What is the difference between BITMAPFILEHEADER and BITMAPINFOHEADER?

BITMAPFILEHEADER is the first 14 bytes of a BMP — the BM signature, total file size and the offset where pixel data begins. BITMAPINFOHEADER (usually the next 40 bytes) describes the image itself: width, height, color planes, bit depth, compression and resolution.

Why is the BMP height sometimes negative?

A positive height means the rows are stored bottom-up (the standard for BMP). A negative height means top-down row order. The inspector reports the sign and shows the absolute pixel height.

How does an ICO store multiple sizes?

An .ico begins with a 6-byte header (reserved, type=1, image count) followed by one 16-byte directory entry per image. Each entry gives the width, height, color count, bit depth and the byte offset and size of that image, so a single file holds many resolutions.

What do the BMP compression codes mean?

0 is BI_RGB (uncompressed), 1 is BI_RLE8, 2 is BI_RLE4, 3 is BI_BITFIELDS (custom channel masks, common for 16 and 32-bit), 4 is BI_JPEG and 5 is BI_PNG. The inspector labels the code it reads from the header.

Is my image uploaded anywhere?

No. All parsing happens in your browser with FileReader and DataView. No bytes are transmitted, so the inspector works offline and keeps your files private.

What is the BMP / ICO File Inspector?

Free BMP and ICO inspector that decodes the BITMAPFILEHEADER and BITMAPINFOHEADER of a .bmp, or the icon directory of a multi-resolution .ico, showing width, height, bit depth, compression and per-image entries. Runs locally with DataView, no upload. It runs free in your browser on Gera Tools, with nothing uploaded.

BMP / ICO File Inspector

Name: BMP / ICO File Inspector
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Decode the binary header of a Windows bitmap (.bmp) or icon (.ico) file — its dimensions, bit depth, compression and, for icons, every resolution packed inside. The inspector reads the BITMAPFILEHEADER, BITMAPINFOHEADER and ICO directory directly with the browser’s DataView API, entirely on your device. No file is uploaded; everything happens in your browser’s memory.

When to use this tool

You have a .bmp file from a legacy scanner, imaging device, or old graphics pipeline and need to confirm its bit depth or compression mode before converting it.
A favicon .ico looks correct in one browser but broken in another and you want to verify whether the 16×16 or 48×48 variant is actually present in the file.
You are debugging a custom BMP writer and need to audit the exact bytes your code produces.
You are checking whether an ICO’s large-size entry is PNG-compressed (modern) or an uncompressed DIB (legacy).

How it works

BMP structure

A BMP starts with a 14-byte BITMAPFILEHEADER:

bytes 0-1  : "BM" signature
bytes 2-5  : total file size (LE)
bytes 10-13: offset to pixel data

then a 40-byte BITMAPINFOHEADER (the common DIB header):

biSize         (4)  header size, usually 40
biWidth        (4)  pixel width (signed)
biHeight       (4)  pixel height (signed; negative = top-down)
biPlanes       (2)  color planes, always 1
biBitCount     (2)  bits per pixel: 1, 4, 8, 16, 24, 32
biCompression  (4)  0=RGB, 1=RLE8, 2=RLE4, 3=BITFIELDS, 4=JPEG, 5=PNG
biSizeImage    (4)  pixel data size in bytes
biClrUsed      (4)  palette entries used

ICO structure

An icon file begins with a 6-byte header: a reserved word, a type word (1 = icon, 2 = cursor) and the image count. Each image then has a 16-byte directory entry:

width  (1)  0 means 256
height (1)  0 means 256
colors (1)  palette size, 0 if ≥256
reserved(1)
planes (2)
bitCount(2)
bytesInRes(4)  size of this image's data
imageOffset(4) where its data starts

The inspector walks the directory and lists every entry, so you can confirm a favicon really contains 16×16, 32×32 and 48×48 variants.

Worked example — reading a 24-bit BMP

Imagine a 100×80 pixel, 24-bit uncompressed image. The inspector would show:

Field	Value	What it means
Signature	BM	Valid Windows bitmap
File size	24,054 bytes	14 + 40 + (100 × 80 × 3) + row padding
biWidth	100	100 pixels wide
biHeight	80	80 pixels tall, stored bottom-up
biBitCount	24	3 bytes per pixel, RGB, no palette
biCompression	0 (RGB)	Uncompressed
biSizeImage	24,000	Pixel data bytes (rows padded to 4-byte boundaries)

A negative biHeight would mean the rows are stored top-down instead — many screen-capture tools write top-down BMPs that some older programs cannot read.

Tips and edge cases

A 32-bit BMP with compression 3 (BITFIELDS) carries an alpha channel via custom channel masks — check the mask values if the transparency isn’t loading correctly.
ICO entries whose bytesInRes starts with the PNG signature (89 50 4E 47) are PNG-compressed icons, common for 256×256 sizes — modern browsers handle these but some older frameworks do not.
If biSizeImage is 0, that is legal for uncompressed images; the actual size can be computed from width, height, bit depth, and row padding (each row rounds up to the next 4-byte boundary).
A cursor .ico (type word = 2) uses the same directory structure; the inspector reports the type.
For very old BMPs, biSize may be 12 (BITMAPCOREHEADER format) rather than 40; the inspector reports the header version so you know which fields are valid.