BMP Sizing Data

What is a BMP file?

A Bitmap (BMP) file is a raster graphics image format widely used on Windows platforms. It stores pixel data in an uncompressed or lightly compressed form, making it simple to read but often large in size. Because the format is straightforward, it is a popular teaching example for imageprocessing concepts.

How to calculate BMP file size

The total file size is the sum of the file header, the DIB header (also called the bitmap info header), optional color tables, and the pixel array.

General formula:

FileSize = HeaderSize + DIBHeaderSize + PaletteSize + PixelArraySize

Where:

• HeaderSize 14 bytes (the BMP file header).
• DIBHeaderSize varies (most common is 40 bytes for BITMAPINFOHEADER).
• PaletteSize 0 for truecolor images; otherwise 4(2^bpp) bytes.
• PixelArraySize calculated per scan line, padded to a 4byte boundary.

The pixel array size is the most critical part, because BMP stores each row (scan line) aligned on a 32bit boundary. The formula for a single scan line:

RowSize = floor((bitsPerPixel * width + 31) / 32) * 4

Then:

PixelArraySize = RowSize * height

Header details

When bitsPerPixel is 24 or 32, the color table is omitted. For 1, 4, and 8bpp images the palette must be present, adding 4(2^bpp) bytes.

Pixel data organization

• Scan line direction: By default BMP stores rows from bottom to top. Setting a negative height makes the image topdown.
• Padding: Each scan line ends on a 4byte boundary. The padding bytes are not part of the image content and may contain any value.
• Color ordering: For 24bpp images each pixel is stored as B, G, R (blue first). For 32bpp an extra alpha byte follows (often unused).

Practical examples

RowSize = floor((24 * 800 + 31) / 32) * 4         = floor((19200 + 31) / 32) * 4         = floor(19231 / 32) * 4         = floor(600.96875) * 4         = 600 * 4 = 2400 bytes

PixelArraySize = 2400 * 600 = 1440000 bytes

FileSize = 14 (file header) + 40 (DIB) + 0 (palette) + 1440000 = 1440054 bytes

RowSize = floor((8 * 128 + 31) / 32) * 4         = floor((1024 + 31) / 32) * 4         = floor(1055 / 32) * 4         = floor(32.96875) * 4         = 32 * 4 = 128 bytes

PixelArraySize = 128 * 128 = 16384 bytes

FileSize = 14 + 40 + 1024 + 16384 = 17462 bytes

RowSize = 1024 * 4 = 4096 bytes

PixelArraySize = 4096 * 768 = 3145728 bytes

FileSize = 14 + 40 + 3145728 = 3145782 bytes

Best practices for managing BMP size

• Choose the smallest adequate bit depth. If the image contains only a few colors, use 1, 4, or 8bpp with a palette.
• Prefer topdown orientation. Using a negative height removes the need for later vertical flipping in many APIs.
• Take advantage of compression. The BMP format supports RLE compression for 4 and 8bpp images (BI_RLE4, BI_RLE8). Although not as common as PNG, it can cut size dramatically for simple graphics.
• Trim unused padding. When generating BMPs programmatically, calculate the padded row size correctly; writing excess padding bytes will make the file larger without visual benefit.
• Store metadata separately. BMP does not embed EXIF or ICC profiles. If you need such data, keep it in a sidecar file or switch to a format that supports embedded metadata.