Standard HVAC Duct Dimensions: Quick Reference Chart for Ordering
Calculators give exact answers — 13.5 inches — and suppliers sell stock — 14 inches. This quick reference covers every stock round duct size, the sheet metal gauge that goes with it, rectangular stock widths and heights, and the rounding rules that keep velocity from spiking when you translate theory into an order.
Last updated September 2026
Advertisement
Stock round duct sizes (4 to 60 inches)
North American round duct is manufactured in a fixed ladder of diameters: 1-inch steps through 12 inches, 2-inch steps through 24 inches, then progressively larger jumps. These are the sizes carried as stock by most sheet metal shops:
| Round duct sizes (inches) | ||||||
|---|---|---|---|---|---|---|
| 4" | 5" | 6" | 7" | 8" | 9" | 10" |
| 11" | 12" | 14" | 16" | 18" | 20" | 22" |
| 24" | 26" | 28" | 30" | 32" | 34" | 36" |
| 38" | 40" | 42" | 44" | 48" | 52" | 56" |
| 60" |
Standard stock diameters. 4–12 in 1-in steps, 14–24 in 2-in steps, 26 in and up in larger jumps.
Spiral pipe follows the same ladder but is more available in the larger jump sizes and odd diameters, since it is rolled to order. Fittings (elbows, take-offs, dampers) are stocked on the same schedule as pipe — one more reason to land on a standard size rather than a custom diameter.
Sheet metal gauge by duct size
Larger duct needs thicker metal to hold its shape and resist denting. These are the common pairings for low-pressure commercial work — always confirm against the SMACNA pressure class table called out on the job:
| Gauge | Thickness | Typical duct size range | Common use |
|---|---|---|---|
| 30 ga | 0.305 mm | Up to 8 in | Residential branch runs |
| 28 ga | 0.378 mm | Up to 10 in | Residential trunks |
| 26 ga | 0.551 mm | Up to 18 in | Most commercial low-pressure duct |
| 24 ga | 0.701 mm | 19 – 30 in | Commercial trunks |
| 22 ga | 0.853 mm | 31 – 42 in | Large mains |
| 20 ga | 1.006 mm | 43 in and up | Large mains, high-pressure |
Rule of thumb for low-pressure systems: move one gauge heavier for duct under positive pressure or above roughly 1,500 FPM.
From calculated size to stock size — without a velocity spike
A duct calculator solves the physics exactly and hands you a number like 13.5 inches. Two rules translate that into an order:
- Round up, always. 13.5 → 14 in. The next stock size up slightly lowers velocity, friction rate and noise relative to the design point — all in the safe direction.
- Never round down to a "close" size. Dropping 13.5 to 13 in (or worse, 12 in, since 13 is not stock) raises velocity and friction rate above the design, and the error compounds across every downstream branch.
Worked example: 1,200 CFM at a 1,200 FPM design velocity needs 13.5 in. Order a 14 in round and the actual velocity is a comfortable 1,115 FPM. Order 12 in because it was on the truck, and velocity jumps to roughly 1,528 FPM — 27% over design, with the noise and friction rate to match.
Order this size
14 in round
Next stock size up. Actual velocity 1,123 FPM — slightly below the design point.
| Choice | Stock size | Actual velocity | Note |
|---|---|---|---|
| Round up (recommended) | 14 in | 1,123 FPM | Meets or beats the design velocity |
| Nearest (rounds up) | 14 in | 1,123 FPM | Also acceptable here |
| Exact theoretical | 13.5 in | 1,207 FPM | Not a stock size — for reference only |
Rectangular duct stock dimensions
Rectangular duct is ordered as a width × height pair matched to the equivalent round diameter. Stock widths and heights step on their own ladders, so a 13.5 in equivalent diameter becomes, for example, a 20" × 10" section rather than an odd custom size:
| Dimension | Stock increments |
|---|---|
| Widths | 8", 10", 12", 14", 16", 18", 20", 22", 24", 26", 28", 30", 32", 34", 36", 42", 48", 54", 60", 72", 84", 96" |
| Heights | 6", 8", 10", 12", 14", 16", 18", 20", 22", 24", 30", 36" |
Any width × height pair with the right equivalent diameter works hydraulically, but keep the aspect ratio under about 4:1 — beyond that, friction rate and metal weight per CFM rise faster than the space savings justify. For the exact conversion math, see the round vs rectangular sizing guide or run your numbers through the rectangular ↔ round converter.
Ordering checklist
- Convert every calculated diameter to the next stock size up before the takeoff.
- Assign gauge by the largest size on each run, not size by size.
- Confirm fittings stock on the same ladder — a 15 in elbow does not exist; a 16 in one does.
- Note pressure class on the order: gauge and reinforcement both follow it, not just diameter.
- For rectangular, state width × height, gauge, and joints (TDC/TDF or slide-on flange) together.
Frequently asked questions
What are the standard round duct sizes in HVAC?
Stock round duct in North America runs in 1-inch increments from 4 through 12 inches, then 2-inch increments through 24 inches, then larger jump sizes: 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 48, 52, 56 and 60 inches.
A calculator says 13.5 inches — what size duct do I order?
Round up to the next stock size, 14 inches. Never round down: a 13-inch equivalent section may not exist in stock, and the smaller diameter raises velocity and friction rate above the design point.
What gauge sheet metal do I need?
Small residential duct is commonly 30 or 28 gauge; 26 gauge covers most commercial low-pressure duct up to about 18 inches; 24 gauge for 19 to 30 inches; 22 gauge above that. Duct under positive pressure, or any duct over 1,500 FPM, moves one gauge heavier. Always confirm against SMACNA pressure class tables for the job.
Do rectangular ducts come in standard sizes too?
Yes. Stock widths step 2 inches through about 36 inches, then 6-inch increments up to 96 inches and beyond; heights commonly run 6 through 36 inches. Since any rectangular section is ordered to match an equivalent round diameter, there are many valid width × height pairs — aspect ratio under 4:1 keeps friction and metal weight reasonable.