❄️ Duct Sizing Calculator
Calculate HVAC duct dimensions based on CFM and Velocity
📊 Quick Reference: CFM to Duct Size (Round Metal)
| Duct Diameter | Max CFM (Supply) | Max CFM (Return) |
|---|---|---|
| 4 inch | 30-40 | 20-25 |
| 5 inch | 50-60 | 40-50 |
| 6 inch | 85-110 | 60-80 |
| 7 inch | 125-150 | 90-110 |
| 8 inch | 180-210 | 140-160 |
Based on 0.1" WC friction rate. Flex duct handles ~10-15% less airflow than rigid metal.
"The biggest mistake I see in DIY ductwork is ignoring the 'Equivalent Length' of fittings. A 90-degree elbow isn't just 6 inches of duct—it creates as much resistance as 10 to 35 feet of straight pipe! If you have a run with four elbows, you need to size that duct much larger than a straight run. Also, never trust the 'rated' CFM of flex duct unless it's pulled tight and straight. Sagging flex duct kills airflow."
📐 Understanding Duct Sizing Principles
Proper duct sizing balances airflow (CFM), velocity (FPM), and friction loss (Static Pressure). The goal is to deliver the right amount of air without excessive noise or energy waste.
Key Concepts
- CFM (Cubic Feet per Minute): The volume of air moving through the duct. 1 ton of AC typically requires 400 CFM.
- Velocity (FPM): The speed of the air. High velocity = noise. Low velocity = poor mixing.
- Static Pressure: The resistance to airflow. Standard residential design is 0.5" to 0.8" WC total external static pressure.
Recommended Velocity Limits (ACCA Manual D)
- Main Supply Trunk: 700 - 900 FPM
- Supply Branches: 600 - 700 FPM
- Return Air Trunk: 600 - 700 FPM
- Return Air Grilles: 400 - 600 FPM (Keep low to reduce noise)
⚠️ Common Mistakes
1. Undersizing Return Air: Most systems are starved for air. If you have a 3-ton unit (1200 CFM), you need enough return grille area to handle 1200 CFM quietly.
2. Using "Rule of Thumb" Sizing: "6-inch for everything" is a recipe for hot/cold spots. A small bedroom needs 80 CFM (5-inch), while a master suite might need 250 CFM (8-inch or two 6-inch).
3. Ignoring Flex Duct Compression: Squeezing a 12-inch flex duct through a 10-inch truss space restricts airflow significantly more than a 10-inch duct would.
4. Too Many Bends: Air hates to turn. Every turn adds resistance. Use long-sweep elbows where possible.
❓ FAQ
How many CFM is a 1-ton AC?
Standard rule is 400 CFM per ton of cooling. A 3-ton unit needs 1200 CFM.
What happens if ducts are too big?
Velocity drops too low. Air may not "throw" far enough into the room to mix properly, and ducts gain/lose more heat due to slower travel time. It also costs more and wastes space.
Can I use flex duct for everything?
Technically yes, but rigid metal is better for main trunks to maintain pressure. Flex is best for the last 5-10 feet to branches to reduce noise.