Enter the pipe diameter (in), the length of the pipe (ft), and the purge rate (cfh) into the Purge Time Calculator. The calculator will evaluate the Purge Time. 

Purge Time Formula

The following formula is used to calculate the Purge Time. 

PT = pi*D/12*L/PR*60


  • PT is the Purge Time (minutes)
  • D is the pipe diameter (in) 
  • L is the length of the pipe (ft) 
  • PR is the purge rate (cfh,ft^3/hr) 

To calculate the purge time, multiply the diameter by pi, multiply again by the length, divide by the purge rate, then finally multiply by 5.

How to Calculate Purge Time?

The following steps outline how to calculate the Purge Time.

  1. First, determine the pipe diameter (in).
  2. Next, determine the length of the pipe (ft). 
  3. Next, determine the purge rate (cfh). 
  4. Next, gather the formula from above = PT = pi*D/12*L/PR*60.
  5. Finally, calculate the Purge Time.
  6. 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.

pipe diameter (in) = 4

length of the pipe (ft) = 100

purge rate (cfh) = 2.5

PT = pi*D/12*L/PR*60 =  ?