Ductwork Calculator
HVAC & ClimateCalculate HVAC duct size, diameter, and airflow (CFM) for round and rectangular ductwork. Get friction loss estimates and proper duct sizing for your system.
Enter Airflow & Velocity
Duct Sizing Results
Created by: ToolsWonder Team
Last Updated: 7/10/2026
What is CFM?
CFM (Cubic Feet per Minute) measures the volume of air that flows through your HVAC system. It's the primary metric for sizing ducts, fans, and air handlers. Higher CFM means more air circulation for larger spaces or more aggressive heating/cooling.
A typical residential home requires 300-1,200 CFM depending on square footage and climate. Commercial HVAC systems can handle 5,000+ CFM. Proper ductwork sizing ensures your system delivers the designed CFM without excessive noise or energy loss.
Duct Sizing Guide
Undersized ducts restrict airflow and reduce system efficiency. Oversized ducts waste space and money. The right size balances airflow velocity, noise, and system performance.
| Velocity (FPM) | Application | Noise Level | Duct Size |
|---|---|---|---|
| 600 FPM | Quiet residential (bedrooms, libraries) | Very Low | Larger |
| 800 FPM | Standard residential (common spaces) | Low | Standard |
| 1,000 FPM | Commercial / High-capacity systems | Medium | Compact |
| 1,200+ FPM | High-velocity systems (tight spaces) | High | Very Compact |
How We Calculate Duct Size
Step 1: Calculate Required Area
We divide CFM by velocity (FPM) to get the cross-sectional area needed. Formula: Area (sq in) = (CFM ÷ Velocity) × 144
Step 2: Convert to Duct Diameter/Size
For round ducts, we use the area to calculate diameter. For rectangular, we calculate width and height. You then round to the nearest standard size (4", 5", 6", 8", etc.)
Step 3: Estimate Friction Loss
Friction loss represents pressure drop in the duct. Higher velocity and smaller ducts increase friction loss. Rougher ducts (fiberglass) have more loss than smooth metal ducts.
Standard Ductwork Sizes
| Round Duct Diameter | Area (sq in) | Typical Use |
|---|---|---|
| 4" | 12.6 | Small supply branches |
| 6" | 28.3 | Main supply ducts |
| 8" | 50.3 | Medium systems |
| 10" | 78.5 | Large residential |
| 12" | 113 | Commercial systems |
Duct Material Comparison
| Material | Durability | Friction Loss | Cost |
|---|---|---|---|
| Galvanized Steel | Excellent | Low (smooth) | Medium |
| Fiberglass Lined | Good | Medium (rough) | Low |
| Aluminum | Good | Low (smooth) | High |
| Flex Duct | Fair | High (wrinkled) | Low |
How to Use This Calculator
- Enter your HVAC system's CFM (find this on your system specs)
- Choose desired velocity (800 FPM typical for quiet residential)
- Select duct type (round or rectangular)
- Get recommended duct size and friction loss estimate
- Round up to the nearest standard duct size available
- Consult an HVAC professional for final design and installation
Frequently Asked Questions
What is CFM in HVAC?
CFM stands for Cubic Feet per Minute. It measures the volume of air flowing through your ductwork. Typical residential HVAC systems handle 400-1,200 CFM depending on home size and climate. Higher CFM means more cooling/heating capacity.
What's the difference between round and rectangular ductwork?
Round ducts are more efficient, require less material, and have lower friction loss. Rectangular ducts are easier to fit in walls and ceilings, making them common in residential construction. For the same CFM, rectangular ducts are typically taller and narrower.
What velocity should I use?
Residential systems typically use 600-800 FPM (feet per minute) to minimize noise. Commercial systems may use 1,000-1,200 FPM for compact ductwork. Higher velocity increases friction loss and noise. Lower velocity requires larger ducts but operates quieter.
What is friction loss in ductwork?
Friction loss is the pressure drop as air moves through ducts. Measured in inches of water column per 100 feet of duct. Higher friction loss requires a more powerful fan to maintain airflow. Smooth, larger ducts reduce friction loss. This calculator provides an approximation.
How do I calculate duct size for my HVAC system?
Divide your system's CFM by the desired velocity and multiply by 144 to get cross-sectional area in square inches. For round ducts, convert area to diameter. For rectangular, calculate width and height. Our calculator does this automatically—just enter CFM and velocity.
What are standard ductwork sizes?
Common round duct diameters: 4", 5", 6", 7", 8", 10", 12", 14", 16". Rectangular ducts come in widths/heights like 3×4", 4×8", 6×8", 8×10". Always round up to the nearest standard size for proper airflow.