cross section in feet per minute (FPM)
  • A = required internal duct cross-sectional area in square feet
  • S = square return duct side in inches
  • D = round return duct diameter in inches

Important assumption: 400 CFM per US ton of refrigeration is an illustrative airflow assumption here, not a required blower setting. Choose Enter airflow to use the actual equipment airflow. A grille’s nominal neck area, core area and free area are different; its published free-area percentage may use core area rather than nominal width × height. Use the manufacturer’s stated area and velocity basis for grille and filter selection.

How to Use the Calculator

  1. For duct size, pick the system size in tons (or enter the actual airflow) and a design velocity.
  2. For airflow or velocity from dimensions, use the clear internal round or rectangular duct dimensions.
  3. Read the result in its displayed units; the calculated size is a geometric cross section.
  4. Check available static pressure, fittings, length, grille and filter performance, noise and balancing before selecting an installation size.

The 300 FPM and 350 FPM values below are illustrative mean duct velocities, not universal limits or recommended grille/filter velocities. At fixed airflow, 300 FPM requires more internal area than 350 FPM. Choose design criteria from the equipment, components and complete duct-system design.

Rectangular Return Ducts

For a rectangular internal duct cross section, if one side is fixed, solve for the other side with the same required area:

W = (144A) / (H)

Here, W and H are internal dimensions in inches. For example, 4.0 ft2 is 576 in2. Matching or exceeding area alone does not establish an acceptable duct design. Equal-area round and rectangular ducts have the same mean velocity at the same airflow, but are not necessarily friction-equivalent; shape equivalency for design considers equal airflow, resistance and length.

Example

For an illustrative 3-US-ton-of-refrigeration system, assume 400 CFM per US ton, giving 1,200 CFM. This is an example airflow, not a prediction for every 3-ton system.

Q = 400(3) = 1200

At an assumed mean internal duct velocity of 300 FPM, the required cross-sectional area is 4.0 ft2.

A = (1200) / (300) = 4.0

That corresponds geometrically to an internal square side of 24.0 inches or an internal round diameter of about 27.1 inches; it does not establish equal friction or select a nominal grille.

S = √(144(4.0)) = 24.0
D = 2√((144(4.0)) / (π)) ≈ 27.1

At an assumed mean duct velocity of 350 FPM, the same 1,200 CFM gives equal-area dimensions of about 22.2 inches square or 25.1 inches round. Confirm the final selection with the complete system design.

Quick Size Chart

This conditional geometry chart assumes 400 CFM per US ton of refrigeration and the stated mean internal duct velocities. Values are rounded to the nearest 0.1 inch for display, not selected nominal products. The unrounded required area is the basis for comparison; final products and pressure performance require separate checks.

Conditional equal-area internal duct dimensions at 400 CFM per US ton of refrigeration
Cooling Capacity (US tons of refrigeration) Square Side @ 300 FPM Round Diameter @ 300 FPM Square Side @ 350 FPM Round Diameter @ 350 FPM
1 ton 13.9 in 15.6 in 12.8 in 14.5 in
1.5 tons 17.0 in 19.1 in 15.7 in 17.7 in
2 tons 19.6 in 22.1 in 18.1 in 20.5 in
2.5 tons 21.9 in 24.7 in 20.3 in 22.9 in
3 tons 24.0 in 27.1 in 22.2 in 25.1 in
3.5 tons 25.9 in 29.3 in 24.0 in 27.1 in
4 tons 27.7 in 31.3 in 25.7 in 29.0 in
5 tons 31.0 in 35.0 in 28.7 in 32.4 in

Practical Sizing Notes

  • Do not size only the duct. The return grille, filter, filter rack, and transition should also support the same airflow without becoming the restriction.
  • Use actual airflow when available. Variable-speed systems, heat pumps, zoning, and special blower settings may not operate at exactly 400 CFM per ton.
  • Multiple returns can share the load. Split the total required airflow or area between the returns rather than forcing everything through one opening.
  • Keep geometric rounding separate from product selection. Do not round a required area down. Select available products only after checking their internal dimensions and pressure, airflow and noise performance.
  • Check component performance. Grille face velocity, free-area velocity, filter velocity and mean duct velocity use different area bases. Manufacturer pressure-drop and sound data are needed for the selected component.
  • This calculator is a first-pass geometry tool. It does not perform a full ACCA Manual D design. Final design includes the blower and available static pressure, layout, fittings, length, component resistance, noise and system balancing.