Duct Size Calculator

The friction rate is the number everyone skips

Almost every duct-sizing page and every chart on the internet uses a fixed 0.1 inch of water column per 100 feet. That number is a default, not a design. Manual D computes it from the equipment and the house: the available static pressure divided by the total effective length of the longest run, times 100. A blower with 0.5 inch of total external static pressure that loses 0.2 to the coil, 0.1 to the filter and 0.06 to the registers and grilles has about 0.14 left to spend on duct. Spread over a 250 foot effective length, that is a friction rate of 0.056 — barely half the default, and every duct in the system has to be larger to hit it.

This matters because undersizing a duct shows up as a capacity problem, not a duct problem. The blower sits on its curve and delivers whatever airflow the duct system allows. A trunk designed at 0.1 when the system could only afford 0.06 delivers roughly 20 percent less air than the equipment needs, and the diagnosis appears months later as a frozen coil or a hot bedroom.

Total effective length is longer than the tape measure

Every fitting costs pressure, and Manual D expresses that cost as an equivalent length of straight duct. A 90 degree elbow with a turning vane is worth 10 to 20 feet; a tight radius elbow without one is worth 40 to 60. A boot, a transition, a wall stack, a flex takeoff collar and a grille each add their own. The sum along the longest path — supply plus return — is the total effective length, and it routinely comes to two to three times the physical distance.

This is why replacing a run of rigid duct with flex can ruin an otherwise fine system. Flex is not just a smaller effective diameter because of its corrugation; it also sags, and a sagging flex run is worse than its nominal size suggests. Published equivalent-length data for flex takeoffs and elbows is the reason a 25 foot flex run can behave like 70 feet of straight pipe.

Velocity is the second constraint

Friction sets the size; velocity sets the limit. Above roughly 900 feet per minute in a supply trunk you start to hear the air, and above 1,200 the noise is the least of the problems — the pressure drop rises with the square of velocity, so a slightly small duct costs far more than its diameter suggests. Common targets are 700 to 900 fpm in a trunk, 500 to 700 in a branch behind a bedroom register, and no more than 700 in a return that passes near living space.

Returns are the most commonly undersized part of a residential system, and the failure is subtle. A return that is too small starves the blower, drops total airflow, and creates positive pressure in the rooms the supply serves — which pushes conditioned air out through the envelope and pulls hot attic air in. A house with a single 14 by 20 return grille serving a 3-ton system is a house with a permanent airflow problem, and no amount of equipment replacement fixes it.

Why flex needs to be sized larger

A flexible duct has roughly 20 to 30 percent more friction than rigid duct of the same nominal diameter, because the helix and the liner both disturb the airflow. In practice that means a flex run needs to be one nominal size larger than the rigid duct it replaces: if the calculation calls for a 9 inch rigid trunk, the flex version is 10 inch. The insulated jacket makes the outside diameter larger still, which is why the joist bay that fit a 10 inch rigid duct may not accept a 10 inch flex one.

Duct board behaves differently again: the interior surface is rougher than galvanised steel, and the friction factor is closer to flex than to rigid. Its advantage is that the insulation is built in, so the duct does not lose heat to the attic the way a bare trunk with a wrapped jacket does.

The static pressure budget, itemised

A residential air handler is typically rated at 0.5 inch of water column total external static pressure, and that budget is spent before the air ever reaches a room:

Evaporator coil — 0.20 to 0.30 inch when clean, more when dirty. A clogged coil is the single most common cause of low airflow.
Filter — 0.08 to 0.15 for a clean 1 inch filter at 400 CFM per ton, rising sharply as it loads. A 4 inch media filter starts lower and stays lower.
Supply and return grilles — 0.03 to 0.06 each, depending on free area and face velocity.
Duct system — whatever is left, which is often less than 0.15 inch on a system with a wet coil and a loaded filter.

That last line is why this calculator asks for available static rather than assuming it. On a system with a 4 inch media filter and a clean coil you might legitimately have 0.20 inch for duct. On a system with a 1 inch filter that has not been changed since spring, you may have 0.10 — and the duct sizes that worked on paper will not deliver the airflow.

What this calculator does not do

It sizes one run at a time. A real design balances every branch against every other, because the blower does not know which room is which — it pushes air down the path of least resistance, and an unbalanced system overheats the rooms closest to the plenum while starving the far end. That balancing is what Manual D and a commissioning visit are for. This page gets a single run into the right nominal size and tells you whether the velocity is defensible, which is enough to catch a design that will never work.

Frequently asked questions

What friction rate should I use for duct sizing?

Whatever your blower can afford. Divide the available static pressure by the total effective length of the longest run and multiply by 100. A system with 0.14 inch available over 250 feet runs at 0.056, not the 0.1 that charts assume. Using 0.1 on a system that cannot support it is the most common cause of undersized residential ductwork.

How many CFM will a 10 inch duct carry?

About 370 CFM at a friction rate of 0.08 and 420 at the chart default of 0.1, which is roughly 680 to 780 feet per minute. At the tighter 0.06 rate that a long run forces, the same 10 inch duct carries only 320 CFM, which is why run length changes the size.

Do I need a bigger duct for flex?

Yes. Flex has 20 to 30 percent more friction than rigid duct of the same nominal size, so go up one size. A 9 inch rigid requirement becomes 10 inch flex, and the insulated jacket makes the outside diameter larger still.

Why is my return duct so important?

Because a blower can only move what the return path allows. An undersized return starves the system, lowers total airflow, and pressurises the house so conditioned air escapes through the envelope. Returns are usually sized at a lower velocity than supplies for exactly this reason.

What velocity is too high in a supply duct?

Above about 900 feet per minute in a trunk it becomes audible, and the pressure drop rises with the square of velocity. Branches behind bedroom registers are usually kept to 500 to 700 fpm.

Can I just use one size bigger everywhere?

It is a reasonable safety margin on a short run and a poor strategy overall, because larger ducts change how the system balances. The branch with the least resistance gets more air, and the far rooms get less. Sizing to a real friction rate is the fix; guessing larger is not.

Does duct insulation change the size?

Not the size, but the heat gain. An uninsulated trunk in a 130°F attic can lose 15 to 20 percent of the cooling it carries. R-8 is the usual specification for attic ducts, and it is the reason duct location appears in load calculations.

Disclaimer: This tool estimates duct diameter for planning purposes. It does not replace ACCA Manual D, a room-by-room duct design, or a static pressure measurement on the installed system.

Citing this page? FigureMill, “Duct Size Calculator” (last reviewed October 30, 2026). Round and rectangular duct sizes at 0.06, 0.08, and 0.10 in. w.g. per 100 ft of friction rate, with the CFM each size carries and the velocity that implies. https://figuremill.com/hvac/duct-size-calculator

Last reviewed: October 30, 2026 · Sources: ACCA Manual D friction-rate method, ASHRAE duct friction approximations, manufacturer flex-duct equivalent-length data

Duct Size Calculator