How to Convert BMP to ICO in Your Browser

How BMP becomes ICO

1. The browser reads the BMP's pixel data directly — BMP files are typically stored uncompressed, so this is a straightforward unpack rather than a decompression step, with alpha present only if the source uses a newer V4/V5 header that actually includes it.
2. That single image is resized to each icon resolution, typically 16, 32, 48, and 256 pixels.
3. Each resized version is compressed as PNG data — introducing real compression to an image that started out uncompressed — and written into the ICO's ICONDIR as its own entry.

What changes when you convert

Most BMP files carry no alpha channel at all. Older BMP headers (BITMAPINFOHEADER and similar) have no field for it, and even files saved with a newer V4 or V5 header that could carry alpha often don't, depending on what tool created them. When there's no alpha in the source, the resulting icon is simply opaque at every size — there's nothing to lose because there was nothing there to begin with.

When a source BMP does carry genuine alpha — typically from a V4 or V5 header written by image-editing software — it's preserved into the icon's PNG-compressed frames the same as any other source. The header version determines this, not anything about the icon output itself.

The bigger change is compression: BMP is essentially raw pixel data with no meaningful compression, so converting to ICO's PNG-compressed frames is often the first time this image has been compressed at all. Combined with the multi-size package (16 through 256px bundled into one file), the resulting ICO is very likely smaller than the original BMP, even though it now contains several resolutions instead of one.

When ICO is the right choice — and when it isn't

ICO exists to give Windows a single file containing an icon at every size the OS needs — a 16px title-bar icon up to a 256px high-DPI shortcut — with the OS picking automatically. If a bitmap is destined to become an app icon, desktop shortcut, or favicon, that's exactly the job ICO does.

BMP itself is usually kept around for legacy Windows compatibility or because some tool demands raw uncompressed pixels, so converting it further to ICO only makes sense if the actual end goal is an icon. If the goal is just a modern, smaller general-purpose image, converting to WebP or AVIF is the more useful direction — BMP-to-ICO is worth doing specifically when you're building a Windows app icon or favicon from an existing bitmap asset.

Limitations and privacy

Because alpha presence in BMP files is inconsistent and depends on header version and the originating tool, it's worth checking whether your source actually has transparency before assuming it will carry through — there's no reliable way to tell just from the file extension.

Reading the BMP, resizing, and PNG-compressing into the ICO container all happen locally in your browser. The file is never uploaded to a server for this workflow.