Size a round supply or return duct from an airflow rate, the way a traditional Ductulator slide rule does. Enter the airflow (CFM) and either a target velocity or friction rate, and the calculator gives the required round diameter, the resulting friction loss per 100 ft, the nearest standard round duct size, and an equivalent rectangular duct size for a given side dimension.
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Assumes standard air density (0.075 lb/ft³) and galvanized-steel roughness — flex duct, duct board, and non-standard elevations/temperatures need different inputs. Does not include fitting (elbow/transition/takeoff) losses — a full system design needs total pressure drop along the whole run, not just straight-duct friction rate.
How the calculation works
- Round diameter. D(in) = 13.54×√(CFM/V), from airflow and target velocity.
- Friction rate. Estimated with a power-law fit approximating the standard duct friction chart, ΔP per 100 ft as a function of CFM and diameter.
- Standard size. The calculated diameter is rounded up to the nearest common standard round duct size.
- Rectangular equivalent. Uses the Huebscher/ASHRAE equal-friction equivalent-diameter formula, De = 1.30×(a×b)0.625/(a+b)0.25, to relate a rectangular duct’s two sides to an equivalent round diameter.
What to have ready
- Design airflow for the duct run (CFM)
- A target velocity (or friction rate) appropriate to the duct’s location — main trunks, branches and low-noise spaces typically use different targets
- One side dimension, if you want a rectangular equivalent size
Notes and limitations
The friction-rate result is a curve-fit approximation of the standard ASHRAE duct friction chart and is intended for quick sizing, not for final system static-pressure design — verify against manufacturer data or full duct-sizing software for the final design. Sound criteria, fitting losses, and duct material roughness are not accounted for here. Final acceptance of the design rests with the responsible design professional.
HVAC Duct Sizing Reference Table — ASHRAE / SMACNA
| Duct Application | Recommended Velocity (FPM) | Max Velocity (FPM) | Friction Rate (in.w.g./100 ft) | Standard Reference |
|---|---|---|---|---|
| Main Supply Trunk (Residential) | 600–900 | 1,000 | 0.08 | ACCA Manual D |
| Supply Branch (Residential) | 500–700 | 900 | 0.08 | ACCA Manual D |
| Return Air Trunk (Residential) | 400–600 | 700 | 0.08 | ACCA Manual D |
| Main Supply Duct (Commercial) | 1,000–1,500 | 2,000 | 0.10 | SMACNA HVAC Duct Design |
| Supply Branch (Commercial) | 800–1,200 | 1,500 | 0.10 | SMACNA HVAC Duct Design |
| Return Air (Commercial) | 600–900 | 1,200 | 0.10 | SMACNA HVAC Duct Design |
| Outside Air Intake | 400–600 | 800 | — | ASHRAE 62.1 |
| Exhaust / Relief Air | 500–800 | 1,000 | — | ASHRAE 62.1 |
| Flexible Duct (residential) | ≤700 | 900 | 0.08 | ACCA Manual D, max 6 ft run |
| High-Pressure System (VAV) | 2,000–3,000 | 4,000 | 0.30–0.60 | SMACNA / ASHRAE 90.1 |
Source: SMACNA HVAC Duct Construction Standards (2018), ACCA Manual D (2016), ASHRAE Handbook — Fundamentals.
Duct Sizing Calculator (Ductulator) FAQ
What is a ductulator and how does it work?
A ductulator is a circular or digital slide-rule tool — originally developed by ASHRAE — used to size HVAC ducts based on airflow (CFM), friction rate (inches of water gauge per 100 ft), and duct velocity (FPM). You set any two known values and the tool returns the remaining unknowns, including equivalent round duct diameter or rectangular duct dimensions. Digital ductulator calculators use the Darcy-Weisbach equation with air density corrections to replicate the same process. The friction rate is set first (commonly 0.08 in.w.g./100 ft for residential, 0.10 for commercial), and the duct size is read from the CFM and friction-rate intersection.
What friction rate should I use for duct design?
For residential systems, ACCA Manual D recommends a friction rate between 0.06 and 0.12 in.w.g./100 ft, with 0.08 as the most common design value. For commercial systems, SMACNA allows 0.10 in.w.g./100 ft for low-pressure systems. The friction rate is derived from the available static pressure of the air handler or fan divided by the equivalent total length of the longest duct run (including fittings expressed as equivalent length). Higher friction rates produce smaller ducts but more noise and fan energy; lower rates produce larger, quieter, more energy-efficient systems.
How do I convert between round and rectangular duct sizes?
The equivalent diameter for a rectangular duct is calculated using the formula: De = 1.30 × (a × b)^0.625 / (a + b)^0.25, where a and b are the rectangular duct dimensions in inches (ASHRAE Handbook — Fundamentals). For example, a 12×8-inch rectangular duct has an equivalent round diameter of approximately 9.8 inches. Most ductulator calculators perform this conversion automatically. Rectangular ducts are preferred in space-constrained applications; round ducts have lower friction loss per unit area and are preferred when headroom is available.
What is the maximum duct velocity for a residential system?
ACCA Manual D recommends a maximum supply trunk velocity of 900–1,000 FPM and a maximum supply branch velocity of 700–900 FPM for residential systems to keep noise levels below NC-30. Return ducts are sized for 400–700 FPM to prevent grille noise. Exceeding these velocities increases system noise (particularly at supply registers), accelerates duct wear, and can cause resonance in thin-gauge sheet metal. Flexible ducts should not exceed 700 FPM and should be kept as straight as possible to minimize friction losses.
What is the difference between Manual D and SMACNA duct design?
ACCA Manual D is the ANSI-accredited standard for residential duct system design in the U.S., used in conjunction with Manual J load calculations and Manual S equipment selection. SMACNA (Sheet Metal and Air Conditioning Contractors’ National Association) duct design standards are used for commercial HVAC systems and cover a broader range of static pressure classes and duct construction details. ENERGY STAR and most building codes require residential duct systems to be designed per Manual D, while Title 24 (California), ASHRAE 90.1, and commercial codes reference SMACNA for commercial work.