Ductwork Calculator

Size supply trunks, branch runs and the return with this ductwork calculator: enter tons, friction rate and duct material to get diameters, velocity and grille area.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: The equal friction method turns tons into CFM at 400 CFM per ton, then reads the diameter that carries that airflow at the design friction rate. A 3 ton system moving 1,200 CFM at 0.08 in w.c. per 100 ft needs a 16 in round sheet metal trunk, with 7 in branch runs for the 150 CFM each of eight runs carries.

Your measurements

A length built from stock pieces: total length divided by piece length, rounded up, with the last piece cut.

System size (tons of cooling)One ton is 12,000 BTU per hour. For a furnace with no cooling, divide the blower CFM on the rating plate by 400 and enter that.Airflow per ton (CFM)400 CFM per ton is the standard residential design airflow. Humid climates run 350 for deeper dehumidification, dry climates up to 450, and a heat pump often wants 400 to 450 in heating.Duct materialFlex carries less air than metal at the same diameter because the wire helix roughens the wall, so a flex run sizes up. Branch takeoffs off a rectangular trunk are still round metal here.Friction ratePressure drop per 100 ft of equivalent length. Manual D derives it from the blower's available static; 0.08 and 0.10 are the everyday residential values, and 0.06 buys quieter, larger duct.Number of supply runsCount the branch takeoffs, not the registers, unless each register has its own run. Airflow is split evenly here; a room-by-room Manual J gives the real split.Rectangular trunk depthinOnly used when the material is rectangular. 8 in clears the drywall in a 2 x 10 joist bay; 10 and 12 in belong in a basement or a soffit.What are you sizing?

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Results

Main trunk, round: 16 in round; Carries 1,200 CFM at 0.08 in w.c. per 100 ft, 859 FPM in the duct
Branch duct per supply run: 7 in round; 150 CFM per run across 8 runs, 561 FPM in the branch
Velocity check: 859 FPM trunk, 561 FPM branch; Manual D holds a trunk near 700 FPM to 900 FPM, a branch under 900 FPM and a return under 700 FPM, because that is where noise starts
Total system airflow: 1,200 CFM; 3 tons x 400 CFM per ton
Return duct: 18 in round; 679 FPM carrying the full 1,200 CFM, one size up from what 0.08 in w.c. per 100 ft alone gives, to hold the return under 700 FPM
Return grille face area: 576 sq in; 432 sq in of free area at 400 FPM, divided by the 75% free area of a filter grille. Split it between two grilles where one that size will not fit. A Manual D layout by the installer governs

Ductwork formula

Total CFM = Tons x CFM per ton (400 standard, 350 to 450 range)
Diameter (in) = (0.109 x CFM^1.9 / friction rate)^(1 / 5.02)   equal friction, round metal
Flex duct: about 1.5x the friction of metal, so use 0.164 in place of 0.109
Velocity (FPM) = CFM / duct area (sq ft);  Branch CFM = Total CFM / number of runs

Worked example

A 4 ton system at 400 CFM per ton moves 1,600 CFM and is run in flex at a 0.10 friction rate with 10 supply runs. In smooth metal the trunk diameter is (0.109 x 1,600^1.9 / 0.10)^(1/5.02) = 16.6 in. Flex carries about 1.5 times the friction, which lifts the diameter to 18.0 in, so the trunk is 18 in flex.

Each of the 10 runs carries 1,600 / 10 = 160 CFM. In metal that comes to 6.9 in, and in flex 7.5 in, so the branches are 8 in flex. Air moves through an 8 in duct at 160 / 0.349 sq ft = 458 feet per minute, which is quiet. The return at the same 1,600 CFM needs 22 in round to stay under 700 FPM. The return grille needs 576 sq in of free area, so 768 sq in of face: two 24 x 16 filter grilles.

Quick reference

Round duct size by airflow and friction rate, and the velocity in the metal size at 0.08
Airflow	Metal at 0.08	Metal at 0.10	Flex at 0.08	Velocity, metal at 0.08
100 CFM	6 in	6 in	7 in	509 FPM
150 CFM	7 in	7 in	8 in	561 FPM
200 CFM	8 in	8 in	9 in	573 FPM
300 CFM	10 in	9 in	10 in	550 FPM
400 CFM	12 in	10 in	12 in	509 FPM
600 CFM	12 in	12 in	14 in	764 FPM
800 CFM	14 in	14 in	16 in	748 FPM
1,200 CFM	16 in	16 in	18 in	859 FPM
1,600 CFM	18 in	18 in	20 in	905 FPM
2,000 CFM	20 in	18 in	22 in	917 FPM

Sizing trunks and branches by equal friction

Start with airflow, not duct size. Multiply tons by 400 CFM per ton, which is the standard residential design airflow; 350 suits a humid climate where longer runs pull more moisture, and 450 suits dry air or a heat pump working hard in heating. A 3 ton system is 1,200 CFM. Split that across the supply runs to get the airflow each branch carries, then read the diameter that passes that airflow at the chosen friction rate.

The friction rate is the pressure the design spends per 100 feet of equivalent length. Manual D derives it from the blower's available static after the coil, filter and balancing dampers take their share, and 0.08 to 0.10 in w.c. per 100 ft covers most houses. Equivalent length is the part people skip. A 90 degree elbow, a boot and a takeoff each add tens of feet, so a run that measures 40 ft often prices out at 90 ft or more. Size from the longest run, then check velocity. A trunk between 700 and 900 feet per minute and branches under 900 stay quiet; past that the registers start to whistle and the fittings roar.

The return is where most houses fall short

Supply gets the attention and the return gets one grille in a hallway. A blower has to pull back everything it pushes, so the return path carries the same 1,200 CFM as the supply. Size the return duct to stay under 700 feet per minute rather than at the supply friction rate, because return noise arrives right at the grille where people stand. On a 3 ton system that is an 18 in round trunk, not the 16 in the friction rate alone would give.

The grille matters as much as the duct. Hold the face velocity to about 400 FPM through free area, and remember a filter grille passes only about three quarters of its face. A 3 ton system wants roughly 432 sq in of free area, which is a 24 x 24 filter grille or two smaller ones. Bedrooms with solid-core doors and no return path lose the same fight: undercut the door 3/4 in, or add a jumper duct or a transfer grille. Doors closed on an undersized return show up as a pressurized bedroom, a cold hallway and a blower running at high static all season. Sealing the ducts that exist is the cheapest fix on the list, typically $600 to $2,000 for a mastic and tape job on an accessible system.

What this calculator leaves out

This sizes duct, not a duct system. It does not compute equivalent length for your fittings, run a Manual J room-by-room load, or subtract the static pressure the coil, filter and grilles take from the blower. Balancing dampers, register and grille selection, zoning dampers and controls, and the plenum and transitions at the air handler all sit outside it. So do duct insulation to R-8 in an attic (IRC N1103.3.1), the mastic and UL 181 tape for sealing, and hangers and strap. Also the duct leakage test at 4 CFM per 100 sq ft of floor area, and the mechanical permit, typically $50 to $250. It also assumes the runs split airflow evenly, which they do not until someone balances them.

Code limits that apply

ACCA Manual Dchecked
Design velocity: trunk 700 to 900 FPM, branch under 900 FPM, return under 700 FPM
If you miss it: Undersized duct raises static pressure, starves the far rooms and makes the registers whistle at every call
IRC M1601.1.1
Ducts are listed factory-made, galvanized steel, aluminum or rigid fibrous duct board
If you miss it: Unlisted duct or a taped-up framing cavity fails the rough-in and has to come out before drywall
IRC M1601.4checked
Flex duct supported at most every 4 ft with no more than 1/2 in of sag per foot
If you miss it: Sagging flex loses half its airflow and the room it serves never reaches the thermostat setting
IRC M1601.4.1
Joints, seams and connections sealed with mastic, mastic tape or a listed UL 181 tape
If you miss it: Cloth duct tape lets go within a few years and the system dumps conditioned air into the attic
IRC M1602.2
Return air is not taken from a garage, a bathroom, a toilet room or a closet
If you miss it: A return in the garage or the furnace closet pulls exhaust and flue gas into the living space
IRC N1103.3.1
Ducts in a vented attic insulated to R-8 at 3 in and larger, R-6 below that
If you miss it: Bare metal in a 130 F attic gives away a large share of the capacity that was paid for

Provision	Limit	If you miss it
ACCA Manual Dchecked	Design velocity: trunk 700 to 900 FPM, branch under 900 FPM, return under 700 FPM	Undersized duct raises static pressure, starves the far rooms and makes the registers whistle at every call
IRC M1601.1.1	Ducts are listed factory-made, galvanized steel, aluminum or rigid fibrous duct board	Unlisted duct or a taped-up framing cavity fails the rough-in and has to come out before drywall
IRC M1601.4checked	Flex duct supported at most every 4 ft with no more than 1/2 in of sag per foot	Sagging flex loses half its airflow and the room it serves never reaches the thermostat setting
IRC M1601.4.1	Joints, seams and connections sealed with mastic, mastic tape or a listed UL 181 tape	Cloth duct tape lets go within a few years and the system dumps conditioned air into the attic
IRC M1602.2	Return air is not taken from a garage, a bathroom, a toilet room or a closet	A return in the garage or the furnace closet pulls exhaust and flue gas into the living space
IRC N1103.3.1	Ducts in a vented attic insulated to R-8 at 3 in and larger, R-6 below that	Bare metal in a 130 F attic gives away a large share of the capacity that was paid for

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

Flex snaked between joists with slack delivers a third of its rating. Pull it tight and strap every 4 ft.
Sizing off measured length: elbows, boots and takeoffs often double the equivalent length, and the friction rate is per 100 ft of equivalent length.
One 20 x 25 filter grille pushes a 3 ton system's 1,200 CFM through 375 sq in of free area, and it roars.
Return pulled from the furnace closet or the garage fails IRC M1602.2 and can draw flue gas into the living space.
Cloth duct tape on joints dries out within a few years; IRC M1601.4.1 wants mastic, mastic tape or a listed UL 181 tape.
Bare metal trunk in a vented attic: IRC N1103.3.1 calls for R-8 on 3 in and larger, and an uninsulated run sweats in July.

Key facts

The residential design airflow is 400 CFM per ton of cooling, and ACCA Manual S accepts 350 to 450 CFM per ton depending on the climate and the coil.
Residential duct systems are designed at a friction rate of 0.06 to 0.10 inches of water column per 100 feet of equivalent length. The everyday values are 0.08 and 0.10.
Flexible duct compressed about 15 percent loses roughly half its airflow at the same pressure. The Air Duct Council standard requires it pulled tight and supported at least every 4 feet.
A 16 in round metal duct carries about 1,290 CFM at 0.08 in w.c. per 100 ft; the same size in flex carries about 1,040 CFM, so 1,200 CFM in flex needs 18 in.
IRC M1602.2 prohibits taking return air from a garage, a room containing a fuel-burning appliance, a bathroom, a toilet room or a closet.

Frequently asked questions

What size duct do I need for a 3 ton system?

A 3 ton system moves about 1,200 CFM. At a 0.08 friction rate that is a 16 in round metal trunk, 18 in if the trunk is flex, with 7 in branch runs for 150 CFM each. The return wants 18 in round and about 432 sq in of grille free area.

How many CFM per ton should I use?

400 CFM per ton is the standard. Humid climates run 350 CFM per ton so the coil pulls more moisture, and dry climates or a heat pump in heating run up to 450. ACCA Manual S accepts the whole 350 to 450 range.

Does flex duct have to be bigger than metal?

Usually one size. The wire helix roughens the wall, so flex carries roughly 1.5 times the friction of smooth metal at the same diameter. A 16 in metal trunk becomes 18 in in flex, and a 7 in metal branch becomes 8 in.

Why do my registers whistle?

Air moving faster than about 900 feet per minute in a branch makes noise at the register. Either the branch is a size too small for the CFM it carries, or the balancing dampers are shut and forcing air through the open runs. Check the velocity before blaming the register.

Sources and references

Manual D Residential Duct Systems, the ACCA duct design standard (ACCA)
Chapter 16 Duct Systems, Sections M1601 duct construction and M1602 return air, 2021 IRC (ICC)
Flexible Duct Performance and Installation Standards, 5th Edition (Air Duct Council)
QuietR Rotary Duct Liner and duct board product data (Owens Corning)

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