Duct Size Calculator

Free online duct size calculator for HVAC systems. Calculate rectangular or round duct dimensions based on CFM and velocity. Essential for air conditioning and ventilation design.

The Duct Size Calculator helps you determine the correct duct dimensions for HVAC systems based on required air flow (CFM) and desired air velocity. Calculate for rectangular or round ducts.

What is a Duct Size Calculator?

A Duct Size Calculator is an essential HVAC tool that determines the proper dimensions of air ducts based on required airflow (measured in CFM - Cubic Feet per Minute) and desired air velocity (measured in FPM - Feet per Minute). Properly sized ducts ensure efficient air distribution, optimal system performance, reduced energy costs, and quieter operation. Undersized ducts cause high pressure drops and noise, while oversized ducts waste materials and increase installation costs.

How to Use the Duct Size Calculator

Select duct type: round (circular) or rectangular ductwork
Enter the required air flow rate in CFM (Cubic Feet per Minute)
Choose air velocity based on application: 500-700 fpm for quiet residential, 700-1000 fpm for standard commercial, 1000-1500 fpm for industrial
Click Calculate to see the recommended duct dimensions
For rectangular ducts, the calculator provides optimal width and height ratios
Round ducts are more efficient but rectangular ducts fit better in tight spaces

Duct Sizing Formulas

1. Cross-Sectional Area

Area (sq ft) = CFM ÷ Velocity (fpm)

2. Round Duct Diameter

Diameter (inches) = √(Area × 183.35) or √(CFM × 183.35 ÷ FPM)

3. Rectangular Duct Dimensions

Width × Height = Area (with typical aspect ratio 1:1 to 4:1)

Air Velocity Guidelines

Residential: 500-700 fpm - Quieter operation, lower pressure drop

Commercial: 700-1200 fpm - Balanced noise and efficiency

Industrial: 1200-2000 fpm - Higher velocity acceptable, focus on efficiency

Return Air: 400-600 fpm - Lower velocity to minimize noise

Round vs Rectangular Ducts

Round Ducts: More efficient airflow, less surface area (less heat loss), easier sealing, stronger structure

Rectangular Ducts: Fit in tight spaces (between joists/studs), easier to run in walls, more common in residential

Duct Sizing Tips

Lower velocity = quieter system but larger ducts required
Standard residential main trunks: 600-900 fpm
Branch ducts can go up to 700-1000 fpm
Return air ducts should be sized for 400-600 fpm
Friction rate typically 0.08-0.15 inches per 100 feet
Always use Manual D calculations for complete system design
Account for fittings and transitions that increase pressure drop

Common Duct Sizing Mistakes

Using too high velocity to save material costs - causes noise and pressure issues
Not accounting for fittings and bends that restrict airflow
Undersizing return ducts - causes system imbalance
Ignoring duct leakage - can lose 20-30% of airflow
Not considering available space for duct installation
Mixing round and rectangular without equivalent diameter calculations