px - Pixels - point per scale corresponds to actual pixels on the screen.
i - Inches - based on the physical size of the screen.
mm - Millimeters - based on the physical size of the screen.
pt - Points - 1/72 of an inch based on the physical size of the screen.
dp - Density-independent Pixels - an abstract unit that is based on the physical density of the screen. These units are relative to a 160 dpi screen, so one dp is one pixel on a 160 dpi screen. The ratio of dp-to-pixel will change with the screen density, but not necessarily in direct proportion. Note: The compiler accepts both dip and dp, though dp is more consistent with sp.
sp - scalable pixels - this is like the dp unit, but it is also scaled by the user's font size preference. It is recommended that you use this unit when specifying font sizes, so they will be adjusted for both the screen density and user's preference.
Take the example of two screens that are the same size but one has a screen density of 160 dpi (dots per inch, i.e. pixels per inch) and the other is 240 dpi.
Lower resolution screen Higher resolution, same size
Physical Width 1.5 inches 1.5 inches
Dots Per Inch (“dpi”) 160 240
Pixels (=width*dpi) 240 360
Density (factor of baseline 160) 1.0 1.5
Density-independent pixels 240 240
(“dip” or “dp” or “dps”)
(“sip” or “sp”) Depends on user font size settings same