HVAC Duct Sizing Calculator
Calculated Duct Specifications:
Recommended Round Diameter: 0 inches
Estimated Air Velocity: 0 FPM
Rectangular Equivalent (Standard Height 10″): 0" x 10″
Recommended Rectangular Size: 0
How to Use the Duct Sizing Calculator
Proper duct sizing is critical for maintaining efficiency, reducing noise, and ensuring consistent comfort throughout a building. This calculator utilizes the "Equal Friction Method," which is the industry standard for sizing low-pressure HVAC systems.
Step 1: Determine CFM (Cubic Feet per Minute)
Identify the volume of air that needs to move through the duct. For residential systems, this is typically based on the cooling capacity (roughly 400 CFM per ton of air conditioning).
Step 2: Choose Friction Rate
The standard design friction rate for residential and light commercial systems is usually 0.10 inches of water column (in. w.c.) per 100 feet of duct length. For very quiet systems, 0.08 may be used.
Typical Velocity Limits
| Application | Max Velocity (FPM) |
|---|---|
| Residential Main Trunk | 700 – 900 |
| Residential Branch Ducts | 600 |
| Commercial Main Trunk | 1,000 – 1,500 |
Sizing Formula and Logic
The calculator uses the Darcy-Weisbach and Colebrook equations simplified for standard air conditions. The primary calculation for round diameter ($D$) is derived from the Friction Rate ($f$) and Airflow ($Q$):
Diameter = 0.1091 × (Q0.42 / f0.19)
Real-World Example
If you have a 3-ton AC unit, you generally need 1,200 CFM (3 tons x 400 CFM). If we use a standard friction rate of 0.10:
- CFM: 1,200
- Friction: 0.10
- Result: Approx 14.5″ Round Duct or a 16″ x 12″ Rectangular Duct.
Using a duct that is too small will cause high static pressure, leading to blower motor failure and excessive whistling noises. Conversely, a duct that is too large will result in low air velocity, making it difficult to "throw" air into the center of a room.