HVAC Duct Sizing Calculator
The HVAC Duct Sizing Calculator computes room-by-room CFM requirements and recommends round duct diameters (4"–36") using the equal friction method. It includes 10 room presets, rectangular duct equivalents via Huebscher equation, and an interactive SVG duct layout diagram — free, no signup required.
Room & Airflow Setup
Duct Velocity Reference
| Application | Max FPM | Max m/s |
|---|---|---|
| Residential (main) | 900 | 4.6 |
| Residential (branch) | 600 | 3 |
| Commercial (main) | 1500 | 7.6 |
| Commercial (branch) | 1000 | 5.1 |
| Industrial | 2500 | 12.7 |
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What is HVAC Duct Sizing?
HVAC duct sizing is the process of determining the correct duct diameter to deliver the required airflow (CFM) to each room while keeping air velocity and noise within acceptable limits. The equal friction method, recommended by ASHRAE, sizes ducts so that friction loss per unit length is constant throughout the system, typically 0.08–0.1 inches of water gauge per 100 feet. Residential main ducts typically operate at 600–900 FPM, while commercial systems may reach 1,500 FPM. Proper sizing prevents excessive noise, energy waste, and uneven temperature distribution.
How to Use This Calculator
- Add rooms and set the area (sq ft) for each — select the room type preset for appropriate CFM per square foot
- Add or remove rooms as needed using the Add Room button
- Set the maximum duct velocity limit (default 900 FPM for residential)
- Click Calculate to see the main duct size and per-room branch duct recommendations
- Review the duct layout diagram and equivalent rectangular duct sizes for each round duct
Frequently Asked Questions
How many CFM per square foot do I need?
The typical rule of thumb is 1 CFM per square foot for standard rooms (bedrooms, offices, living rooms). Kitchens, restaurants, and gyms need 1.5–2 CFM/ft² due to higher heat loads. Server rooms may require 3+ CFM/ft². These values can vary based on climate, insulation, window area, and occupancy.
What is the maximum duct velocity for residential HVAC?
ASHRAE recommends 600 FPM for branch ducts and 900 FPM for main trunks in residential systems to maintain noise levels below NC 35. Higher velocities (up to 1,500 FPM for commercial) are acceptable where noise is less critical. Exceeding these limits causes whistling, rumbling, and reduced comfort.
Round or rectangular duct — which is better?
Round ducts are more efficient: they have less friction loss per unit of airflow, use less material, and are easier to seal. Rectangular ducts are used where ceiling space is limited (height-restricted areas). For the same airflow, a rectangular duct with 4:1 aspect ratio needs 40% more metal than the equivalent round duct.
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