Duct Size Calculator: Round Duct Diameter from CFM and Velocity

How duct sizing by velocity works

Air moving through a duct obeys a simple relationship: flow equals area times speed. If you know how much air a room needs (see the CFM calculator) and how fast you are willing to let it travel, the duct area falls straight out. Velocity limits exist for two reasons: faster air is noisier at the register, and friction loss climbs steeply with speed, which the blower has to overcome.

Area (ft²) = CFM ÷ Velocity (FPM)
Area (in²) = Area (ft²) × 144
Diameter (in) = 2 × √(Area in² ÷ π)
Friction, metal ≈ 0.109 × CFM^1.9 ÷ D^5.02 (in. w.c. per 100 ft, D = chosen standard size)
Friction, chosen material = Friction, metal × Material factor
Pressure drop (in. w.c.) = Friction × Length (ft) ÷ 100

The friction expression is a widely used curve fit to the standard friction chart for clean, round galvanized duct, evaluated at the standard diameter you will actually install. The material factor scales it for rougher surfaces:

These factors are typical planning values; the true penalty for flex depends heavily on how much it is compressed and how it is supported. The pressure drop covers the straight length only. Elbows, boots and takeoffs add equivalent length, often more than the straight duct itself, so a full Manual D design adds those before dividing the available static pressure.

Worked example

A branch needs 400 CFM and you want a standard 800 FPM velocity, using smooth galvanized metal over a 50 ft straight run.

1. Area = 400 ÷ 800 = 0.50 ft², or 72 in².
2. Diameter = 2 × √(72 ÷ 3.1416) = 2 × 4.79 = 9.57 in.
3. Round up to the next standard size: 10 in.
4. Actual velocity in a 10-inch duct (78.5 in², 0.545 ft²): 400 ÷ 0.545 = 733 FPM.
5. Friction rate: 0.109 × 400^1.9 ÷ 10^5.02 ≈ 0.092 in. w.c. per 100 ft, within the usual residential design range. The material factor for metal is 1.0, so it stays 0.092.
6. Pressure drop over 50 ft: 0.092 × 50 ÷ 100 = 0.046 in. w.c.

Those are the results the calculator gives with its default inputs. Switch the material to flex duct with typical sag (× 2.5) and the same 10-inch run loses about 0.229 in. w.c. per 100 ft, or 0.115 in. w.c. over 50 ft, two and a half times as much. Notice that rounding up always lowers velocity and friction, which is why designers round up rather than to the nearest size.

Typical CFM by round duct size

The table gives two capacities for each common size: the airflow at a friction rate of 0.08 in. w.c. per 100 ft (a typical Manual D design value for metal duct) and the airflow at 700 FPM. Use the smaller of the two for a quiet, conservative branch.

Small ducts are limited by friction; large trunks are limited by velocity. For flex duct, step up one size from what the metal column suggests.

Recommended velocities

These are typical residential figures; ACCA Manual D sets maximum velocities and works from the blower's available static pressure, so the final design can differ. Return ducts are run slower because return grilles are often in living spaces where noise is noticed. Undersized returns are one of the most common faults in existing homes; see cold air return vents.

Velocity method vs friction method

The velocity method used here is quick and keeps noise in check, which makes it ideal for checking an individual run or picking a register duct. A full Manual D design uses the friction rate method: it starts with the blower's available static pressure, subtracts the losses of the coil, filter and grilles, divides what is left by the total effective length of the longest run, and sizes every duct to that friction rate. The friction output in the calculator lets you see whether a velocity-based size also lands in a sensible friction range (about 0.05–0.10 in. w.c. per 100 ft for most homes).

Rectangular trunks can be converted to an equivalent round diameter; a rectangle with the same area does not carry the same air, because its corners add friction. Long, flat ducts are the worst.

Common duct sizing mistakes

• Sizing to the collar of the register. Register boots come in fixed sizes, but the duct feeding them should be sized to the airflow.
• Kinked or sagging flex duct. A flex run with sharp bends can carry half its rated airflow.
• Ignoring the return. The supply side gets attention, while a single small return grille throttles the whole system.
• Oversized equipment, undersized ducts. A larger AC on the same ducts raises velocity and noise and often gives short cycling. Size the system with the AC tonnage calculator first.

If some rooms are always warm and others cold, measure supply temperatures and look at the duct runs before adding capacity. Balancing dampers and an extra branch usually solve more than a bigger unit.

Frequently asked questions