Duct System Planner

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Design complete HVAC duct systems from plenum to registers with auto-sizing and pressure drop calculation.

Units:
Application:
Building Type:
Friction Rate: in.wg/100ft
100%
Add branches and registers to see the system diagram
System Tree
Tree Preview
Add branches and registers to see the system diagram
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Sizing Results
Total Pressure Drop: -
Branch CFM Max CFM Size Velocity
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About This Tool

The Duct System Planner is a comprehensive HVAC duct design tool that lets you build, size, and analyze complete duct systems from the supply plenum to every register outlet. Unlike basic duct sizing calculators that handle one segment at a time, this tool models the entire system topology as an interactive tree structure, giving you a complete picture of airflow distribution, pressure drops, and velocity performance across every branch and run. The sizing engine uses the equal friction method per ASHRAE Fundamentals, automatically calculating duct dimensions for each segment based on CFM requirements that propagate upward from registers through branch nodes to the main trunk. Duct sizes are determined using the Huebscher equivalent diameter equation and Darcy-Weisbach friction formula at your chosen friction rate. The tool computes pressure drops for every segment, identifies the critical path — the duct run with the highest total pressure drop from plenum to register — and validates velocities against SMACNA thresholds for residential, commercial, or industrial applications. A powerful lock mechanism lets you fix specific parameters — CFM, duct size, or velocity — at any node to match available materials or design constraints, while the system recalculates all unlocked nodes around them. The interactive diagram uses the Reingold-Tilford tree layout algorithm with color-coded velocity indicators for instant visual feedback on system performance. You can export your complete design as JSON for documentation, collaboration, or future editing. This tool serves HVAC contractors sizing residential and commercial systems, mechanical engineers validating duct layouts, energy auditors assessing existing installations, and students learning duct design principles. All calculations run entirely in your browser — no project data is sent to any server, ensuring your system designs remain completely private and available offline.

Sources: ASHRAE · SMACNA

How to Use

  1. Select supply or return air system and set your friction rate.
  2. Build your system by adding branches to the plenum, then add registers to each branch with their CFM values.
  3. Click 'Size All' to auto-calculate duct dimensions and view total pressure drop. Export your design as JSON for sharing or backup.

Principles of Duct System Design

HVAC duct system design follows two primary methods: equal friction and static regain. The equal friction method, implemented in this tool, maintains a constant friction rate throughout the system — typically 0.08 in.w.g. per 100 feet for residential applications. This approach naturally reduces duct sizes as airflow decreases toward terminal outlets, creating an efficient tapered system. The static regain method, used in large commercial systems, sizes ducts so that the static pressure regain from velocity reduction at each branch takeoff offsets the friction loss in the next section. This produces more uniform static pressure at all outlets but requires more complex calculations. Proper system design also considers fitting losses — a typical residential duct system has 50-70% of its total pressure drop in fittings rather than straight duct. The equivalent length method converts each fitting (elbows, tees, transitions) into an equivalent length of straight duct for simplified calculations. A 90-degree elbow equals approximately 15 feet of equivalent straight duct, while a branch tee can equal 30-50 feet.

How to Use

  1. Select supply or return air system and set your friction rate.
  2. Build your system by adding branches to the plenum, then add registers to each branch with their CFM values.
  3. Click 'Size All' to auto-calculate duct dimensions and view total pressure drop. Export your design as JSON for sharing or backup.

Methodology

The planner uses bottom-up traversal to size ducts: register CFM values propagate upward, with each branch node summing its children's airflow. Duct sizes are calculated using the Huebscher equivalent diameter equation and Darcy-Weisbach friction formula at the system's target friction rate (default 0.08 in.w.g./100ft for residential). The lock mechanism allows fixing any parameter (CFM, duct size, velocity) at a specific node — locked values are preserved while the system recalculates unlocked nodes around them. Pressure drop is computed for each segment as delta-P = friction_rate x equivalent_length / 100. The critical path is identified as the duct run with the highest total pressure drop from plenum to register, which determines the required fan static pressure. Velocity validation uses SMACNA thresholds appropriate for the application type (residential, commercial, or industrial).

Sources: SMACNA

Understanding Your Results

The system diagram shows your duct layout with color-coded velocity indicators. Green nodes have optimal velocity for the application type. Yellow indicates borderline velocity — acceptable but may warrant attention. Red signals velocity outside the recommended range. The pressure summary shows total pressure drop along the critical path, which must not exceed your air handler's rated external static pressure (typically 0.3-0.5 in.w.g. for residential). If total pressure is too high, consider increasing duct sizes on the critical path, reducing equivalent lengths by minimizing fittings, or splitting high-CFM branches. The system automatically recalculates when you modify any node, showing the cascade effect of design changes.

Practical Examples

A 3-ton residential system (1,200 CFM) typically has: a 14-16 inch main trunk from the plenum, reducing to 12 inches after the first branch, with 8-inch branches to bedrooms (100-150 CFM each) and a 10-inch branch to the living area (300-400 CFM). Total critical path pressure drop should be 0.25-0.40 in.w.g. For a 5-ton system (2,000 CFM), the main trunk starts at 18-20 inches. Keep velocities in the main trunk between 700-900 FPM and branch ducts at 400-600 FPM for quiet operation.

Tips for Duct System Design

Start your design at the registers and work backward to the plenum — this ensures each room receives its required airflow. Use the lock feature to fix duct sizes to commercially available dimensions (6, 7, 8, 10, 12, 14, 16, 18, 20, 24 inch rounds are standard). Keep the total equivalent length of the critical path under 200 feet for residential systems. Minimize branch takeoff angles — 45-degree tees have roughly half the pressure drop of 90-degree tees. Size return ducts to be at least as large as supply trunks. When pressure drop exceeds the air handler's capacity, identify the critical path and increase duct sizes on that specific path first, as other paths are unaffected.

Sources

All calculations are performed locally in your browser. No data is sent to any server.

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Frequently Asked Questions
How do I design a duct system with this tool?
Start by selecting supply or return air system. The tool creates a plenum automatically. Add branches to the plenum by clicking '+Add Branch'. For each branch, set the duct length and add registers (terminals) with their CFM values. The tool automatically calculates the required duct sizes for each segment based on the cumulative CFM flowing through it.
How is total pressure drop calculated?
The tool calculates friction loss for each duct segment using the Darcy-Weisbach equation, based on CFM, duct diameter, and length. It then traces all paths from plenum to registers and identifies the critical path (longest pressure drop). This helps size your blower/fan. Note: This calculation includes duct friction only - add fitting losses separately.
Can I save and share my duct system design?
Yes! Use 'Export JSON' to download your complete system design as a JSON file. This includes all branches, registers, CFM values, and duct lengths. To reload a design, use 'Import JSON' and select your saved file. You can also share the design with others by sending them the JSON file.
What is the tree diagram showing?
The tree diagram visualizes your duct system hierarchy. The plenum is at the top, with branches and registers shown as connected nodes below. Each node displays its CFM value. Nodes are color-coded: green for acceptable velocity, yellow for warnings (too low or high velocity). This helps you quickly identify problem areas.
What duct velocity should I use for residential design?
Per ACCA Manual D Table 3-1 for rigid duct: supply trunks 700 FPM typical (900 max), supply branches 600 FPM typical (700 max), runouts 500 FPM typical (600 max). Return ducts are typically 100 FPM lower. At the standard residential friction rate of 0.08 in.wg/100ft, these velocities balance noise control, energy efficiency, and duct cost. The tool automatically flags velocities outside recommended ranges with color-coded indicators.
What is the difference between round and rectangular duct sizes?
Round ducts are more aerodynamically efficient because they have less surface area per unit of airflow, resulting in lower friction losses. Rectangular ducts are often used where ceiling space is limited because they can be made with a lower profile. According to SMACNA guidelines, when using rectangular ducts, the aspect ratio (width to height) should ideally stay below 4:1 to maintain efficiency. This tool calculates equivalent round duct diameters so you can compare performance between the two shapes.
How does the locking system work?
The locking system lets you fix specific values while the calculator adjusts others. You can lock CFM (size and velocity are derived), lock duct size (velocity is derived from CFM), lock velocity (size is derived), or lock both size and velocity (CFM is derived). This is useful when you have a physical constraint, like a fixed duct size in an existing chase, and want the system to calculate around it.
Can I design multi-zone commercial systems?
Yes. Use the building type selector to switch between residential, commercial, and industrial presets. Commercial systems use higher velocity limits and friction rates. Build your duct tree with a plenum at the root, main trunk branches for each zone, and registers for individual diffusers. The planner automatically applies the appropriate velocity guidelines from ASHRAE and SMACNA based on your building type and duct location in the system.
How do I calculate duct size from CFM?
Enter the CFM (cubic feet per minute) airflow for each register, and the tool calculates the required round duct diameter automatically. The calculation uses your chosen friction rate — at the standard residential rate of 0.08 in.w.g./100ft, a 150 CFM bedroom register sizes to an 8-inch standard duct, while a 300 CFM living room branch sizes to a 10-inch duct. The tool also shows equivalent rectangular sizes using the Huebscher equation. For a quick single-duct lookup, try the related Duct Calculator, or use this planner to size an entire multi-branch system at once.
What is the maximum recommended duct run length?
For residential systems using the equal friction method at 0.08 in.w.g./100ft, ACCA Manual D recommends keeping the total equivalent length of the critical path — the longest duct run from plenum to register — under 200 feet. This includes both straight duct and equivalent lengths for fittings: a 90-degree elbow adds about 15 equivalent feet, and a branch tee adds 30 to 50 feet. The planner automatically identifies your critical path and calculates its total pressure drop. If your longest run exceeds limits, increase duct sizes on that specific path or reduce the number of fittings.
What is the Huebscher equation used in duct sizing?
The Huebscher equation converts rectangular duct dimensions into an equivalent round duct diameter for accurate friction loss calculations. The formula is De = 1.30 × (a × b)^0.625 / (a + b)^0.250, where a and b are the width and height of the rectangular duct in inches. This industry-standard equation from the ASHRAE Handbook of Fundamentals ensures that a rectangular duct and its equivalent round duct produce the same friction loss at the same airflow. The planner uses this equation to display equivalent rectangular sizes alongside every round duct diameter it calculates, helping you choose between round and rectangular options for each segment.
Can I use this tool for flexible duct sizing?
Yes, with an important adjustment. Flexible duct has significantly higher friction losses than rigid galvanized steel duct — ACCA Manual D documents substantially higher pressure drops for flex duct, especially when compressed or not fully extended. The tool defaults to a friction rate of 0.08 in.w.g./100ft, which is standard for rigid sheet metal. When designing with flex duct, increase the friction rate using the input field or select a higher preset. As a general guideline, fully stretched flex duct needs roughly double the friction rate of rigid duct, and poorly installed flex may need three times or more. The tool instantly recalculates all duct sizes when you change the friction rate.