Spiral Duct Sizing Chart

How to read this chart

Given an airflow requirement from the mechanical schedule, find the row with your CFM, then pick a column by your target velocity. The cell shows the duct diameter that delivers that velocity at that airflow. Typical design velocities for commercial HVAC are 1,000–1,500 FPM for trunks and 700–1,000 FPM for branches.

Diameter vs CFM vs Velocity — imperial

Airflow (CFM)	700 FPM	1,000 FPM	1,500 FPM	2,000 FPM	2,500 FPM
100	5" (125 mm)	4" (100 mm)	4" (100 mm)	3" (80 mm)	3" (80 mm)
200	7" (180 mm)	6" (150 mm)	5" (125 mm)	4" (100 mm)	4" (100 mm)
400	10" (250 mm)	8" (200 mm)	7" (180 mm)	6" (150 mm)	5" (125 mm)
600	12" (300 mm)	10" (250 mm)	8" (200 mm)	7" (180 mm)	6" (150 mm)
800	14" (355 mm)	12" (300 mm)	10" (250 mm)	8" (200 mm)	7" (180 mm)
1,000	16" (400 mm)	14" (355 mm)	11" (280 mm)	10" (250 mm)	8" (200 mm)
1,500	20" (500 mm)	17" (430 mm)	14" (355 mm)	12" (300 mm)	10" (250 mm)
2,000	23" (585 mm)	19" (480 mm)	16" (400 mm)	14" (355 mm)	12" (300 mm)
3,000	28" (710 mm)	24" (600 mm)	19" (480 mm)	17" (430 mm)	14" (355 mm)
5,000	36" (915 mm)	30" (760 mm)	25" (635 mm)	21" (535 mm)	19" (480 mm)
8,000	46" (1170 mm)	38" (965 mm)	31" (790 mm)	27" (685 mm)	24" (610 mm)
12,000	56" (1420 mm)	47" (1200 mm)	38" (965 mm)	33" (840 mm)	30" (760 mm)
20,000	72" (1830 mm)	61" (1550 mm)	50" (1270 mm)	43" (1090 mm)	38" (965 mm)

Round diameters to the next manufactured size above the calculated value. SBKJ SBTF-1602 produces standard round diameters Φ80 to Φ1600 mm; SBTF-2020 extends up to Φ2000 mm.

The three rules of thumb that actually matter

1. Velocity first, friction second. Above 2,500 FPM in commercial settings, noise becomes the complaint even when friction loss is fine. Size for velocity, check friction, not the other way around.
2. Round duct is smaller than you think. A 14" round duct handles the same airflow as a 20" × 10" rectangular with less friction and less metal. If headroom allows, round wins.
3. Upsize before downsize. When in doubt between two sizes, go up. An oversized duct runs quieter and has lower pressure drop; an undersized duct is a permanent complaint you cannot fix without replacing the run.

Friction loss — rule-of-thumb numbers

For round galvanized spiral duct at typical commercial velocities:

• 700 FPM — roughly 0.04–0.06 in. w.g. per 100 ft
• 1,000 FPM — roughly 0.08–0.10 in. w.g. per 100 ft
• 1,500 FPM — roughly 0.15–0.20 in. w.g. per 100 ft
• 2,000 FPM — roughly 0.25–0.32 in. w.g. per 100 ft
• 2,500 FPM — roughly 0.40–0.50 in. w.g. per 100 ft

Actual friction depends on diameter, roughness and fittings. Use a duct sizing calculator or ASHRAE 2021 HVAC Fundamentals Chapter 21 for final design.

Metric equivalents

• 100 CFM ≈ 170 m³/h
• 1,000 CFM ≈ 1,700 m³/h
• 700 FPM ≈ 3.5 m/s
• 1,000 FPM ≈ 5.1 m/s
• 1,500 FPM ≈ 7.6 m/s
• 2,000 FPM ≈ 10.2 m/s
• 2,500 FPM ≈ 12.7 m/s

Pressure drop across fittings (equivalent length rule of thumb)

• 90° smooth radius elbow — equivalent to 10 × diameter of straight duct
• 90° gored elbow (5-piece) — equivalent to 15 × diameter
• 45° elbow — equivalent to 5 × diameter
• Tee branch (branch flow) — equivalent to 30–60 × diameter depending on geometry
• Reducer (gradual) — equivalent to 5 × diameter

Add equivalent lengths to actual straight run length before computing total friction loss.

Why spiral over rectangular?

For the same airflow, a round duct has:

• 15–25% less sheet metal — lower friction perimeter, less surface area
• 30–40% lower friction loss — smoother flow path, less turbulence at corners
• Lower leakage — one continuous helical seam vs four longitudinal Pittsburgh locks + four corners per joint
• Better acoustic performance — round geometry damps low-frequency roar
• Stronger under pressure — round cross-section resists deformation without internal bracing

See our spiral forming guide for how round duct is actually manufactured.

Ask SBKJ about an SBTF spiral tubeformer →

FAQ

What is the recommended velocity for round HVAC duct?

For low-pressure commercial HVAC, 1,000 to 1,500 FPM (5 to 7.5 m/s) is standard for supply trunks. Branches run 700 to 1,000 FPM. For high-pressure variable air volume systems, trunk velocities up to 2,500 FPM (12.5 m/s) are acceptable.

How do I size a round duct for a given CFM?

Divide CFM by your target velocity in FPM to get the cross-section area in square feet. Convert to diameter with D = 2 × √(Area / π). Alternatively, use the sizing chart above.

What is friction loss for round spiral duct?

Friction loss for round galvanized spiral duct is approximately 0.08 to 0.15 in. w.g. per 100 feet at typical commercial velocities (1,000 to 1,500 FPM). Spiral has slightly higher friction than smooth rigid round, but lower than equivalent rectangular duct.

Why is round duct preferred over rectangular for the same airflow?

Less metal per square meter of airflow, lower friction loss, lower leakage, stronger under pressure, and better acoustics. The downside is clearance height in drop ceilings, where rectangular still wins.