android - What is the difference between "px", "dip", "dp" and "sp"?

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Top 5 Answer for android - What is the difference between "px", "dip", "dp" and "sp"?

vote vote

97

From the Android Developer Documentation:

  1. px
    > Pixels - corresponds to actual pixels on the screen.

  2. in
    > Inches - based on the physical size of the screen.
    > 1 Inch = 2.54 centimeters

  3. mm
    > Millimeters - based on the physical size of the screen.

  4. pt
    > Points - 1/72 of an inch based on the physical size of the screen.

  5. dp or dip
    > 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".

  6. sp
    > Scaleable Pixels OR scale-independent pixels - this is like the dp unit, but it is also scaled by the user's font size preference. It is recommended you use this unit when specifying font sizes, so they will be adjusted for both the screen density and the user's preference. Note, the Android documentation is inconsistent on what sp actually stands for, one doc says "scale-independent pixels", the other says "scaleable pixels".

From Understanding Density Independence In Android:

Density Bucket Screen Density Physical Size Pixel Size
ldpi 120 dpi 0.5 x 0.5 in 0.5 in * 120 dpi = 60x60 px
mdpi 160 dpi 0.5 x 0.5 in 0.5 in * 160 dpi = 80x80 px
hdpi 240 dpi 0.5 x 0.5 in 0.5 in * 240 dpi = 120x120 px
xhdpi 320 dpi 0.5 x 0.5 in 0.5 in * 320 dpi = 160x160 px
xxhdpi 480 dpi 0.5 x 0.5 in 0.5 in * 480 dpi = 240x240 px
xxxhdpi 640 dpi 0.5 x 0.5 in 0.5 in * 640 dpi = 320x320 px
Unit Description Units Per Physical Inch Density Independent? Same Physical Size On Every Screen?
px Pixels Varies No No
in Inches 1 Yes Yes
mm Millimeters 25.4 Yes Yes
pt Points 72 Yes Yes
dp Density Independent Pixels ~160 Yes No
sp Scale Independent Pixels ~160 Yes No

More info can be also be found in the Google Design Documentation.

vote vote

88

Pretty much everything about this and how to achieve the best support for multiple screens of different sizes and densities is very well documented here:

Screen size
Actual physical size, measured as the screen's diagonal. For simplicity, Android groups all actual screen sizes into four generalized sizes: small, normal, large, and extra-large.

Screen density
The number of pixels within a physical area of the screen; usually referred to as dpi (dots per inch). For example, a "low" density screen has fewer pixels within a given physical area, compared to a "normal" or "high" density screen. For simplicity, Android groups all actual screen densities into six generalized densities: low, medium, high, extra-high, extra-extra-high, and extra-extra-extra-high.

Orientation
The orientation of the screen from the user's point of view. This is either landscape or portrait, meaning that the screen's aspect ratio is either wide or tall, respectively. Be aware that not only do different devices operate in different orientations by default, but the orientation can change at runtime when the user rotates the device.

Resolution
The total number of physical pixels on a screen. When adding support for multiple screens, applications do not work directly with resolution; applications should be concerned only with screen size and density, as specified by the generalized size and density groups.

Density-independent pixel (dp)
A virtual pixel unit that you should use when defining UI layout, to express layout dimensions or position in a density-independent way. The density-independent pixel is equivalent to one physical pixel on a 160 dpi screen, which is the baseline density assumed by the system for a "medium" density screen. At runtime, the system transparently handles any scaling of the dp units, as necessary, based on the actual density of the screen in use. The conversion of dp units to screen pixels is simple: px = dp * (dpi / 160). For example, on a 240 dpi screen, 1 dp equals 1.5 physical pixels. You should always use dp units when defining your application's UI, to ensure proper display of your UI on screens with different densities.

If you are at all serious about developing an Android app for more than one type of device, you should have read the screens support development document at least once. In addition to that, it is always a good thing to know the actual number of active devices that have a particular screen configuration.

vote vote

80

I will elaborate more on how exactly does dp convert to px:

  • If running on an mdpi device, a 150 x 150 px image will take up 150 * 150 dp of screen space.
  • If running on an hdpi device, a 150 x 150 px image will take up 100 * 100 dp of screen space.
  • If running on an xhdpi device, a 150x150 px image will take up 75 * 75 dp of screen space.

The other way around: say, you want to add an image to your application and you need it to fill a 100 * 100 dp control. You'll need to create different size images for supported screen sizes:

  • 100 * 100 px image for mdpi
  • 150 * 150 px image for hdpi
  • 200 * 200 px image for xhdpi
vote vote

63

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 the 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”)  Scale-independent pixels   (“sip” or “sp”)                  Depends on user font size settings    same 
vote vote

60

Moreover you should have a clear understanding of the following concepts:

Screen size:

Actual physical size, measured as the screen's diagonal. For simplicity, Android groups all actual screen sizes into four generalized sizes: small, normal, large, and extra-large.

Screen density:

The number of pixels within a physical area of the screen; usually referred to as dpi (dots per inch). For example, a "low" density screen has fewer pixels within a given physical area, compared to a "normal" or "high" density screen. For simplicity, Android groups all actual screen densities into four generalized densities: low, medium, high, and extra high.

Orientation:

The orientation of the screen from the user's point of view. This is either landscape or portrait, meaning that the screen's aspect ratio is either wide or tall, respectively. Be aware that not only do different devices operate in different orientations by default, but the orientation can change at runtime when the user rotates the device.

Resolution:

The total number of physical pixels on a screen. When adding support for multiple screens, applications do not work directly with resolution; applications should be concerned only with screen size and density, as specified by the generalized size and density groups.

Density-independent pixel (dp):

A virtual pixel unit that you should use when defining UI layout, to express layout dimensions or position in a density-independent way. The density-independent pixel is equivalent to one physical pixel on a 160 dpi screen, which is the baseline density assumed by the system for a "medium" density screen. At runtime, the system transparently handles any scaling of the dp units, as necessary, based on the actual density of the screen in use. The conversion of dp units to screen pixels is simple: px = dp * (dpi / 160). For example, on a 240 dpi screen, 1 dp equals 1.5 physical pixels. You should always use dp units when defining your application's UI, to ensure proper display of your UI on screens with different densities.

Reference: Android developers site

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