Duct Size Calculator: Round or Rectangular from CFM

How to use the duct size calculator

1. Enter the airflow in cubic feet per minute. That figure usually comes from a room-by-room load or from the CFM calculator — this page does not calculate a heating or cooling load.
2. Choose the run type (supply or return, trunk or branch, rigid or flex). The form fills the recommended feet-per-minute from ACCA Manual D Table 3-1. You can override the speed; the table maximum is shown in the hint.
3. Read the exact diameter and the next standard round size (4 through 20 inches). For a rectangular duct, enter a height and the width is area ÷ height.

Room capacity stays on the BTU calculator. A full Manual D also sets a friction rate, fitting losses and blower available static pressure, which this page does not.

The velocity formula and Table 3-1

Required cross-section is airflow ÷ velocity: area in square feet equals CFM ÷ feet per minute. The exact round diameter in inches is 12 × √(4 × area / π). The calculator then steps up to the next common round size.

ACCA Manual D Table 3-1, as reproduced in a Montana Department of Labor and Industry Manual S and D training handout, recommends 700 fpm (maximum 900) for a rigid supply trunk and 600 fpm (maximum 700) for a flexible supply trunk. Supply branches are 600 fpm recommended (900 rigid / 700 flex maximum). Return trunks are 600 fpm recommended and 700 fpm maximum; return branches are 400 fpm recommended and 700 fpm maximum.

A worked example: 200 CFM at 700 fpm

A 200 CFM rigid supply trunk at the recommended 700 fpm needs 200 ÷ 700 = 0.286 sq ft, or about 41.1 sq in. The exact round diameter is 7.24 inches, so the next standard size is an 8-inch round duct. (A rigid supply branch at its Table 3-1 recommended 600 fpm is 7.82 inches, still an 8-inch next size. The form opens on a rigid branch at 600 fpm; switch the run type or the velocity to match this trunk example.) An 8-inch-high rectangular duct would need a width of about 5.14 inches — in practice you would choose a standard rectangular size that is at least that wide.

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• Cross-section area in square feet equals CFM ÷ design velocity in fpm. Exact round diameter in inches is 12 × √(4 × area / π), then stepped up to the next common round size from 4 to 20 inches. Rectangular width is area ÷ the height you enter.
• Default velocities are the recommended feet-per-minute from ACCA Manual D Table 3-1 for the selected run type. You can override them. The table maximum is shown in the field hint and flagged if exceeded.
• This is a velocity-area size check, not a full Manual D friction-rate design and not a room heating or cooling load.

Sources

1. Montana Department of Labor and Industry, Building and Commercial Measurements Bureau, Residential HVAC Design: Manual S and D (training handout). Public training reprint labeled “Copy of Table 3-1 from ACCA Manual D.” Recommended / maximum FPM: supply trunk rigid 700 / 900, supply trunk flex 600 / 700; supply branch rigid 600 / 900, supply branch flex 600 / 700; return trunk 600 / 700; return branch 400 / 700 (rigid and flex share the return figures).
2. Air Conditioning Contractors of America / Building America Solution Center (PNNL), Manual D — Residential Duct Systems. Identifies ANSI/ACCA 1 Manual D as the residential duct design standard from which Table 3-1 is taken. The standard itself is not reproduced here.

Last reviewed November 12, 2026. A full Manual D design and the airflow from a load calculation win.