Bitmap file header size

Question

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!!

Padding for alignment? Use SizeOf instead of calculating it yourself. (Also, it helps if you mention the programming language you're asking about; it makes it much easier to provide an answer.) — Ken White
– Ken White, Commented Sep 9, 2012 at 6:25
@nneonneo: "I'm a newbie in programming bmp files" is the opening sentence of the question. — Ken White
– Ken White, Commented Sep 9, 2012 at 6:30

nneonneo · Accepted Answer · 2012-09-09 06:25:59Z

16

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.

answered Sep 9, 2012 at 6:25

nneonneo

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2 Comments

user1657743 Over a year ago

Hi, Thanks a lot for your answer. That makes sense... but in my program, i have a matrix of 10x10 pixels, each pixel is RGB. So i have an extra of 20 bytes of raster data to calculate ? I'm a little bit confused ... thanks again for your help!

nneonneo Over a year ago

@user1657743: you don't have to put anything specific in the padding; it's just garbage bytes that nobody (except steganographers) will see.

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