On testing application by QA Eng it is not that every-times all the defect is get raised or find by QA Eng , sometimes defects get slip out from the QA end and it is raised by the customer or the end user .
Defect seepage is the ratio of post delivery defect against the total defects . To find the defect seepage we apply the following formula :

  Total Number of post delivery defect
  ---------------------------------------------- * 100
  Total Defect

  Example :
  Defect find while testing : 15
  Defect find post delivery : 3

  so according to formula defect seepage is :
      3
  ----------- * 100 = 16.67 %
  (15+3)