HVAC Duct Size & CFM Calculator

HVAC Duct Sizing Guide

Proper duct sizing ensures efficient airflow and comfortable temperatures throughout your space. Undersized ducts create noise, reduce efficiency, and cause uneven cooling. Oversized ducts waste materials and may create drafts.

CFM Requirements

Airflow is measured in Cubic Feet per Minute (CFM). Standard requirements:

• Residential cooling: 400 CFM per ton
• Commercial cooling: 350-400 CFM per ton
• By room type: 1 CFM per sq ft (bedrooms), 1.5 CFM/sq ft (kitchen)

Duct Velocity Guidelines

Air velocity should be maintained within acceptable ranges to control noise and ensure efficiency:

• Main/Trunk ducts: 700-900 FPM (Feet Per Minute)
• Branch ducts: 500-700 FPM
• Supply registers: 300-500 FPM
• Return grilles: 300-500 FPM

Round Duct Size Reference

Common round duct sizes for various CFM:

• 100 CFM: 5-inch diameter
• 200 CFM: 7-inch diameter
• 400 CFM: 9-inch diameter
• 600 CFM: 10-inch diameter
• 800 CFM: 12-inch diameter
• 1,200 CFM: 14-inch diameter

Rectangular Duct Equivalents

When space constraints require rectangular ducts, use equivalent round diameter. A 10-inch round duct (600 CFM) is equivalent to approximately 8x12 or 10x10 rectangular duct.

Duct Design Best Practices

• Minimize length: Shorter runs reduce pressure drop
• Avoid sharp turns: Use gradual transitions
• Insulate: Prevent condensation and heat gain/loss
• Seal joints: Air leaks waste energy
• Balance system: Ensure equal airflow to all rooms