Enter the mean free path length and the characteristic length into the Knudsen Number Calculator. The calculator will evaluate the Knudsen Number. 

Knudsen Number Formula

The following two example problems outline the steps and information needed to calculate the Knudsen Number.

Kn = FPL / CL 


  • Kn is the Knudsen Number ( )
  • FPL is the mean free path length 
  • Cl is the characteristic length 

How to Calculate Knudsen Number?

The following steps outline how to calculate the Knudsen Number.

  1. First, determine the mean free path length. 
  2. Next, determine the characteristic length. 
  3. Next, gather the formula from above = Kn = FPL / CL .
  4. Finally, calculate the Knudsen Number.
  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.

mean free path length = 2

characteristic length = 5

Kn = FPL / CL  = ?