How to use
- Paste the Base64 string or the data:image/bmp;base64 URI. PNG, JPEG, WebP and other formats are accepted too.
- Check the preview, the dimensions and the decoded size.
- BMP is preselected as the download format. Choose Original instead if the data is already a BMP and you want it unchanged.
- Download the .bmp file.
Spotting a bitmap in Base64
Windows bitmap files begin with the two letters BM, which encode to Qk at the very start of the Base64 string. The header that follows stores the file size, the offset of the pixel data, the dimensions and the bit depth. BMP data shows up in older Windows applications, clipboard and screenshot exports, point-of-sale and industrial systems, microcontroller display tools and test fixtures. The MIME type is usually image/bmp, although image/x-ms-bmp is also seen.
Browsers display the common BMP variants, but very old OS/2 bitmaps and rare compression modes may not render in every browser. If the preview stays empty although the string starts with Qk, download it as Original and open it in a desktop image viewer.
Why BMP files are so large
The converter writes uncompressed 32-bit pixels, four bytes for every pixel, so the file size depends only on the dimensions and not on the content:
| Dimensions | BMP size | As Base64 |
|---|---|---|
| 640 × 480 | about 1.2 MB | about 1.6 MB |
| 1920 × 1080 | about 8.3 MB | about 11 MB |
| 3840 × 2160 | about 33 MB | about 44 MB |
The same screenshot as a PNG is often ten times smaller. Use BMP only when the receiving software requires it.
About the 32-bit output
The output keeps an alpha channel, so transparent areas stay transparent in programs that read it. Some older programs ignore the alpha channel and may show those areas as black or white. If that happens, remove the transparency from the source image before converting.
To decode a Base64 BMP on Windows without a browser, PowerShell is enough:
$b64 = Get-Content .\image.txt -Raw
[IO.File]::WriteAllBytes("$PWD\image.bmp", [Convert]::FromBase64String($b64))Frequently asked questions
Why is the BMP so much bigger than the original?
BMP output is uncompressed and uses four bytes per pixel. A compressed PNG or JPEG of the same image is usually far smaller.
Does BMP support transparency?
Yes, the 32-bit BMP files this tool creates include an alpha channel. Whether the transparency is displayed depends on the program that opens the file.
How can I tell that a Base64 string is a BMP?
Qk, the Base64 form of the letters BM. The tool confirms the format by reading the header bytes.