Bitmap file header size

file-format bmp
user1657743 picture user1657743 · Sep 9, 2012 · Viewed 20.3k times · Source

I'm a newbie in programming bmp files and i checked this web site to learn about bmp header.. http://www.daubnet.com/en/file-format-bmp

it seems that the header of a bmp file is 54 bytes.

Using paint, i created a simple 10x10 image, and i saved it in 24 bits. so according to simple math, the file size should be 10*10*3 + 54 = 354 bytes.

but hex editor and file explorer returned a size of 374 bytes.

So i have a difference of 20 bytes, and i don't know why.

could you tell me why please?

thanks a lot!!

Answer

nneonneo picture nneonneo · Sep 9, 2012

Lines in BMPs are padded out to multiples of 4 bytes.

Without padding, each line is 3*10 = 30 bytes. With padding, each line is 32 bytes, so the image data is 320 bytes in size. Thus, the file size is 54+320 = 374 bytes.

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