Construction & Architecture / Energy & HVAC
Ductwork Sizing Calculator
Leave blank to calculate from room sizes below
Room-by-Room Sizing
Ductwork Sizing Guide
How to Use: Enter your total system CFM (from your HVAC unit specs) OR enter the square footage of each room to auto-calculate CFM (1 CFM per sq ft rule of thumb).
Key Concepts:
- CFM: Cubic Feet per Minute - volume of air moved
- FPM: Feet Per Minute - speed of air through duct
- Static Pressure: Resistance in duct system (inches of water column)
- Main Trunk: Large duct from furnace/air handler to house
- Branch Ducts: Smaller ducts from trunk to individual rooms
Duct Velocity Guidelines:
- Too Fast (>900 FPM): Noisy, high pressure, energy waste
- Ideal (600-900 FPM): Quiet, efficient, good balance
- Too Slow (<500 FPM): Oversized ducts, poor air distribution
Quick Sizing Rules:
- 400 CFM per ton of AC (e.g., 3-ton AC = 1200 CFM)
- 1 CFM per square foot of living space
- Main trunk: 700 FPM for residential
- Branch ducts: 400 FPM for quiet operation
Common Duct Sizes:
- Small room (100-150 sq ft): 6" round
- Medium room (150-250 sq ft): 7-8" round
- Large room (250-400 sq ft): 9-10" round
- Main trunk (1200-2000 CFM): 16-20" round
Disclaimer: This calculator provides simplified estimates. Professional Manual D duct design is recommended for optimal system performance. Consult a licensed HVAC contractor for final duct design.
Use this ductwork sizing calculator to compute energy & hvac results from your inputs.
Formula
For rectangular ducts: Area = CFM / Velocity (900 FPM supply, 700 return) For round ducts: Diameter = √(4 × Area / π) Approximation: Diameter ≈ 1.3 × (CFM / 100)^0.45 Aspect ratio for rectangular: Keep between 1:1 and 4:1