Rectangular Duct Sizing Chart

The equal-friction method in one paragraph

Pick a target friction loss per 100 ft of duct (0.10 in. w.g. is the industry default). For every section, size the duct so friction at the section's airflow equals that target. The result is a duct system where every section contributes the same friction per unit length, fan static pressure is the sum of all friction losses plus fitting losses, and you do not need heavy balancing dampers to match the fan curve. This is why 90% of commercial HVAC ductwork is sized with equal friction.

Step-by-step procedure

1. Read the mechanical schedule to find the CFM at every section of the branch and trunk.
2. Start at the fan discharge with total system CFM. Pick an initial trunk velocity — 1,200 FPM is typical. Use the table below to find the duct diameter that delivers total CFM at 1,200 FPM.
3. Read off the friction rate at that diameter/CFM from the friction chart. That is now your design friction rate — typically 0.08–0.12 in. w.g. per 100 ft. Use this same rate for every other section.
4. For each downstream section, find the round diameter that delivers the section CFM at the same friction rate. Velocity will fall naturally as CFM decreases.
5. Convert each round size to rectangular using the equivalent diameter equation: De = 1.30 × (ab)^0.625 / (a+b)^0.25. Aim for aspect ratios 2:1 or lower; never exceed 4:1.
6. Sum the friction losses over the longest run to find total system static pressure. Add fitting losses (elbows, tees, transitions). Verify against the fan curve with a 25% safety margin.

Round duct diameter vs CFM at 0.10 in. w.g. per 100 ft

Use this table to find the round diameter for a given CFM at the standard design friction. Convert to rectangular dimensions with the equivalent-diameter table below.

Values calculated with the Darcy-Weisbach equation at 0.10 in. w.g./100 ft and galvanized steel roughness (ε = 0.0003 ft). Actual friction varies with duct condition and fitting density.

Round-to-rectangular equivalent diameter — quick reference

Once you have a round diameter, use this table to find rectangular dimensions at a 2:1 aspect ratio (SMACNA's preferred maximum for balanced installation).

Rectangular sizes produce the same friction loss per unit length as the equivalent round. Dimensions rounded to standard 1" or 25 mm increments. Aspect ratios above 2:1 increase sheet metal quantity per CFM and raise pressure drop from turbulence at the inside corners.

Why aspect ratio matters more than you think

A round duct has the smallest perimeter per cross-sectional area. As you flatten a rectangular duct, you add perimeter (more sheet metal cost) and increase the friction contribution from the walls relative to the bulk flow. At 4:1 aspect ratio, sheet metal quantity per CFM is roughly 35% higher than at 1:1. At 8:1 it is nearly double, with the bonus problems of drumming, lower reinforcement effectiveness, and difficult site installation in ceiling spaces.

Rule of thumb: use 1:1 where clearance allows, 2:1 where you need to fit above a corridor ceiling, 3:1 where you are flat-packed between joists, and never 4:1 or higher unless you have a space constraint that leaves no alternative.

Velocity targets by section

• Supply main trunk: 1,000–1,500 FPM (5–7.5 m/s). Going above raises noise; going below wastes sheet metal.
• Supply branch: 600–900 FPM (3–4.5 m/s). Balances noise and duct size.
• Return main trunk: 800–1,200 FPM (4–6 m/s). Slightly lower than supply to compensate for return plenum losses.
• Return branch: 400–600 FPM (2–3 m/s). Low enough to be silent at diffusers.
• Fresh air intake: 500–700 FPM (2.5–3.5 m/s). Limits entrainment of rain and debris.
• Exhaust discharge: 2,000–3,000 FPM (10–15 m/s). High enough to eject plume cleanly above roof.

Static pressure rules of thumb

• A typical equal-friction design at 0.10 in. w.g./100 ft adds 0.3–0.6 in. w.g. of friction over the longest run in a small-to-medium office. Add coil, filter and terminal losses to get total external static pressure.
• Each 90° elbow adds the equivalent of 10–15 duct diameters of straight length. Radius elbows are about half that. Long-radius elbows (R/D = 1.5) are the sweet spot.
• A branch takeoff with a conical fitting adds roughly 0.10–0.15 in. w.g.. A straight-cut takeoff can double that — specify conical.
• Round the calculated fan static pressure up by 25% before selecting the fan. Site installations never match paper calculations because of reducer fittings, field changes, and dirty filters.

Matching duct sizing to machinery on the floor

Once sizes are locked, the question is which machine on the fabricator's floor will roll each section. SBKJ's SBAL-V fully automatic duct line handles sheet widths from 800 to 1,500 mm, which covers the vast majority of commercial HVAC duct (rectangular duct up to roughly 1,250 × 1,250 mm). For larger industrial sections, the line is reconfigurable with wider tooling. The SBAL-III semi-automatic line covers the same range at lower throughput — typical for fabricators doing less than 1,000 m per day. Specifications with aspect ratios above 2:1 are best handled by the auto duct line because it tracks the cut length and seam position automatically; manually folded duct above 2:1 tends to develop twist and out-of-square corners.

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FAQ

What is the equal friction method for duct sizing?

The equal friction method sizes every section of a duct system so that friction loss per unit length is constant — typically 0.08 to 0.12 inches of water gauge per 100 feet of duct. It is the default method for low and medium pressure HVAC duct because it is quick, produces balanced systems that do not require heavy field damper adjustment, and matches how fans are curved.

What is the maximum aspect ratio for rectangular duct?

SMACNA recommends a maximum aspect ratio of 4:1 for rectangular duct; 2:1 or lower is preferred for efficient installation and lower pressure drop. Above 4:1, sheet metal quantity per CFM rises sharply, the duct becomes harder to reinforce, and vibration and drumming become more likely at typical fan operating points.

What velocity should I target in rectangular HVAC duct?

For commercial HVAC: supply main trunks 1,000–1,500 FPM, supply branches 600–900 FPM, return main trunks 800–1,200 FPM, return branches 400–600 FPM. Going above 1,500 FPM raises noise and pressure loss sharply; going below 600 FPM creates oversized duct and wasted sheet metal.

How do I convert round duct diameter to rectangular dimensions?

Use the equivalent diameter equation De = 1.30 × (a×b)^0.625 / (a+b)^0.25 where a and b are the rectangular duct inside dimensions. Solve for rectangular a and b that give the same De as your target round diameter. Most designers use a pre-calculated table rather than solving the equation directly.

What pressure class should I specify for rectangular duct?

Low-pressure (up to 2 in. w.g.): SMACNA pressure class 1 or 2. Medium-pressure (2–4 in. w.g.): class 3 or 4. High-pressure (4–10 in. w.g.): class 6 and 10. Choose based on the total static pressure the fan must deliver plus a safety margin of roughly 25%. Specify the pressure class on the drawings because it drives gauge, seam type, reinforcement spacing and seal class.