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Air Duct Size Calculator Free and Instant

A ductulator for the field: enter CFM and friction rate, get a duct diameter back. No email, no signup.

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Updated August 2026

What this calculator does

This calculator sizes round ductwork the same way a physical ductulator wheel has done for decades: the equal-friction method, using your required CFM and a target friction rate to solve for duct diameter. It is the same underlying approach referenced in ACCA Manual D duct design, simplified down to the two numbers a tech has on hand at the truck.

Getting duct size wrong costs money later. Undersized ducts choke airflow and wear out the blower early. Oversized ducts drop velocity too low for good air mixing. Both show up as a comfort complaint or a callback, on a job that already looked finished when you left it.

The sections below walk through where the two calculator inputs, CFM and friction rate, come from: a room's square footage, a Manual J load calculation, the tonnage of the equipment, and the static pressure budget of the system. Skip straight to the calculator if you already have both numbers.

Every duct size starts with room square footage

Airflow demand scales with floor area, so square footage is where the math begins. For a rectangular or square room, multiply length by width. A bedroom that measures 12 by 10 feet is 120 square feet. A blueprint, a zoning drawing on file with the local permitting office, or a recent real estate listing all work as a source if you cannot measure the room directly.

An L-shaped or irregular room does not fit that formula cleanly. Split it into two or three rectangular sections, find the area of each, and add them together. This square footage number becomes an input to the room's load calculation, which drives the CFM this calculator needs.

How duct CFM comes from a load calculation

A duct's required airflow comes from a Manual J heating and cooling load calculation, run once for the whole building and once for each individual room. Skip the room-by-room step and you end up with a house that is comfortable in one zone and off in every other one, even though the total equipment size was correct.

Step 1

Convert the whole-building load to equipment tonnage: divide total BTU/h by 12,000, since one ton of HVAC capacity equals 12,000 BTUs. Round up on an uneven result; there is no such thing as a 2.33-ton unit.

Step 2

Convert tonnage to total system CFM by multiplying tons by 400, the standard residential cooling airflow rate per ton.

Step 3

Give each room its share of that total, proportional to its share of the whole-house load: Room CFM = (Room load / Whole house load) x Equipment CFM.

Worked example

A house comes back from a Manual J load calculation at 36,000 BTU/h total. That is a 3-ton system (36,000 / 12,000), so equipment CFM is 3 x 400 = 1,200 CFM. One bedroom in that same load calculation carries 3,600 BTU/h of the total, 10 percent of the house. That bedroom's duct CFM is (3,600 / 36,000) x 1,200 = 120 CFM.

As a rough field check afterward: 1 CFM covers roughly 1 to 1.25 square feet of floor area on a typical room, closer to 2 CFM per square foot on a room with heavy sun exposure or a lot of glass.

Friction rate: the other half of the sizing math

CFM tells you how much air a duct needs to move. Friction rate tells you how much pressure drop the system can afford to spend getting it there, and that number decides how large or small the duct can be. A system with more static pressure to spare supports a smaller, more restrictive duct; a system with little to spare needs a larger one.

Friction Rate = (Available Static Pressure x 100) / Total Effective Length

Available static pressure (ASP) starts with the blower's total external static pressure rating, listed on the equipment's spec sheet at a given CFM. Subtract the pressure drop of every component the air has to pass through: the filter, the coil, supply registers, return grilles, dampers. What is left over is the ASP the duct system is allowed to spend.

Worked example

An air handler is rated at 0.60 in.wg of total external static pressure at the design CFM. The supply register drops 0.03 in.wg, the return grille drops 0.03 in.wg, and the filter drops 0.12 in.wg. Available static pressure is 0.60 - 0.03 - 0.03 - 0.12 = 0.42 in.wg.

The longest run in the system measures 80 feet of straight duct, and its fittings, two 90-degree elbows, a tee, and a damper, add up to 145 equivalent feet, for a total effective length of 225 feet. Friction rate is (0.42 x 100) / 225 = 0.187 in.wg per 100 ft.

Feed that friction rate and the bedroom's 120 CFM from the section above into the calculator, and the exact diameter comes back at 5.5 inches, which rounds to the nearest stocked size: 6 inches.

What total effective length measures

Total effective length (TEL) is the physical length of the longest duct run, from the farthest supply outlet through the equipment to the farthest return, plus the equivalent length that every fitting along that run adds on top of it. A straight section of duct and a 90-degree elbow do not create the same pressure drop per foot, so fittings get converted into the number of straight-duct feet that would produce an equal drop.

To build a TEL number, add up the physical length of straight duct in the most restrictive run, then add the equivalent length of every elbow, tee, transition, damper, register, and grille sitting in that same run. Manufacturers and duct design references publish equivalent-length values for common fitting types, so this comes down to a lookup and a sum. Once TEL is set, it plugs directly into the friction rate formula above.

Metal, flex, or rectangular: does it change the number?

Rigid galvanized metal offers the least resistance to airflow of the common duct materials, so it stays closest to the friction rate the calculator assumes. Flexible duct resists airflow more, and that resistance grows if the flex is not pulled fully taut, sags between supports, or carries sharp bends packed into a short run. It is common practice to round up one stocked size on a flex branch that has several bends rather than trust the exact calculated diameter.

A rectangular duct carries the same airflow as a round one when both are sized to the same equivalent diameter, which is what the rectangular-to-round converter below the main calculator solves for. Use it when framing forces a rectangular shape but you have already worked out the round diameter you need.

Quick reference: round duct size to CFM

This table runs the same equal-friction formula behind the calculator above at a friction rate of 0.10 in.wg per 100 feet, the ACCA Manual D default for most residential systems. Use it as a sanity check on a stocked size before you cut and hang it, then re-run the exact numbers through the calculator whenever your friction rate is not 0.10.

Round duct diameterAirflow at 0.10 in.wg/100ft
4"37 CFM
5"67 CFM
6"109 CFM
7"163 CFM
8"232 CFM
9"317 CFM
10"419 CFM
12"678 CFM
14"1019 CFM
16"1450 CFM
18"1980 CFM
20"2615 CFM

ACCA Manual D standard for most residential systems is 0.08-0.1.

Round duct diameter

10"

Exact calculated size: 9.8", rounded to nearest stocked size.

Rectangular to round equivalent

Round equivalent

8.4"

From a duct number to a booked airflow visit

A homeowner calls because a spare bedroom never cools down like the rest of the house. AutoRev answers, pulls up the system on file at that address, and can tell them the branch feeding that room was already sized to the load, so a blocked return or a closed damper is more likely than a resize. It gets the details on when someone can be home, checks the calendar, and books the diagnostic visit before the call ends.

By the time the tech pulls up, the duct numbers from that call are already sitting on the job instead of waiting to be measured on-site again.

You are not building a script for that call. AutoRev is a general-purpose AI coworker you hand tasks to: answer this line, price this repair, follow up on that quote, book the next opening. It plugs into whatever FSM or CRM your shop already runs, so a call like this one lands in the same system your techs already check, not a separate app you have to babysit.

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FAQ

Duct sizing questions

What techs ask before and after they run the numbers.

Duct size follows CFM = Area x Velocity. For a round duct, area is 0.785 times the diameter squared, so once you know the required CFM and the target air velocity (usually 600 to 900 FPM for a residential supply run), you can solve for the diameter. A ductulator does this same math against a friction-rate chart so you do not have to solve it by hand on every job.

A ductulator is the tool HVAC techs have used for decades, originally a paper slide wheel, to size ductwork from CFM, velocity, and friction rate without doing the math manually. This calculator is the digital version of that same equal-friction method: enter your CFM and friction rate, get a duct diameter back.

Most residential systems default to a friction rate of 0.05 to 0.10 inches of water column per 100 feet of duct. 0.05 is a common conservative default when you have not calculated total effective length and available static pressure for the specific run. If you have those numbers, plug them into the calculator's static pressure mode instead of guessing.

Most residential systems are designed around 400 CFM per ton of cooling, with a typical range of 350 to 450 CFM per ton depending on climate and humidity. Humid regions run closer to 350 for better moisture removal, dry regions run closer to 450. Heating airflow needs run lower than cooling, so size the ductwork to the cooling number and it covers both seasons.

Room CFM = (Room load / Whole house load) x Equipment CFM. Run a Manual J load calculation for the whole house and for each room, convert the whole-house load to equipment CFM (tonnage x 400), then give each room its proportional share of that airflow. A room carrying 10 percent of the house's total load gets 10 percent of the equipment's total CFM.

TEL is the length of the longest duct run, measured from the farthest supply outlet through the equipment to the farthest return, plus the equivalent length every fitting and bend adds to that run. Every elbow, tee, and transition creates pressure drop equal to some number of feet of straight duct, and those equivalent feet get added to the physical length. TEL feeds directly into the friction rate formula, so getting it wrong throws off every duct size downstream.

Friction Rate = (Available Static Pressure x 100) / Total Effective Length. Start with the blower's total external static pressure rating from the equipment's spec sheet, subtract the pressure drop of every component in the system (filter, coil, registers, grilles, dampers), and what is left is your available static pressure. Divide that by TEL and multiply by 100 to get the friction rate in inches of water column per 100 feet, then feed that number into the calculator above.

Yes, indirectly. Rigid metal duct offers the least resistance to airflow, so it holds closer to the calculated friction rate at a given size. Flexible duct resists airflow more, and that resistance gets worse if the flex is not pulled fully taut or has sharp bends packed into a short run. Techs commonly round up one stocked size on a flex run with several bends rather than trust the exact calculated diameter.

Yes. Undersized ductwork restricts airflow, raises static pressure, and strains the blower, which shows up as uneven room temperatures, a noisy system, and a shorter equipment life. Oversized ductwork drops air velocity too low for good mixing, which also reads as an uneven, uncomfortable house even though the equipment itself is fine.

Round up when the exact calculated diameter falls close to the midpoint between two stocked sizes, or when the run has more fittings than a typical straight branch. A slightly oversized duct costs a little more in material and loses a bit of velocity; a slightly undersized one raises static pressure on the whole system and shows up as a callback.

This tool solves for round duct diameter directly, and includes a rectangular-to-round converter below the main calculator. Enter both sides of a rectangular run to see its round equivalent, or work the round number you get back into a rectangular shape sized to fit the framing cavity you are running through.

Now turn that number into an estimate.

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