Duct Size Calculator

Duct size depends on the airflow (CFM) plus a design target — either a friction rate (commonly 0.08–0.10 inches of water column per 100 feet for residential) or a target air velocity. The console solves for the round diameter that meets your target, then shows the rectangular and flat-oval equivalents and the resulting velocity. As a feel for scale, a 14-inch round duct moves about 800 CFM at roughly 750 feet per minute, and a 28-inch round moves about 6,000 CFM near 1,400 feet per minute. Always confirm against a full ACCA Manual D design.

Two exact relationships drive it. Air velocity in feet per minute equals 183.3 × CFM ÷ diameter² (diameter in inches), so for a target velocity you solve diameter = √(183.3 × CFM ÷ velocity). For a friction-rate target the tool applies the Darcy-Weisbach friction equation on the ASHRAE chart basis and iterates the diameter until the pressure loss per 100 feet matches your chosen friction rate. It then steps up to the nearest standard duct size.

Tonnage sets the airflow before it sets the duct. A common design figure is roughly 400 CFM per ton, so a 2-ton system moves about 800 CFM, a 3-ton about 1,200 CFM, and a 4-ton about 1,600 CFM. Size the trunk for that airflow: at a typical 900 feet per minute, 1,200 CFM lands on about a 16-inch round trunk that runs near 860 feet per minute, which stays inside the residential range. Enter your system's CFM with a friction rate or velocity target and the console returns the round, rectangular, and flat-oval options. The 400-CFM-per-ton figure is only a rule of thumb — a Manual J load calculation should set the real airflow.

Many residential supply designs land in the 0.08–0.10 inches of water column per 100 feet range, and 0.10 is a common default. But the friction rate is not a universal constant. Proper ACCA Manual D derives it from the blower's Available Static Pressure divided by the Total Effective Length of the run, then multiplied by 100. Use a value derived from your actual system when you can; treat 0.08–0.10 only as a reasonable starting estimate.

Friction rate is the duct's pressure loss per 100 feet of equivalent length, expressed in inches of water column. In Manual D it is Friction Rate = (Available Static Pressure ÷ Total Effective Length) × 100, where Total Effective Length adds the equivalent length of every fitting, the coil, the filter, and the registers to the straight-duct length. Fittings often dominate that total, which is why a correctly derived friction rate is usually lower than people assume.

It depends on the velocity you allow, because CFM equals velocity multiplied by cross-sectional area. At a typical 700 feet per minute, a 6-inch round carries roughly 140 CFM, an 8-inch about 245 CFM, a 10-inch about 380 CFM, and a 12-inch about 550 CFM. Allow a higher velocity and each carries proportionally more air — along with more noise and static pressure. The console reports the exact figure for any size and velocity.

Airflow is velocity times cross-sectional area: CFM equals the air velocity in feet per minute multiplied by the duct area in square feet. For a round duct the area is the diameter squared divided by 183.3, so a 12-inch round is about 0.785 square feet and at 800 feet per minute it carries roughly 628 CFM. In the field you usually get the velocity from velocity pressure measured with a manometer or pitot tube — velocity equals 4005 times the square root of the velocity pressure in inches of water column, so 0.04 inches reads as about 800 feet per minute. Multiply that velocity by the area for the CFM; the console reports the velocity and the resulting airflow for any size.

Use the ASHRAE circular-equivalent (Huebscher) relationship: equivalent diameter Dₑ = 1.30 × (a·b)^0.625 ÷ (a+b)^0.25, where a and b are the rectangular sides in inches. Choose rectangular dimensions whose equivalent diameter matches your round size. The match preserves friction loss and air-carrying capacity, not velocity, and you should keep the aspect ratio at or below about 4 to 1.

No. The equivalent-diameter conversion preserves equal friction loss and equal capacity for the same airflow, but a rectangular duct has a different cross-sectional area, so its velocity differs — usually higher than the round duct it replaces. If you need a specific velocity, size by velocity directly. The console reports the actual velocity for every shape so the difference is visible.

Flat-oval uses its own correlation: equivalent diameter Dₑ = 1.55 × A^0.625 ÷ P^0.25, where A is the flat-oval cross-sectional area and P is its perimeter. The tool computes A and P from the major and minor dimensions and returns the equal-friction round diameter, so you can swap flat-oval for round or rectangular without changing the duct's pressure drop.

Common guidance keeps residential supply trunks around 700–900 feet per minute, with branches and returns a step slower for quiet operation; light commercial supply often runs 1,200–1,500 feet per minute. Higher velocity buys smaller duct at the cost of noise and static pressure. The console grades your result against the velocity cap you select and flags a size that runs too fast.

Size a return for the same airflow it carries back to the air handler, but aim for a lower velocity so it stays quiet — commonly around 600 to 700 feet per minute for residential return trunks. Because a slower target needs more area, a return generally comes out larger than the supply duct for the same CFM: a 1,000-CFM return sized for about 700 feet per minute works out to roughly a 16-inch round, or its rectangular equivalent — about a size up from what the same airflow needs on the supply side. Enter the return airflow with a 600 to 700 feet-per-minute velocity target and the console sizes it and grades the result.

Flexible duct has a corrugated, much rougher interior, so it loses noticeably more pressure than smooth galvanized steel at the same diameter — often two to three times the friction, and far more when it is compressed or sagging. To carry the same airflow at the same friction rate, flex generally needs to step up at least one size. Select the material in the console and the effective roughness changes with it.

No. It is an educational estimating aid for sizing straight, clean galvanized round duct on the ASHRAE friction-chart basis. A complete design accounts for the whole system — fittings, the coil and filter, register equivalent lengths, the blower curve, zoning, and your local mechanical code — which is the work of an ACCA Manual D design and, where required, a licensed mechanical engineer. Verify every result before fabrication.