Duct Calculator

This duct calculator sizes an air duct from the airflow and the air speed you want. The area the duct needs is CFM ÷ FPM (× 144 for square inches); it then gives the round size, the next standard size, and a rectangular duct with the same friction. It is an estimate for one duct, not a Manual D design.

Use the calculator HVAC invoice template

Reviewed October 2026 · by the BrickByClick team

At a glance

What it does: round and rectangular duct sizes from CFM and FPM, the velocity in each, and the area needed.
Velocities: presets from ACCA Manual D, Table 3-1, for supply and return trunks and branches.
What it is not: a full duct design. It does not include friction rate, fittings, duct length or blower static pressure.

Air duct size calculator

Enter the airflow the duct must carry and the air speed you want. Pick a duct type to load the ACCA Manual D velocity for it, or type your own.

An estimate, not a duct designThis sizes one duct by velocity. A proper residential design follows ACCA Manual D: it starts from a Manual J room-by-room load, uses the blower’s available static pressure, and accounts for duct length, fittings, filters and registers. Use this to sanity-check a size or plan a quote, and have the system designed before you build it.

How to calculate duct size from CFM

Airflow is speed times area. Rearranged, the area a duct needs is the airflow divided by the speed you want:

area (sq ft) = CFM ÷ FPM area (sq in) = CFM ÷ FPM × 144
round diameter (in) = √(4 × area in sq in ÷ π)
velocity in a chosen duct (FPM) = CFM ÷ (duct area in sq in ÷ 144)

Worked example

A supply trunk carrying 400 CFM at the recommended 700 FPM:

Area: 400 ÷ 700 = 0.571 sq ft, × 144 = 82.3 sq in.
Round diameter: √(4 × 82.3 ÷ 3.1416) = 10.24 in.
A 10 in duct (78.5 sq in) would run at 400 ÷ 0.545 = 733 FPM, over the target, so the calculator picks the next standard size up, 12 in (113.1 sq in), which runs at 509 FPM.
With an 8 in height limit, a rectangle with the same friction as the 10.24 in round is 8 × 12 in (equivalent round 10.7 in), running at 600 FPM.

Those are the numbers the calculator opens with.

Round to rectangular duct conversion

A rectangle with the same area as a round duct carries the same air at the same speed, but it has more wall for the air to rub on, so it loses more pressure. To match the round duct’s friction, size the rectangle by its equivalent round diameter, the Huebscher formula published by ASHRAE:

equivalent round diameter = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25

where a and b are the two sides in inches. The calculator keeps your height and widens the rectangle an inch at a time until its equivalent diameter reaches the round size the airflow needs. That is why the rectangle it gives is bigger than the same-area rectangle. Flat, wide ducts need the biggest increase.

Recommended duct velocity (FPM)

These are the residential velocities in ACCA Manual D, Table 3-1, for rigid duct. Faster air means smaller ducts but more noise and more pressure for the blower to overcome.

Residential velocities, rigid duct (ACCA Manual D, Table 3-1)
Where	Recommended	Maximum
Supply trunk	700 FPM	900 FPM
Supply branch	600 FPM	900 FPM
Return trunk	600 FPM	700 FPM
Return branch	400 FPM	700 FPM
Return grille face	–	500 FPM
Filter grille face	–	300 FPM

Manual D also lists lower limits for flexible duct. Check the current edition of Manual D for the full table.

Duct size chart: CFM by round duct size

How much air each round size carries at three common velocities (CFM = area in sq ft × FPM). This is velocity only; friction and fittings are not included.

CFM = (π × d² ÷ 4 ÷ 144) × FPM
Diameter	Area (sq in)	At 600 FPM	At 700 FPM	At 900 FPM
4 in	12.6	52	61	79
5 in	19.6	82	95	123
6 in	28.3	118	137	177
7 in	38.5	160	187	241
8 in	50.3	209	244	314
9 in	63.6	265	309	398
10 in	78.5	327	382	491
12 in	113.1	471	550	707
14 in	153.9	641	748	962
16 in	201.1	838	977	1,257
18 in	254.5	1,060	1,237	1,590
20 in	314.2	1,309	1,527	1,963

From duct size to an HVAC quote

Once the sizes are set, the quote is materials, labour and margin. The margin calculator turns cost into a price, and the HVAC invoice template bills the job when it is done. To email invoices with a card payment link, BrickByClick Free is $0 a month; Starter, $99 a month, adds estimates, jobs, a calendar and the crew app.

Sources

Velocities: ACCA Manual D, Table 3-1, recommended and maximum velocities (as reproduced in an ACCA document hosted by AHRI). Check the current edition of Manual D for the full table.
Equivalent round diameter of a rectangular duct (Huebscher): The Engineering ToolBox, Fluid Flow – Equivalent Diameter; the same equation is in the ASHRAE Handbook – Fundamentals, Duct Design chapter.
Airflow = velocity × area, with 144 sq in to the square foot (12 in × 12 in).

Questions people ask

How do I calculate duct size from CFM?

Divide the CFM by the velocity in feet per minute to get the area in square feet, multiply by 144 for square inches, then find the diameter: √(4 × area ÷ π). 400 CFM at 700 FPM needs 82.3 sq in, a 10.24 in round, so use a 12 in duct.

What size duct do I need for 400 CFM?

At 700 FPM, the Manual D recommended speed for a supply trunk, 400 CFM needs 82.3 sq in: a 12 in round (a 10 in round would run at about 733 FPM) or about 8 × 12 in rectangular. At 600 FPM it needs 96 sq in, still a 12 in round.

What is the recommended air velocity in residential ducts?

ACCA Manual D, Table 3-1, recommends about 700 FPM for rigid supply trunks and 600 FPM for supply branches, with maximums of 900 FPM. For returns it recommends 600 FPM in trunks and 400 FPM in branches, with a maximum of 700 FPM.

How do I convert a round duct to rectangular?

Use the equivalent round diameter: 1.30 × (a × b)^0.625 ÷ (a + b)^0.25. Choose the rectangle whose equivalent diameter matches the round duct. A rectangle with the same area as the round duct has more friction, so it needs to be a little bigger.

What is CFM and FPM in ductwork?

CFM is cubic feet per minute, the volume of air moving through the duct. FPM is feet per minute, how fast the air is moving. CFM equals FPM times the duct’s cross-section area in square feet.

Is this the same as a Manual D calculation?

No. Manual D sizes the whole system from the room loads, the blower’s available static pressure, and the friction of every duct and fitting. This calculator sizes one duct by velocity, which is useful as a check or a first estimate.

What happens if a duct is too small?

The air moves faster, which makes noise at registers and raises the pressure the blower must push against. The system can then deliver less air than it should, which hurts comfort and efficiency.

How much air does a 6 inch duct carry?

A 6 in round has 28.3 sq in of area, so it carries about 118 CFM at 600 FPM and about 137 CFM at 700 FPM. Long runs, flex duct and elbows reduce what it really delivers.

Why are standard round duct sizes 10, 12, 14 inches?

Above 10 in, round duct and fittings are usually stocked in 2 in steps. The calculator picks the smallest standard size that keeps the speed at or below your target, and shows the next size down so you can see its speed.

How do I price ductwork for a customer?

Add the duct, fittings and registers to labour and your margin. The margin calculator turns cost into a price, and BrickByClick Starter sends estimates customers can accept online.

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