About

BMP (Bitmap) files store pixel data uncompressed, producing large file sizes that complicate embedding in HTML, CSS, or JSON payloads. Converting a BMP to Base64 encodes the binary data as an ASCII string using a 6-bit alphabet (64 characters: A - Z, a - z, 0 - 9, +, /). The output is approximately 33% larger than the source because every 3 input bytes map to 4 output characters. This tool performs the conversion entirely in your browser using the native FileReader API. No data leaves your machine.

Incorrect Base64 encoding breaks inline images, corrupts API payloads, and triggers CORS failures in production. A common mistake is omitting the Data URI prefix (data:image/bmp;base64,) when embedding in <img> tags, or including it when the backend expects raw Base64. This converter handles both cases explicitly. Note: BMP files lack compression. For web embedding, consider converting to PNG or WebP first if bandwidth matters. This tool approximates final string size before conversion so you can verify payload limits.

Formulas

The Base64 encoding maps every group of 3 input bytes to 4 output ASCII characters. The output length is computed as:

L_out = 4 ⋅ ceil(L_in3)

Where L_in = file size in bytes, L_out = Base64 string length in characters. The size overhead ratio is:

R = L_outL_in ≈ 1.333

This means a 1MB BMP file produces approximately 1.33MB of Base64 text. When the Data URI prefix data:image/bmp;base64, (22 characters) is included, the total becomes L_out + 22. BMP file validity is confirmed by checking the first 2 bytes equal 0x42 0x4D (ASCII "BM"). The pixel data offset is read from bytes 10 - 13 as a 32-bit little-endian integer.

Reference Data

Property	BMP Format Detail	Base64 Impact
Magic Bytes	0x42 0x4D ("BM")	Validated before conversion
Bit Depth: 1-bit	Monochrome, palette-indexed	Smallest output per pixel
Bit Depth: 4-bit	16 colors, palette-indexed	~33% larger than source
Bit Depth: 8-bit	256 colors, palette-indexed	~33% larger than source
Bit Depth: 16-bit	High color, 65,536 colors	Moderate Base64 size
Bit Depth: 24-bit	True color, 16.7M colors	Large Base64 output
Bit Depth: 32-bit	True color + alpha channel	Largest Base64 output
Compression: BI_RGB	No compression (most common)	Direct byte-to-Base64 mapping
Compression: BI_RLE8	Run-length encoding, 8-bit	Smaller source, smaller output
Compression: BI_RLE4	Run-length encoding, 4-bit	Smaller source, smaller output
Max File Size (practical)	~50MB in-browser	~66.7MB Base64 string
MIME Type	image/bmp or image/x-bmp	Used in Data URI prefix
Data URI Prefix	N/A	data:image/bmp;base64,
Base64 Alphabet	N/A	A-Z a-z 0-9 + / = padding
Encoding Ratio	3 bytes input	4 characters output
Padding	BMP rows padded to 4-byte boundary	= or == appended if needed
Header Size (BITMAPINFOHEADER)	54 bytes minimum	72 Base64 characters
Endianness	Little-endian	Preserved in Base64 encoding
Color Space (v5)	sRGB / calibrated RGB	Metadata preserved in binary
Browser Support	Universal (all browsers)	FileReader API: IE10+

Frequently Asked Questions

Base64 encoding uses 6 bits per character to represent data, while the source binary uses 8 bits per byte. Every 3 input bytes (24 bits) are split into 4 groups of 6 bits, producing 4 output characters. The ratio is 4÷3 ≈ 1.333, yielding a 33.3% size increase. If your BMP is 3MB, expect approximately 4MB of Base64 text.

Include the prefix (data:image/bmp;base64,) when embedding directly in HTML <img src> attributes, CSS background-image: url() declarations, or Markdown image syntax. Use raw Base64 (without the prefix) when sending binary payloads to REST APIs, storing in database BLOB fields, or when the receiving system adds its own MIME header. Mixing these up is the most common cause of broken inline images.

No. The conversion reads the raw bytes of the BMP file and encodes them directly to Base64. No pixel manipulation, color space conversion, or resampling occurs. The decoded Base64 output will produce a byte-identical copy of the original file. This is a lossless encoding of the binary, not an image transformation.

The practical limit is approximately 50MB, constrained by browser memory allocation for the FileReader API and the resulting Base64 string (which will be ~66.7MB). Files beyond this may cause tab crashes on devices with limited RAM. For BMP files exceeding 50MB, use a server-side tool or command-line utility like OpenSSL (openssl base64 -in file.bmp).

The tool reads the first 2 bytes of the file as an ArrayBuffer and checks for the BMP magic number: 0x42 0x4D (ASCII characters "B" and "M"). This is the same check used by image libraries and operating systems. If the magic bytes do not match, the conversion is rejected with an error message, preventing corrupt or misnamed files from producing invalid Base64 output.

Yes, but with caveats. In CSS custom properties, the string must be wrapped correctly: --bg: url(data:image/bmp;base64,...). In JavaScript template literals, Base64 is safe because the alphabet (A-Z a-z 0-9 + / =) contains no backtick or ${ sequences. However, very large strings (over 1MB) may cause JavaScript parser slowdowns in some bundlers. Consider loading from a separate file if performance is critical.

Some systems (MIME email, PEM certificates, certain legacy APIs) require Base64 to be wrapped at 76 characters per line per RFC 2045. Other systems (JSON, Data URIs, modern web APIs) require a single continuous string with no line breaks. Inserting line breaks where they are not expected corrupts the data. This tool defaults to no wrapping for web use but offers configurable wrapping at 76 or 64 character widths for email/PEM compatibility.