Stock spiral duct for HVAC supply runs spans roughly 80 mm to 900 mm (3 in to 36 in nominal) in outside diameter across the major UK and US vendor catalogs, with the 80–710 mm band being the most commonly stocked range for residential and light commercial work [S1][S4]. Wall thickness on galvanized spiral product runs 0.6 mm to 1.6 mm as a standard mill range, stepping up from light residential gauge at small diameters to heavier commercial gauge past 24 in [S7].
Larger custom runs reach 1524 mm (60 in nominal) outside diameter for industrial HVAC and process exhaust, and equivalent-diameter spiral product is used to size rectangular equivalents for air flows between 100 and 50,000 cfm [S2][S7]. The spiral lock-seam format is the dominant galvanized round product because it ships in long lengths, cuts cleanly on site, and mates to standard fittings and couplings without flanges in low-pressure systems.
Diameter Windows by Application Class
The 80–710 mm band covers essentially all residential supply trunk and branch runs, with 80 mm (3 in) typically feeding a single register boot, 150–200 mm (6–8 in) feeding a branch with 2–3 outlets, and 250–400 mm (10–16 in) running as a main trunk in a 1,200–2,000 cfm residential air handler [S1][S4][S5]. Above 710 mm the product shifts to commercial and light-industrial use: 500–900 mm (20–36 in) is the workhorse range for office building mains, classroom ventilation, and retail RTU supply risers, and 900–1524 mm (36–60 in) is typically only seen in central plant headers, large AHU discharge, or industrial ventilation where the duct doubles as a structural plenum [S2][S7].
Equivalent rectangular sizing for the same cfm class shows the round-vs-rectangular trade-off clearly. A 1,000 cfm residential main at 0.10 in.w.c. per 100 ft friction rate sizes to a 14 in round, or to 18 x 8 in rectangular at the same friction rate, or to 12 x 12 in if the designer is willing to spend more on sheet metal to gain plenum height [S5]. The round equivalent is usually 15–25% smaller in cross-section than the rectangular for the same pressure drop, which is the underlying reason designers reach for spiral on retrofits where the ceiling cavity is tight.
Wall Gauge, Pressure Class, and Material Options
Standard galvanized spiral duct follows a stepped gauge schedule tied to diameter, not to a single project pressure class. Small diameters (3 in to 18 in nominal) typically run 0.0276 in (0.7 mm) wall; 20 in to 36 in step up to 0.0336 in (0.85 mm); 38 in to 48 in to 0.0396 in (1.0 mm); and 50 in to 72 in to 0.0516 in (1.3 mm) [S2]. Australian and European stock follows the same logic expressed in metric: 76–1524 mm OD with 0.6–1.6 mm wall, where 0.6 mm is the minimum for low-pressure galvanized and 1.6 mm is specified for commercial mains and any duct exposed to positive pressure above the default 0.5 in.w.c. class [S7].
Stainless steel (typically 304 or 316) and aluminum spiral are both available as specials in the same diameter range, mainly for cleanrooms, kitchen exhaust make-up air, coastal installations, and pharmaceutical or food-grade supply where galvanized is not acceptable. Fittings inventory matters as much as the duct itself: standard 90° elbows, 45° angles, register boots, square-to-round transitions, and round-to-round adjustable couplings are all kept in stock to match the same diameter grid, and an entire spiral system fails economically if even one size falls outside the standard coupling range [S6].
Sizing Inputs: CFM, Velocity, Friction Rate
Three numbers drive every spiral duct diameter decision: required CFM per run, target velocity, and target friction rate. The industry-standard residential baseline is 0.1 in.w.c. per 100 ft of equivalent length, with 0.08–0.10 used for quiet supply mains and 0.10–0.12 tolerated on return drops [S3][S5]. Velocity targets split by duct role: residential supply at 600–900 FPM, residential return at 500–700 FPM, and commercial main runs often pushed to 1,200–1,800 FPM where acoustic treatment is built into the duct boot and diffuser selection rather than into the trunk [S3].
The 20–30% energy-cost penalty cited for mis-sized ductwork comes from two compounding effects: undersized trunks push the fan into a higher static pressure curve, and undersized returns starve the air handler until the coil approaches freeze or the heat exchanger overheats [S3]. A single sharp 90° elbow adds 15–30 ft of equivalent length to a run, and a square-throat 28–33 in elbow can add 125 ft, so the spiral diameter must be picked after all fittings are tabulated, not before [S3][S5]. Flexible duct, by contrast, runs corrugated and high-friction, so the standard practice is to upsize the flexible run by 1–2 in over the rigid equivalent and to keep flexible runs under about 14 ft to avoid static regain problems at the diffuser boot [S3].
Comparison of Stock Diameter Bands by Project Type
Picking the correct spiral diameter depends on which pressure and velocity window the project sits in. The table below lines the three common stock windows up against the criteria that actually drive the spec: friction rate feasibility, velocity headroom, and what a standard coupling will accept without a custom fab order. [S3]
Residential and light commercial (80–710 mm, 3–28 in): friction rate 0.08–0.10 in.w.c. per 100 ft is easily met at 600–900 FPM supply velocity, gauge is light (0.6–0.85 mm), and every size mates to off-the-shelf elbows, tees, and register boots from any major HVAC supplier. This is the default band for split-system trunks, townhouse ventilation, and small retail RTU supply [S1][S4][S5].
Commercial mains (500–900 mm, 20–36 in): friction rate is still in the 0.10–0.12 in.w.c. range but velocity creeps up to 1,000–1,400 FPM on long runs, wall gauge steps to 1.0–1.3 mm, and the specifier should confirm coupling and flange compatibility with the AHU discharge before ordering. Standard fittings exist for the full range but lead time grows past 36 in [S2][S5][S7].
Industrial and central plant (900–1524 mm, 36–60 in): friction rate is usually set by the AHU curve rather than by a residential benchmark, velocity is often capped by NC rather than by pressure drop, gauge is 1.3–1.6 mm, and most fittings are made-to-order with 2–4 week lead time. Stainless 304/316 is routine in this band, where ductwork also has to handle wash-down, coastal chloride, or high-humidity cleanroom supply [S2][S7].
Common Sizing Failures and Field Fixes
The single most expensive field error is undersized return duct: the air handler pulls negative on the return plenum, the coil runs cold enough to freeze on a low ambient night, and the heat exchanger overheats on every heating call. Sizing returns at the same friction rate as supply (rather than upsizing them) is the wrong reflex; returns are typically sized 1 size larger than supply in the same CFM class [S3].
The second failure mode is treating the spiral main as if it were flexible. Flexible duct needs the 1–2 in upsize and short run length; spiral rigid duct does not, and applying the flexible rule to a rigid spiral main simply inflates sheet metal cost and ceiling cavity size for no gain [S3].
The third is forgetting equivalent length in the takeoff count. A spiral run with four 90° elbows, one square-to-round transition, and one register boot can carry 100–200 ft of equivalent length before the static regain actually matters, and the diameter must be picked off the total effective length, not the geometric run [S3][S5]. A 4 in round duct at 100 cfm at the residential friction rate will carry the air, but the same 4 in duct on a 200 ft effective length run will not, and the fix is to step to 6 or 7 in, not to chase a higher blower speed [S5].
Standards, Sourcing, and Specification Anchors
ACCA Manual D remains the residential duct sizing reference that friction-rate and velocity-limit tables are tied back to, and ACCA Manual J feeds it the load calculation upstream. ASHRAE fundamentals and the ASHRAE Handbook (HVAC Applications) carry the equivalent commercial and industrial methodology, including the static-recovery and NC-driven velocity caps that govern mains above 900 mm [S3][S5][S8]. For spiral duct specifically, SMACNA HVAC Duct Construction Standards sets the gauge and reinforcement tables for galvanized round, and most domestic spiral shops publish their own diameter-vs-gauge schedule against that reference [S2][S7].
Sourcing spiral duct in 2026 is straightforward inside the 80–900 mm window: domestic HVAC wholesalers and steel-service centers in North America, the UK, and Australia all stock the standard diameters in 3 m or 12 ft lengths, with couplings and standard fittings off the shelf. Above 900 mm, lead time is the gating constraint, and the specifier should lock the diameter, gauge, and material (galvanized vs stainless 304 vs stainless 316 vs aluminum) before the mechanical room layout is frozen, because a late change to a 1,200 mm spiral main is a 4-week fab delay in most regional markets [S6][S7]. The coupling and flange class should be specified in the same line as the duct diameter, since mismatched coupling classes are the single most common field-fabrication cost on spiral installs.
Two trackable signals to watch on the next spec cycle: 1) SMACNA and ASHRAE continue to push velocity caps downward in the 1,200–1,800 FPM band for acoustic reasons, which will tend to push commercial mains toward the larger end of the 500–900 mm range rather than allowing the historical 24 in mains to absorb more cfm; 2) stainless 316 spiral is moving from a cleanroom-only product into a coastal-HVAC default in the same 80–710 mm range, and stocked size availability, not lead time, will be the real procurement constraint through 2026 [S3][S7].
For the relevant spec sheets and selection criteria, see spiral duct, marine hvac, and wiring duct.
For related coverage, see Single-Acting vs Double-Acting Plunger Pump: Selection Matrix.