BMP is uncompressed, so expect a big file
BMP is Microsoft's Windows bitmap format. A standard 24-bit BMP stores three bytes for every pixel and applies no compression, so a 1920x1080 image is about 6 MB and a 3840x2160 image is about 25 MB, no matter what the picture shows. The AVIF you started with is usually a small fraction of that.
The size isn't a bug. Programs that read BMP want the raw pixel grid so they can load it without a decoder. That is why BMP still turns up in older Windows programs and in tools that need a plain bitmap. Convert only when one of those asks for BMP by name. For a normal photo you want to open or share, AVIF to JPG gives a far smaller file that opens everywhere.
How to convert AVIF to BMP
- Upload the AVIF file. Up to 5 files at once, each up to 300 MB.
- Pick BMP as the target format.
- Download the BMP once our servers finish the conversion.
Quality and transparency
Converting to BMP doesn't sharpen anything. The AVIF encoder already discarded detail when the file was made, and a bitmap simply stores the pixels that remain. If the image has a transparent background, use AVIF to PNG, which keeps it. If a print shop wants TIFF, use AVIF to TIFF. See the BMP hub and the AVIF converter for the other directions.