Duct Velocity Calculator
The Duct Velocity Calculator figures out how fast air travels through a duct. It uses the airflow rate and duct size to give you the velocity in feet per minute or meters per second. This is great for real-life decisions, like designing ventilation, picking fans, or ensuring air reaches where it’s needed.
This calculator keeps your system running smoothly by making sure air moves at the right speed—not too slow or too fast. It’s reliable for important tasks, like improving air quality or saving energy. Want to know how it’s calculated? Let’s check out the formula next.
Formula for Duct Velocity
The formula for finding duct velocity is simple:
V = Q / A
Where:
- V = Air velocity (feet per minute or meters per second)
- Q = Airflow rate (cubic feet per minute or cubic meters per second)
- A = Duct cross-sectional area (square feet or square meters)
Area for Circular Ducts
For round ducts:
A = π × (D/2)² = π × D²/4
Where:
- D = Duct diameter (feet or meters)
- π = 3.14159
Area for Rectangular Ducts
For rectangular ducts:
A = W × H
Where:
- W = Duct width (feet or meters)
- H = Duct height (feet or meters)
This formula comes from basic airflow science. Use consistent units—like feet and minutes or meters and seconds—for accurate results. Now, let’s make it easier with a table.
Quick Reference Table for Duct Velocity
Why calculate every time? This table shows velocities for common duct sizes and airflow rates. It’s a fast way to check without doing math.
| Duct Type | Size (ft) | Airflow (CFM) | Velocity (FPM) |
|---|---|---|---|
| Circular | 1 (diameter) | 500 | 637 |
| Circular | 0.5 (diameter) | 200 | 1,019 |
| Rectangular | 1×0.5 | 300 | 600 |
| Rectangular | 2×1 | 1,000 | 500 |
How to Use the Table
- Pick your duct type and size.
- Find your airflow rate.
- Check the velocity in feet per minute.
This table helps with searches like “duct velocity for 500 CFM.” For exact results, use the formula. Next, let’s try an example.
Example: Calculating Duct Velocity
Suppose you have a circular duct with a 1-foot diameter and an airflow rate of 800 CFM. You want to know the velocity. Here’s how to do it:
- Calculate the area:
A = π × D²/4
A = 3.14159 × 1²/4 = 3.14159 × 0.25 ≈ 0.785 ft² - Plug into the formula:
V = Q / A
V = 800 / 0.785 ≈ 1,019 FPM
So, the air velocity is about 1,019 feet per minute. This matches engineering standards and helps you plan your system.
Most Common FAQs
Velocity affects how well air moves—too slow wastes energy, too fast can get noisy.
For most ducts, 400-1,200 FPM works well—check your system’s needs.
Yes, just use the right area formula—π × D²/4 for round, W × H for rectangular.