Enter the duct design flow (CFM) and the allowable leakage percent (%) into the Duct Leakage Calculator. The calculator will evaluate the Duct Leakage. 

Duct Leakage Formula

The following two example problems outline the steps and information needed to calculate the Duct Leakage.

DL = DF * AL / 100


  • DL is the Duct Leakage (CFM)
  • DF is the duct design flow (CFM) 
  • AL is the allowable leakage percent (%) 

To calculate the duct leakage, divide the duct design flow by the allowable leakage percentage over 100.

How to Calculate Duct Leakage?

The following steps outline how to calculate the Duct Leakage.

  1. First, determine the duct design flow (CFM). 
  2. Next, determine the allowable leakage percent (%). 
  3. Next, gather the formula from above = DL = DF * AL / 100.
  4. Finally, calculate the Duct Leakage.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

duct design flow (CFM) = 500

allowable leakage percent (%) = 3

DL = DF * AL / 100 = ?