Even after Google's effort to clearly document how to create assets, every Android developer will end up getting the wrong asset from time to time. So here is a dead simple and definitive answer.

    
      MDPI
      1x
    
      HDPI
      1.5x
    
      XHDPI
      2x
    
      XXHDPI
      3x
    
      XXXHDPI
      4x

TIP: By designing your assets based on a multiple of 2 you are all but guaranteed a clean scale at the various sizes. 13px at MDPI would be 19.5px in HDPI which could lead to ugly subpixel rendering.

MDPI	1x
HDPI	1.5x
XHDPI	2x
XXHDPI	3x
XXXHDPI	4x