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Free Windows HVAC duct-sizing tool (equal-friction & velocity method) — the digital ductulator from McQuay/Daikin.
The McQuay Duct Sizer is a free, lightweight Windows program for HVAC duct sizing — the digital equivalent of the classic cardboard ductulator that mechanical engineers, HVAC designers and contractors have relied on for decades. Developed by McQuay International (now part of Daikin Applied), the McQuay Duct Sizer software lets you size round, rectangular and flat-oval ducts in seconds using the equal-friction and velocity methods, instantly returning duct dimensions, air velocity, friction rate and pressure loss. Because it is fast, free and accurate, the Duct Sizer remains one of the most downloaded HVAC design utilities in the world, and this page explains exactly what it does, how to use it, the duct-sizing methods behind it, and how to download it.
Whether you are laying out a residential trunk-and-branch system, a commercial VAV network or an industrial exhaust run, the McQuay Duct Sizer turns the tedious, error-prone chart look-ups of manual duct design into instant calculations. Below you will find a complete guide to the tool, the theory of duct sizing it automates, its key features, step-by-step instructions, common mistakes to avoid, and answers to the questions engineers ask most.
The McQuay Duct Sizer is a standalone duct sizing software that reproduces the function of a mechanical ductulator on your computer. You enter an airflow (in CFM or L/s) and choose a design parameter — a target friction rate (inches of water per 100 ft, or Pa/m) or a maximum air velocity — and the program returns the required duct size along with the resulting velocity and pressure loss. It handles round duct, rectangular duct and flat-oval duct, converts between them using the equivalent-diameter relationship, and lets you lock one rectangular dimension to fit a constrained ceiling space. The tool is freeware, runs on any modern version of Windows, is only a few megabytes in size, and requires no license key or activation.
Because it is based on the same ASHRAE friction data used throughout the HVAC industry, the McQuay Duct Sizer produces results consistent with the friction charts in the ASHRAE Handbook and with SMACNA construction practice. That combination of speed, accuracy and zero cost is why it has become a staple on the desktops of HVAC engineers, estimators, service technicians and students.
Duct sizing is the process of selecting the cross-sectional dimensions of each section of ductwork so that it carries the required airflow while balancing three competing goals: keeping air velocity low enough to avoid noise and excessive friction, keeping the friction loss low enough that the fan can deliver the air without wasting energy, and keeping the duct small enough to fit the available space and control material cost. A duct that is too small runs fast, noisy and high in static pressure; a duct that is too large wastes sheet metal and plenum space. The McQuay Duct Sizer exists to find the sweet spot instantly.
Every duct calculation begins with the fundamental continuity relationship between airflow, velocity and area: the cross-sectional area equals the airflow divided by the velocity (A = Q ÷ V). From that area the tool derives the round diameter, or a rectangular width-by-height, or a flat-oval size. The friction rate then follows from the airflow and diameter using ASHRAE's turbulent-flow friction data. This is exactly the math a ductulator encodes on its rotating scales, and exactly what the McQuay Duct Sizer computes digitally.
Using the Duct Sizer software takes only a few clicks:
The McQuay Duct Sizer automates the classic low-pressure sizing methods from the ASHRAE Handbook. Understanding them helps you get the most from the tool.
The equal-friction method is the workhorse of duct design. You choose a single friction rate — typically 0.08 to 0.10 inches of water per 100 feet for commercial low-pressure systems — and size every duct section to that same rate. Because all sections share the same pressure gradient, the system tends to self-balance, and the design is quick and predictable. The McQuay Duct Sizer makes this trivial: set the friction rate once and size section after section to it.
The velocity method caps the air velocity by application to control noise and, in exhaust systems, to keep particulate entrained. Occupied-space branches near diffusers are usually held below about 700–1,000 FPM for quiet operation, commercial mains run 1,500–2,000 FPM, and industrial ducts run higher. The Duct Sizer lets you set a maximum velocity and sizes the duct to it, which is essential for noise-sensitive spaces where friction alone would allow a smaller, noisier duct.
Larger, higher-velocity systems sometimes use the static-regain method, which sizes each successive section so the static pressure recovered as the air slows at each takeoff balances the friction of the next section, keeping the static pressure nearly constant along the main. While the classic McQuay Duct Sizer focuses on equal-friction and velocity sizing, understanding static regain helps you decide when a more advanced design is warranted for large high-velocity trunks.
Duct shape is both a performance and a cost decision, and the McQuay Duct Sizer converts freely between shapes using the equivalent-diameter relationship. Round duct is the most efficient — least friction and least sheet metal per CFM — and should be the default where ceiling space allows. Rectangular duct fits shallow plenums but pays for it with more metal, more friction and more seams to seal, and the penalty grows as the aspect ratio rises; keep the width-to-height ratio at or below 4:1 (ideally 3:1). Flat-oval duct is the compromise — a flattened round that keeps much of round's efficiency in a shallower profile. The tool's equivalent-diameter conversion lets you size a round duct on the friction chart and then translate it into the rectangular or flat-oval size that fits the building.
The McQuay Duct Sizer software earns its place on the desktop for several reasons. It saves time by replacing chart look-ups and hand calculations with instant results, letting you size a whole system in minutes. It reduces errors by removing the interpolation mistakes that plague manual ductulator use. It standardizes your designs to ASHRAE friction data, so results are defensible and consistent across a team. It is free, so every engineer, estimator and student on a project can use the same tool. And it is fast to learn — anyone familiar with a ductulator is productive in minutes, and newcomers pick it up quickly because the interface mirrors the physical wheel.
For contractors and estimators, the speed translates directly into faster take-offs and more competitive bids. For students, it is an ideal way to learn the relationship between airflow, velocity, friction and duct size by experimenting interactively. For practicing engineers, it is the quick check that confirms a duct layout before it goes into CAD.
The McQuay Duct Sizer is a desktop program you download and install, which is convenient for offline use but tied to a Windows machine. A modern alternative is an online duct sizing calculator that runs in any browser on any device, with no download required and always the latest version. MEPMate's own free Duct Size Calculator offers the same equal-friction and velocity sizing in the browser, with US-first units, round and rectangular support, and instant results — plus companion tools for duct pressure drop, heat load and fan power. Many engineers keep the McQuay Duct Sizer for offline work and use an online calculator for quick checks on the go; the underlying ASHRAE math is the same, so the results agree.
Get the best results from the McQuay Duct Sizer by defaulting to round or square duct and flattening only when headroom forces it, holding occupied-space velocity below about 1,000 FPM for quiet operation, and budgeting roughly half the fan's external static for fitting losses on a typical layout. Keep flexible duct short and pulled tight, size returns generously, and document the index run and static tally so the fan selection is traceable. Size to a consistent friction rate across the system so it self-balances, and provide balancing dampers so the installation can be commissioned to deliver the design airflow to every room.
While the McQuay Duct Sizer is a favorite, several alternatives cover duct sizing. Full HVAC design suites such as ductwork modules in load-calculation software size ducts as part of a complete system model. Manufacturer tools from other equipment makers offer similar ductulator-style utilities. And browser-based duct calculators — like the free MEPMate Duct Size Calculator — provide the same equal-friction and velocity sizing without any download, on any device. For a quick, accurate duct size the McQuay Duct Sizer and a good online calculator are interchangeable; for full system design with fittings, static tally and fan selection, a complete design tool or a spreadsheet built on the same ASHRAE data is the next step.
Yes. The McQuay Duct Sizer is distributed as freeware by McQuay/Daikin, with no cost, no license key and no activation. It is a small Windows program that installs in seconds, which is a large part of why it has been so widely adopted by HVAC engineers and contractors.
It uses the standard low-pressure duct-sizing methods from the ASHRAE Handbook — primarily the equal-friction method (sizing to a fixed friction rate) and the velocity method (capping air velocity) — with ASHRAE turbulent-flow friction data, the same basis as a mechanical ductulator and the friction charts engineers use by hand.
Yes. It sizes round, rectangular and flat-oval ducts and converts between them using the equivalent-diameter relationship, so you can find a round size on the friction chart and translate it into the rectangular or flat-oval dimensions that fit the available space.
It is a Windows program and runs on modern versions of Windows. It is only a few megabytes, uses minimal resources, and does not require a network connection to compute, making it convenient for field and offline use.
It is accurate for sizing individual duct sections because it uses ASHRAE friction relationships. Remember that a complete duct design also requires accounting for fitting losses, the total static-pressure budget of the longest run, sealing and balancing — the tool sizes the duct, and the surrounding design decisions turn those sizes into a working system.
Yes. A browser-based tool such as the free MEPMate Duct Size Calculator performs the same equal-friction and velocity sizing on any device with no download, and because it uses the same ASHRAE math the results match. Many engineers use the McQuay Duct Sizer offline and an online calculator for quick checks.
For low-pressure commercial systems a friction rate of 0.08 to 0.10 inches of water per 100 feet is the common default. Residential systems often use around 0.08, and higher-pressure or industrial systems may run higher. Pick one rate and size every section to it so the system self-balances.
The tool is a mature, stable utility that continues to be used widely even though it is a legacy program. Because the ASHRAE friction physics it encodes does not change, it remains accurate and useful; for the newest interface and cross-device access, a modern online duct calculator is a convenient complement.
To use the McQuay Duct Sizer confidently, it helps to understand the three quantities it juggles. The friction rate is the pressure the air loses to friction per unit length of duct, expressed in inches of water gauge per 100 feet (in. w.g./100 ft) or Pascals per meter (Pa/m). A lower friction rate means larger ducts, quieter operation and lower fan energy, but more material and space; a higher rate means smaller, cheaper ducts but more noise and fan power. The industry default of 0.08–0.10 in. w.g./100 ft for low-pressure commercial systems is a well-tested compromise, and the McQuay Duct Sizer lets you dial it in.
Velocity is how fast the air moves, in feet per minute (FPM) or meters per second (m/s). It matters because noise rises sharply above roughly 1,000 FPM near occupied spaces, and because velocity determines the dynamic (velocity) pressure that fittings convert into losses. The total pressure loss along a duct run is the friction rate multiplied by the length, plus the losses from every fitting — elbows, transitions, dampers and takeoffs — each of which the designer adds as an equivalent length or a loss coefficient. The fan must overcome the total loss of the longest (index) run at the design airflow. The McQuay Duct Sizer gives you the friction rate and velocity for each section; tallying the fittings and the index-run total to select the fan is the designer's next step, and it is where many duct designs succeed or fail.
Before software, HVAC engineers sized ducts with a ductulator — a circular cardboard or plastic slide-rule with concentric rotating scales for airflow, velocity, friction and duct size. Turning the wheel aligned the scales to read the duct dimensions for a given airflow and friction rate. McQuay International, a long-established maker of chillers, air handlers and HVAC equipment (now part of Daikin Applied, one of the world's largest HVAC manufacturers), distributed both physical ductulators and, later, the free McQuay Duct Sizer software that digitized the wheel. The software preserved the ductulator's speed and familiarity while eliminating the interpolation errors and wear of the cardboard tool, and its zero price made it ubiquitous. That heritage is why so many engineers still reach for the McQuay Duct Sizer by name — it is the trusted, digital descendant of a tool the profession grew up with, backed by a major manufacturer's engineering credibility.
Duct sizing is one step in a larger air-side design process, and the McQuay Duct Sizer fits naturally into it. First comes the load calculation — determining each space's heating and cooling load and the airflow (CFM) required to condition it, often at roughly 400 CFM per ton for comfort cooling. Next is equipment selection — the air handler or rooftop unit and its available fan static pressure. Then comes duct sizing, where the McQuay Duct Sizer turns each section's airflow into a duct dimension at the chosen friction rate. After sizing comes the pressure tally — adding straight-duct friction and fitting losses along the index run to confirm the fan can deliver the air — followed by diffuser and grille selection, duct construction to SMACNA standards, and finally balancing to set each outlet to its design airflow. By producing accurate sizes instantly, the Duct Sizer keeps this workflow moving and lets the engineer focus on the judgment-heavy steps of fitting losses, fan selection and balancing.
A few field-tested habits make duct designs reliable. Always check velocity and friction together — a duct that passes friction can still be too noisy. Default to round or low-aspect-ratio rectangular duct to save metal and friction. Reserve fan static for fittings, budgeting roughly half the external static for fitting losses on a typical layout. Treat flexible duct carefully, keeping it short and taut because it has two to three times the friction of smooth metal. If a system underperforms after installation, the usual culprits are undersized returns, unsealed ducts leaking 10–20% of the airflow, sagging or compressed flex duct, or a fan operating far from its rated point because the actual static exceeded the design. Sizing with the McQuay Duct Sizer prevents the sizing errors; the installation and balancing prevent the rest. Providing balancing dampers at branch takeoffs and sealing to the SMACNA leakage class are the two cheapest insurance policies for a quiet, efficient system.
When sizing with the McQuay Duct Sizer, these reference values keep your designs in the right range. Recommended air velocities by application: residential mains 700–900 FPM and branches 600–700 FPM; low-noise commercial mains 1,000–1,500 FPM and branches 600–1,000 FPM; general commercial mains 1,500–2,000 FPM; industrial 2,000–3,000 FPM; and return-air mains 1,000–1,400 FPM. Friction rate for low-pressure systems is typically 0.08–0.10 in. w.g. per 100 ft. As a capacity guide at 0.1 in. w.g./100 ft, round duct carries roughly: 6-inch ≈ 110 CFM, 8-inch ≈ 230 CFM, 10-inch ≈ 420 CFM, 12-inch ≈ 680 CFM, 14-inch ≈ 1,000 CFM, 16-inch ≈ 1,400 CFM, and 20-inch ≈ 2,500 CFM. Keep the rectangular aspect ratio at or below 4:1, and remember that flexible duct carries two to three times the friction of smooth metal, so size it up or account for the added loss. These anchors, combined with the McQuay Duct Sizer's instant calculations, let you size a complete duct system quickly while keeping every section quiet, efficient and code-appropriate. For a browser-based version of the same reference and calculations on any device, the free MEPMate Duct Size Calculator provides identical ASHRAE-based sizing along with duct velocity guidance and companion HVAC tools.
The McQuay Duct Sizer is a fast, free and accurate way to size HVAC ducts by the equal-friction and velocity methods — the digital ductulator that belongs on every HVAC engineer's desktop. Use the download button on this page to get the software, and pair it with MEPMate's free online Duct Size Calculator and HVAC Heat Load Calculator for a complete, browser-based duct-design workflow. Whether you prefer the offline convenience of the desktop tool or the anywhere access of an online calculator, the ASHRAE-based results are consistent, so you can size ductwork quickly and confidently for any project.
Always scan any downloaded installer with your antivirus. MEPMate links to freeware for convenience and is not affiliated with McQuay/Daikin.