What is Duct Sizing?
Duct sizing means choosing the right dimensions for air ducts so they can carry the correct amount of air without being too noisy or wasting energy. It's like choosing the right size pipe for water - too small and you don't get enough flow, too big and you waste space and money.
The Three Key Numbers
1. CFM (Cubic Feet per Minute)
This is how much air needs to move through the duct. Think of it like "gallons per minute" for water, but for air instead.
Example: Room Air Requirements
A 12' × 10' bedroom (120 sq ft) typically needs about 120-150 CFM of air for heating and cooling.
Quick estimate: For learning purposes, you can estimate about 1 CFM per square foot for residential spaces. This gives you a starting point to practice calculations.
2. Velocity (FPM - Feet per Minute)
This is how fast the air moves through the duct. Faster air means you can use smaller ducts, but it also creates more noise.
| Space Type | Recommended Velocity | Why? |
|---|---|---|
| Bedrooms | 500-700 FPM | Quiet for sleeping |
| Living areas | 700-900 FPM | Good balance |
| Utility rooms | 900-1200 FPM | Noise less important |
3. Friction Loss
This measures how much the duct "fights" the airflow. Like water pressure dropping through a garden hose. Common values are 0.08 to 0.15 inches of water per 100 feet of duct.
Simple Starting Point
For most residential systems, use 0.1 inches of water per 100 feet. This gives good performance without excessive cost.
Basic Duct Sizing Steps
- Determine the CFM needed
Start with the air requirements for the space you're conditioning. - Choose your approach
Use either the velocity method (easier) or friction method (more precise). - Pick round or rectangular
Round ducts are more efficient, rectangular ducts fit in tight spaces. - Calculate the size
Use formulas or our calculator to find the right dimensions. - Check your answer
Make sure the size makes practical sense.
Simple Sizing Formulas
For Round Ducts (Velocity Method)
Example Calculation
Need: 400 CFM at 800 FPM
Calculation: 13.54 × √(400 ÷ 800) = 13.54 × 0.707 = 9.6 inches
Answer: Use a 10-inch round duct
For Rectangular Ducts
Then choose width and height that give you that area.
Example: Finding Rectangular Size
Need: 0.5 sq ft of area
Options:
- 6" × 12" = 0.5 sq ft ✓
- 8" × 9" = 0.5 sq ft ✓
- 4" × 18" = 0.5 sq ft (too narrow - avoid)
Round vs Rectangular: When to Use Each
Choose Round Ducts When:
- You have plenty of space
- Energy efficiency matters
- You want lower cost
- System operates at higher pressures
Choose Rectangular Ducts When:
- Space is limited (low ceilings)
- You need specific dimensions
- Appearance matters
- Connecting to rectangular equipment
Understanding the Relationships
Instead of memorizing tables, focus on these key relationships:
- More CFM = Bigger Duct (for same velocity)
- Higher Velocity = Smaller Duct (for same CFM)
- Round ducts perform better than rectangular
- Keep rectangular ducts under 4:1 ratio (width to height)
Ready to Practice?
Use our calculator to try different CFM and velocity combinations and see how duct sizes change.
Try the CalculatorCommon Mistakes to Avoid
- Making ducts too small - Better to go one size larger
- Ignoring noise in bedrooms - Keep velocities under 700 FPM
- Forgetting about fittings - Elbows add resistance
- Extreme rectangular ratios - Keep under 4:1
What's Next?
Once you understand these basics, you can learn about:
- Friction loss calculations
- Equivalent diameter for rectangular ducts
- Static pressure and fan selection
- Professional design methods (Manual D)
Key Takeaways
- Start with CFM requirements
- Choose appropriate velocity for the space
- Round ducts are usually better
- Use calculators to speed up the math
- Practice with different scenarios