How to size ductwork

1. Why duct size matters

An HVAC system is only as good as its ducts. Undersized ducts restrict airflow, force the air handler to work harder, increase electricity consumption, and leave rooms uncomfortable. Oversized ducts move air too slowly, which reduces throw distance and can allow condensation in humid climates. Getting duct size right means the right amount of conditioned air reaches every room at the right pressure — quietly and efficiently.

Duct sizing in residential construction is governed by ACCA Manual D (North America) and equivalent national standards, which define two main methods:

• Velocity method: set a target air speed (FPM or m/s), divide the required airflow by that speed to get the cross-section area, then find the duct diameter. Fast to apply; good for residential branch runs.
• Equal-friction method: size every duct section so the friction loss per unit length is the same throughout the system (typically 0.08–0.10 in w.g./100 ft in North America). More accurate for longer or complex systems; requires a full duct-layout plan.

This guide — and the calculator — uses the velocity method, which is the standard starting point for residential systems.

2. The velocity method — step by step

Before you begin, you need two numbers: the airflow required for the duct section in L/s (from your Manual J load calculation or from your HVAC contractor), and the design velocity in m/s for that duct type.

Step 1 — Choose a design velocity

Pick the velocity from the table below based on the duct type and how much noise is acceptable. For a quiet bedroom, stay toward the lower end. For a utility area or mechanical room, the upper end is fine.

Planning guidance — not code minimums. Source: ACCA Manual D / ASHRAE residential duct design.

Step 2 — Calculate the required cross-section area

Divide the airflow (m³/s) by the velocity (m/s) to get the required area in m².

Step 3 — Find the round duct diameter

For a round duct, the diameter is derived from the area of a circle: A = π × r², so r = √(A ÷ π) and D = 2r.

Step 4 — Choose a standard size

Always round up to the next standard duct size — never down. Going smaller would exceed the design velocity and increase noise and pressure drop.

3. Rectangular duct sizing

If a round duct won't fit in the available space — say, a floor joist bay or a low ceiling cavity — use a rectangular duct with the same cross-section area. The area needed is exactly the same as for a round duct at the same airflow and velocity.

Fix one dimension (the height, limited by the available space) and solve for the other (the width):

Note that rectangular ducts have more surface area per unit of cross-section than round ducts, which means higher friction loss for the same airflow. A rectangular duct that is very wide and shallow (high aspect ratio) is less efficient than a square duct of the same area. Keep the aspect ratio below 4:1 where possible.

The duct size calculator shows rectangular equivalents for several common fixed widths automatically.

4. How much airflow does each room need?

Before you can size a duct, you need the airflow it must carry. The authoritative method is a Manual J load calculation— a room-by-room analysis of heat gain in summer and heat loss in winter based on the building's insulation, windows, orientation, occupancy, and local climate. A qualified HVAC contractor or engineer runs Manual J to determine the exact L/s for each supply register and return grille.

For planning purposes, a widely-used rule of thumb is 1 CFM per square foot of floor area for a standard 8-foot ceiling in a reasonably insulated home, or roughly 0.4–0.6 L/s per m² in metric countries. A bedroom of 150 sq ft (14 m²) would need about 100–150 CFM (47–70 L/s) on this basis. Well-insulated or small rooms may need less; rooms with large south-facing windows or poor insulation need more.

The 1-CFM-per-sq-ft rule is a starting point only. Use it to ballpark duct sizes during early planning, then have a licensed HVAC designer confirm the numbers before installation. Duct sizing is connected to system pressure, equipment selection, and the overall refrigerant circuit — a wrong duct size affects the whole system, not just one room.

For more on heating and cooling load sizing, see the BTU calculator →

5. Common duct sizing mistakes

• Sizing for velocity alone without checking friction loss. A long duct run at a reasonable velocity can still have too much total pressure drop if there are many bends, transitions, and fittings. For runs over 30 ft (9 m), check the total equivalent length and friction loss too.
• Using the rule of thumb for the wrong room type. A kitchen with high cooking loads, or a south-facing sunroom, needs significantly more cooling capacity and airflow than the 1 CFM/sq ft rule suggests. Use Manual J for rooms with unusual loads.
• Sizing round duct then fitting rectangular without recalculating. Round and rectangular ducts are interchangeable only if they have the same cross-section area. A 7-inch round duct has 38.5 in² — a 4 × 10 in rectangular duct has only 40 in² (adequate), but a 4 × 8 has 32 in² (too small).
• Installing flex duct at too high a velocity. Flexible duct has a corrugated liner with much higher friction than smooth metal duct. At velocities above 600 FPM it becomes noisy and develops significant pressure drop — especially if it is not pulled taut.
• Forgetting the return air path. Every supply duct needs a return air path of equal or greater capacity. A common error is to size supply ducts carefully but then rely on the gap under doors for return air — which fails when doors are closed. Size return ducts with the same method, using 600–800 FPM as the design velocity.

Common questions

How do you calculate duct size?

Divide the required airflow (CFM) by the design velocity (FPM) to get the minimum cross-section area in square feet. Multiply by 144 for square inches. For a round duct the diameter equals 2 × √(area ÷ π). For 150 CFM at 700 FPM: area = 150 ÷ 700 = 0.214 ft² = 30.9 in²; diameter = 2 × √(30.9 ÷ π) = 6.3 inches. Round up to the next standard size: 7-inch round duct.

What size duct do I need for a bedroom?

A 150–200 sq ft bedroom typically needs 100–150 CFM. At a branch-supply velocity of 700 FPM, 100 CFM requires a 5.1-inch round duct (use 6") and 150 CFM requires a 6.3-inch round duct (use 7"). For metric: 47 L/s at 3.6 m/s needs a 129 mm duct (use 150 mm). These are starting-point figures — the exact supply air quantity comes from a Manual J load calculation.

What velocity should HVAC ducts be sized at?

Typical design velocities are 700–900 FPM (3.6–4.6 m/s) for supply main trunk ducts, 500–700 FPM (2.5–3.6 m/s) for branch supply ducts, and 400–600 FPM (2.0–3.1 m/s) for return air ducts and flex duct. Higher velocities cause more noise and friction loss; lower velocities need larger duct sizes. These are planning guidelines from ACCA Manual D — not code minimums.

Is round or rectangular duct better for HVAC?

Round duct is more aerodynamically efficient — less friction loss per unit of airflow capacity — and easier to seal airtight. Rectangular duct fits into tighter spaces like joist bays and ceiling cavities where round duct won't fit. For the same airflow and velocity, both shapes need the same cross-section area. Where the geometry allows it, round duct is the better choice; use rectangular where the framing dictates.