Duct Size Calculator: Trunk and Branch Sizes from Your AC or Furnace

Already picked a 3-ton AC or an 80,000 BTU furnace?

Enter it and get the round and rectangular duct size for the trunk and each branch, with velocity and noise flags.

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Results will appear here once you enter your values.

How the equal-friction method sizes a duct system

Once you have chosen an air conditioner or furnace, the ductwork has to carry exactly the airflow the blower is built to move. The equal-friction method, the standard way to do this in homes, picks one friction rate and sizes every duct section to lose the same pressure per 100 feet. Each section is then sized from only two numbers: the CFM it carries and the friction rate.

The tool follows these steps:

  1. System airflow. Cooling: tons x CFM per ton (400 by default). Heating: output BTU/h / (1.08 x temperature rise). The 1.08 is the sensible heat constant for standard air: 60 minutes x 0.075 lb/cu ft x 0.24 BTU/lb-°F.
  2. Trunk diameter. From the ASHRAE round-duct friction equation, loss = 0.109136 x CFM^1.9 / D^5.02 in. w.g. per 100 ft, solved for D. For 1,200 CFM at 0.08 that gives about 15.6 in, so a 16 in stock duct.
  3. Branch diameters. Each branch uses the same equation with its own CFM share. 200 CFM at 0.08 needs about 7.9 in, so an 8 in branch.
  4. Rectangular equivalent. The Huebscher equation converts the round size to a rectangle at the height or aspect ratio you choose.
  5. Velocity check. Velocity = CFM / duct area, compared with quiet and noisy limits for trunks and branches.

The exponents in the friction equation matter: loss climbs with CFM to the 1.9 power but falls with diameter to the 5.02 power. Going from a 14 in to a 16 in trunk cuts friction by about 49%, which is why a modest bump in duct size is the cheapest way to quiet a system.

Reading your results: velocity, noise and what to change

The results table lists each section with its exact round size, the stock size to buy, a rectangular alternative, the air velocity in feet per minute and a noise rating. Velocity is what you hear: air noise rises quickly with speed, so a duct a size too small is the most common cause of a whistling register.

  • Quiet: trunk up to 700 fpm, branch up to 600 fpm. Air noise is unlikely to be noticeable.
  • Acceptable: up to 900 fpm for trunks and branches, the ACCA Manual D maximum for rigid duct. Fine in a basement or attic; consider larger ducts above bedrooms. Flex duct tops out at 700 fpm.
  • Too noisy: anything above 900 fpm. Fix it by lowering the friction rate, adding a branch so each carries less air or switching from a flat rectangle to a rounder shape.

The default 3-ton, six-branch layout lands at roughly 860 fpm in the 16 in trunk and 570 fpm in each 8 in branch, so the branches are quiet and the trunk is acceptable. Change the number of branches to 3 and each branch carries 400 CFM; the exact branch size becomes 10.3 in, so the tool picks a 12 in stock duct running at about 510 fpm. Fewer, bigger branches need bigger ducts, but each one stays quiet.

The pressure loss column shows the friction at the stock size. It is always at or below your chosen friction rate, so you can see how much margin the rounding gives you.

What this friction-only sizing leaves out

Duct friction is only part of the total pressure the blower must overcome. A system whose straight runs are sized perfectly can still be starved of air if the rest of the pressure budget is ignored.

  • Fittings. Elbows, tees, boots and transitions each add an equivalent length of straight duct. A tight 90° elbow can add as much loss as 20 to 40 feet of straight duct, so a long run with many turns needs a lower friction rate than 0.08.
  • Filters, coils and grilles. A dirty filter or a high-efficiency pleated filter can use 0.1 to 0.3 in. w.g. of the blower external static pressure before air reaches the first duct.
  • Blower capability. Residential blowers are usually rated at 0.5 in. w.g. total external static. If the filter, coil and grilles use 0.3, only 0.2 is left for ducts. Spread over a 250 ft total effective length (straight duct plus fitting equivalents), that is 0.2 / 2.5 = 0.08 per 100 ft. Check the blower fan table before you commit to a friction rate.
  • Return air. Returns need sizing too. Undersized returns are a leading cause of low airflow and frozen coils. ACCA Manual D recommends 600 fpm or less in return trunks and 400 fpm or less in return branches.
  • Flex duct. Flexible duct has higher friction than the smooth metal the equation is fitted to, and more again if it is compressed or sagging. Size flex one size up, or keep it short and fully stretched.

Treat the tool as the sizing step. Use ACCA Manual D when you need the full pressure budget, and have the system balanced after installation.

Where ductwork sizing fits in your HVAC project

Duct sizing comes after the equipment is chosen, so this tool picks up where the other HVAC calculators on Energy Toolset stop:

  1. Work out the load. The heating load calculator estimates how much heat the house loses on a design day.
  2. Pick the equipment. The furnace size calculator and AC BTU calculator give a capacity. Compare efficiency choices with the SEER savings calculator.
  3. Find the airflow. The CFM calculator turns a load or room into CFM. Enter that figure in the Total CFM tab here.
  4. Size the ducts. This tool splits the airflow across a trunk and branches and checks velocity.

Common planning cases: replacing a 2.5-ton AC with a 3-ton unit (check whether the existing 14 in trunk still passes: 1,200 CFM through 14 in is about 1,120 fpm, which is well above the 900 fpm trunk limit), adding a branch to feed a new room, and sanity-checking a contractor quote that calls for a very small trunk.

Frequently Asked Questions

Common questions about the Duct Size Calculator: Trunk and Branch Sizes from Your AC or Furnace

About 1,200 CFM. The usual design value is 400 CFM per ton of cooling, so 3 tons x 400 = 1,200 CFM. Installers use 350 CFM per ton in humid climates, where a colder coil removes more moisture, and up to 450 in dry climates. The calculator lets you change the airflow per ton between 250 and 600 CFM.

Sources & References

ASHRAE Handbook - Fundamentals, Duct Design chapter

Source of the round-duct friction equation and the equivalent-diameter method.
- Friction loss: 0.109136 x CFM^1.9 / D^5.02 in. w.g. per 100 ft.
- Rectangular-to-round equivalent diameter (Huebscher).
→Source: ASHRAE(2021)

ACCA Manual D: Residential Duct Systems

The industry procedure for residential duct design, including friction rate selection from available static pressure and total effective length.
→Source: Air Conditioning Contractors of America(2016)

ACCA Manual S: Residential Equipment Selection

Basis for the typical 350 to 450 CFM per ton cooling airflow range.
→Source: Air Conditioning Contractors of America(2014)

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