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).
- 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
- 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
- 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
- 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.
About This Calculator
Ductwork Sizing Calculator is designed to reduce manual errors and give repeatable outputs when you need quick, reliable answers.
Inside energy & hvac, this tool gives you a practical way to model scenarios, compare outcomes, and make better next-step decisions without spreadsheet overhead.
If your workflow expands, pair this calculator with BTU Calculator and Heat Load Calculator to cross-check assumptions and build a stronger analysis chain.
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
Example Calculation
The worked example below demonstrates how the input fields translate into the final output. Use it as a quick validation pass before entering your own numbers.
- CFM (cubic feet per minute) required: 6
- Duct length (feet): 8
- Friction rate (inches of water per 100 ft, typically 0.1): 12
Explanation of Results
Result Interpretation
The ductwork sizing calculator returned calculated value based on CFM (cubic feet per minute) required 6, Duct length (feet) 8, and Friction rate (inches of water per 100 ft, typically 0.1) 12. Use this result as a baseline, then adjust one input at a time to understand how sensitive your outcome is before making decisions.
FAQ
How should I validate the ductwork sizing calculator result?
Run a second scenario with rounded numbers, then compare the direction and magnitude of the change before using the value operationally.
What formula is this based on?
This page uses the following formula logic: 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
Can I bookmark this energy & hvac tool?
Yes. Use the canonical URL /construction-architecture/energy-hvac/ductwork-sizing-calculator to return to this calculator in the Construction & Architecture library.
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