What measurement should I take to buy an atmospheric filter for my espresso machine? I want to buy an atmospheric filter for my espresso machine, but I don't know what part of the filter I should measure. Should I be measuring the radius from the internal part of the border of the filter or the external part of the ring/border?

Sorry, I tried to express myself as well as I could but I am not familiar with some coffee terminology in English.
The portafilter sizes are given as the inner diameter of the basket in mm. This is the same as the diameter a tight fitting tamper would have.