Duct Size Calculator
A duct that delivers the airflow can still be the thing everyone complains about. Air noise is a velocity problem, and velocity is exactly what duct calculators leave out — they give you a diameter and stop. This one reports the velocity your chosen size actually produces, compares it against the accepted ceiling for that duct type, and offers the next size up when it is over.
- Round duct
- 12" diameter 11.1" required, rounded up to the next standard size
- Velocity at this size
- 509 FPM Comfortable for a supply branch — target 600, ceiling 700 FPM
- Rectangular equivalent
- 13" × 8" Equivalent diameter 11.1" by the Huebscher formula, 554 FPM
- Required area
- 96.0 in² 400 CFM ÷ 600 FPM target velocity
- Rule of thumb check
- 1.00 tons Residential systems move roughly 400 CFM per ton of cooling. If this is far from your equipment size, check the airflow figure.
Velocity is what you hear
Duct noise is not caused by the blower. It is caused by air moving fast enough to become turbulent at fittings, dampers and register faces, and turbulence rises sharply with velocity rather than gradually.
The accepted residential ceilings reflect where that becomes audible. A supply trunk stays quiet up to roughly 900 feet per minute and a branch up to 700. Push a branch to 1,200 and you get the rush of air that people describe as the system being "loud" — but the blower is unchanged, and replacing it fixes nothing.
Returns are quieter than supplies at the same velocity because the noise is generated upstream of the occupied space rather than blown into it, which is why the return targets here are lower for a different reason: return runs are usually longer and more restrictive, and velocity there costs static pressure rather than comfort.
Registers matter as much as duct. A correctly sized branch feeding an undersized register will still whistle, because the velocity through the face is what the occupant hears.
Velocity sizing versus equal friction
There are two common ways to size a duct system and they answer different questions.
Velocity sizing, used here, picks a target speed and derives the area from the airflow. It is exact arithmetic, needs no chart, and is the standard first pass. Its weakness is that it says nothing about how much pressure the system will lose over its length.
The equal friction method picks a target pressure drop per hundred feet — typically 0.08 to 0.10 inches of water column in residential work — and sizes every duct to lose the same amount. It naturally balances a system so that distant branches are not starved by nearer ones, but it requires a friction chart or a duct calculator wheel.
ACCA Manual D combines both, plus fitting losses, to produce a design where the total external static pressure lands within what the blower can deliver. Velocity sizing on its own can produce a system where every duct is individually reasonable and the total static is far too high.
Round versus rectangular
Round duct is more efficient than rectangular of the same cross-sectional area, and the difference is larger than people expect.
Friction depends on the ratio of wall surface to flow area. A circle has the least perimeter for a given area of any shape, so a round duct has less surface for the air to rub against. A rectangular duct with the same area always has more perimeter, and a flat one — say 3 inches by 20 — has dramatically more.
That is why the Huebscher equivalent diameter formula used here does not simply match areas. It gives the diameter of a round duct with the same friction loss, and the rectangular equivalent always works out slightly larger in area than the round option it replaces.
The practical rule is to keep the aspect ratio below about 4 to 1. Beyond that the friction penalty grows quickly, and a duct squeezed into a joist bay at 2 by 24 inches performs far worse than its area suggests.
What this is based on
- Continuity: duct area = airflow ÷ velocity
- Huebscher equivalent diameter formula for rectangular to round conversion
- Accepted residential velocity ranges for supply and return, trunk and branch
- ACCA Manual D — the recognised residential duct design procedure
A first-pass estimate by velocity. It does not calculate total static pressure, fitting losses or flexible duct penalties, and is not a substitute for an ACCA Manual D design on a new system.
Frequently asked questions
Why does duct velocity matter more than diameter?
Because velocity is what you hear. Air moving faster than roughly 900 FPM in a supply trunk or 700 in a branch produces audible rush and whistle at registers. Undersizing a duct by one step often doubles the complaint rate without changing measured airflow much.
Is this the same as the equal friction method?
No. This sizes by target velocity, which is exact arithmetic — area equals airflow divided by velocity. The equal friction method sizes from a pressure drop per 100 feet and needs a friction chart. Velocity sizing is the standard first pass and is what most rules of thumb are built on.
How do I convert round to rectangular?
Not by matching area — a rectangular duct has more wall surface and therefore more friction for the same area. The Huebscher equivalent diameter formula used here accounts for that, which is why the rectangular option is slightly larger in area than the round one.
What airflow should I use?
For a whole system, roughly 400 CFM per ton of cooling. For a single room, the load in BTU per hour divided by about 30 gives a workable starting figure. A room-by-room Manual D calculation is the proper method for a full design.
Does duct length affect the size?
It affects the total static pressure the blower must overcome, not the velocity in the duct. Long runs, many fittings and flexible duct all add resistance, and a system with a lot of them may need larger ducts than velocity sizing alone suggests.