A set of six generalized densities:

• ldpi (low) ~120dpi
• mdpi (medium) ~160dpi
• hdpi (high) ~240dpi
• xhdpi (extra-high) ~320dpi
• xxhdpi (extra-extra-high) ~480dpi
• xxxhdpi (extra-extra-extra-high) ~640dpi

• xlarge screens are at least 960dp x 720dp
• large screens are at least 640dp x 480dp
• normal screens are at least 470dp x 320dp
• small screens are at least 426dp x 320dp

l : m : h : xh : xxh : xxh = 2 : 3 : 4 : 6 : 8;

比如mdpi对应图片为48*48px,则hdpi为72*72px,xhdpi为96*96px,xxhdpi为144*144px;xxxdpi为196*196px;

参考 ：http://developer.android.com/guide/practices/screens_support.html

屏幕密度与分辨率