The Duct Leakage Calculator measures how much air leaks out of a duct system. It tells you the leakage flow rate in cubic feet per minute (CFM) and can show it as a percentage of total airflow. You use this to check if your ducts are tight or need fixing. It’s great for real-life decisions, like saving energy, improving air quality, or cutting heating and cooling costs.
This calculator looks at duct pressure and construction quality to give you a clear number. It’s reliable for important tasks, like making sure your HVAC runs smoothly. Ready to see how it’s calculated? Let’s check out the formula next.
Formula for Duct Leakage
The formula for finding duct leakage is simple:
Where:
- Q = Leakage flow rate (CFM)
- C = Leakage coefficient (depends on duct quality)
- P = Static pressure in the duct (inches of water column)
Leakage Percentage
To find the percentage of air lost:
Leakage (%) = (Q / Qtotal) × 100
Where:
- Qtotal = Total system flow rate (CFM)
The leakage coefficient (C) varies by duct type—tight ducts have lower C values (like 5-10), while leaky ones have higher values (like 20-30). These come from HVAC standards. Now, let’s make it easier with a table.
Quick Reference Table for Duct Leakage
Why calculate every time? This table shows leakage rates for common pressures and duct qualities. It’s a fast way to check without doing math.
| Pressure (in. w.c.) | Duct Quality | C Value | Leakage (CFM) | Total Flow (CFM) | Leakage (%) |
|---|---|---|---|---|---|
| 0.5 | Tight | 5 | 3.54 | 500 | 0.71 |
| 1.0 | Average | 15 | 15 | 1,000 | 1.5 |
| 2.0 | Leaky | 25 | 35.36 | 800 | 4.42 |
How to Use the Table
- Pick your duct pressure and quality.
- Check the leakage rate and percentage.
- Use it to plan fixes or upgrades.
This table helps with searches like “duct leakage at 1 inch pressure.” For exact results, use the formula. Next, let’s try an example.
Example of Duct Leakage Calculator
Suppose your duct has a static pressure of 1.5 inches of water column, and it’s average quality with a C value of 15. The total airflow is 600 CFM. You want to know the leakage. Here’s how to do it:
- Plug into the formula:
Q = C × P^0.6
Q = 15 × 1.5^0.6 - Calculate step-by-step:
- 1.5^0.6 ≈ 1.24
- Q = 15 × 1.24 ≈ 18.6 CFM
- Find the percentage:
Leakage (%) = (Q / Qtotal) × 100
Leakage (%) = (18.6 / 600) × 100 ≈ 3.1%
So, the leakage is about 18.6 CFM, or 3.1% of the total flow. This fits HVAC norms and helps you decide if sealing is needed.
Most Common FAQs
Leaks waste air, raise energy bills, and make heating or cooling less effective.
Less than 5% is good for most systems—higher means you should fix the ducts.
Check duct quality—tight ducts have low C (5-10), average are 15-20, and leaky are 25+.