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In typical [[image compression|uncompressed]] bitmaps, image [[pixel]]s are generally stored with a variable number of bits per pixel which identify its color (the [[color depth]]). Pixels of 8 bits and fewer can represent either [[grayscale]] or [[indexed color]]. An [[alpha channel]] (for [[transparency (graphic)|transparency]]) may be stored in a separate bitmap, where it is similar to a grayscale bitmap, or in a fourth channel that, for example, converts 24-bit images to 32 bits per pixel.
The bits representing the bitmap pixels may be [[packed]] or unpacked (spaced out to byte or word boundaries), depending on the format or device requirements. Depending on the color depth, a pixel in the picture will occupy at least ''n''/8 bytes, where {{Var|n}} is the bit depth.
For an uncompressed, packed-within-rows bitmap, such as is stored in Microsoft DIB or [[BMP file format]], or in uncompressed [[TIFF]] format, a lower bound on storage size for a ''n''-bit-per-pixel (2<sup>n</sup> colors) bitmap, in [[byte]]s, can be calculated as:
:<math>\text{size} = \text{width} \cdot \text{height} \cdot n/8</math>
where width and height are given in pixels.
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{{Main|BMP file format}}
[[Microsoft]] has defined a particular representation of color bitmaps of different [[color depth]]s, as an aid to exchanging bitmaps between devices and applications with a variety of internal representations. They called these device-independent bitmaps "DIBs", and the file format for them is called DIB file format or [[BMP file format]]. According to Microsoft support:<ref name=DIBhelp>{{cite web |url= http://support.microsoft.com/kb/q81498/ |title= DIBs and Their Uses |work= Microsoft Help and Support |date= 2005-02-11}}</ref>
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