Convert BMP to GIF

That is the whole of this conversion. A 24-bit BMP stores an actual colour for every pixel — three bytes, one per channel, sixteen and a half million possibilities, written out in an order that betrays the format’s age: rows bottom to top, channels blue then green then red. A GIF stores an index into a palette instead, and the palette holds 256 entries. Everything else on this page follows from that one difference.

It is a limit in the format rather than a setting, which is why there is no control for it. GIF89a dates from 1989 and CompuServe published the first version in 1987, when 256 colours was what a screen could show. The registry marks it legacy for exactly that reason, and it remains in use because a great deal of software has read it for nearly forty years. BMP comes from the same world — Microsoft published it in 1987 as well — which is why the two turn up together in old systems and why this conversion is more often a migration than a choice.

Something has to decide which colours survive, and doing it badly is what gives GIF conversions their reputation. Rounding each channel to a fixed grid is the naive method and it turns a photograph into visible steps regardless of what was in it.

The encoder here quantises by popularity within a k-d tree — the same family of algorithm ImageMagick uses — so the palette is built from the colours the image actually contains rather than from a generic grid. The palette is computed in a reduced colour space to keep it fast, which costs a little accuracy; for a format that ends at 256 colours that trade sits heavily on the side of not making you wait. The practical effect is that the palette follows your picture: a bitmap that is mostly one brand blue spends most of its 256 entries on shades of that blue, which is what makes flat artwork survive intact.

Dithering is the other way to handle a palette: instead of snapping each pixel to the nearest available colour, scatter two neighbouring colours in a pattern so that the eye averages them and the band disappears. It is what made 256-colour screens tolerable in the 1990s, and it is not applied here.

The reason is that it moves the damage rather than removing it. A dithered GIF trades smooth bands for visible speckle, and because the pattern breaks up the long runs of identical pixels that the format compresses, the file grows as well — worse to look at and larger, on material that was already the wrong material. On flat artwork, where this conversion actually belongs, there is nothing to dither in the first place.

A logo, an icon, a diagram, a chart, a scanned line drawing or a screenshot of an interface typically contains a few dozen distinct colours. All of them fit in the palette, and the GIF is indistinguishable from the bitmap. That is the case this conversion is good at, and it is also the material most BMP files contain.

A photograph is the opposite. A sky is a continuous gradient of hundreds of subtly different blues, and reducing it to a handful produces the bands the registry lists as the format’s known problem. The banding is most visible exactly where a picture is smoothest — skies, skin, studio backgrounds, anything softly lit — and no setting on this page prevents it. A useful test before converting a batch: open the bitmap and look for any area where the colour changes gradually across more than a few dozen pixels. Every one of those is somewhere a step will appear.

GIF is the format people associate with animation, and BMP holds exactly one image. So the output is a single-frame GIF: a still picture in a container capable of more, which is a perfectly valid file and not what some people arrive expecting.

If the goal is an animation, the input has to be several images or a video. Converting one bitmap after another and hoping they combine does not work — each conversion produces its own independent file, and nothing here assembles frames.

GIF can be transparent in one coarse sense: a single palette entry may be nominated as invisible, with no partial levels. The encoder here nominates none, so every pixel of the file you download is opaque — the format’s one bit is not used, let alone the 256 levels a PNG would give you.

It rarely comes up from a bitmap, which is the other half of the answer. A BITMAPV5HEADER can describe an alpha channel and 32-bit files carrying one exist, but they are rare enough that this site treats BMP as an opaque format, and this pair offers no background colour to pick. If one does arrive with transparency the browser reports, it is filled with white before the palette is built. Where a cut-out has to survive, PNG keeps it on both ends and is the conversion to use instead.

Much less, in nearly every case. The bitmap spends three uncompressed bytes on every pixel, while the GIF spends one palette index and then compresses the run of indices — and flat artwork, which is mostly long runs of the same index, compresses extremely well.

The comparison that matters is not against the BMP, though. Almost anything beats three uncompressed bytes per pixel, so a comparison against the source flatters every target on this site equally. It is against the alternatives that GIF has to argue: for a photograph, a JPEG or a WebP of the same picture will be smaller than the GIF and look far better, because a lossy photographic codec is built for exactly the material a palette cannot describe. GIF wins on compatibility and on nothing else.

Systems with a fixed list of accepted formats, written when that list was short: older forum and wiki software, some mail and newsletter templates, embedded and industrial interfaces, and internal tools nobody has touched in a decade. Occasionally a specification or an assignment names the format outright.

All of those are requirements rather than preferences, which is the honest framing for this page. Nobody chooses GIF for a still image on its merits any more, and a page that suggested otherwise would be describing 2005. It is worth confirming that the requirement is real before working around it, because "GIF" is sometimes shorthand in a document nobody has revised since the list was written. If the system accepts a PNG when one is offered, the whole of this page becomes unnecessary and the result is better.

PNG keeps every colour in the bitmap, keeps 256 levels of transparency instead of one, is lossless, is read by everything and is usually smaller than the GIF on the same flat artwork. It replaced GIF for still images decades ago and did so on the merits.

So the decision procedure is short. If something told you it needs a GIF, this page is correct. If you are choosing, choose PNG — and if the source is a photograph and the destination is a web page, choose WebP and skip both.

The bitmap is decoded by the browser and the palette and the GIF are built in the same tab, so nothing is uploaded and there is no queue. The ceiling is 100 MB per file, roughly 33 megapixels of uncompressed bitmap.

Drop a folder and each file is quantised and encoded in turn, keeps its name with the extension swapped, and the set comes back as one ZIP. Quantisation is the slowest step here, so a batch of large images takes longer than the file sizes suggest — setting a maximum width, if the destination does not need the full dimensions, cuts both the time and the output. Since the systems that still require GIF are generally old ones displaying small images, that reduction is usually free in every sense.