BMP to PDF Converter

1Drop your BMPs inIt never leaves your machine.

2Set the pageEverything updates as you change it.

3Build and downloadAssembled here and saved from your own browser.

Very large in, very small out

A bitmap stores every pixel uncompressed, which is why these files are enormous. Re-encoding on the way in typically turns megabytes into kilobytes with no visible difference.

How to use this tool

Drop your BMP files onto the box at the top, or click it to browse. Each becomes a page, in the order the thumbnails show, and you can drag a thumbnail onto another to move it.

Set the paper size and margin, then check the preview before downloading. Each thumbnail shows the effective print resolution of that image at the size you have chosen.

The quality slider matters more here than with any other format, because it is doing nearly all of the work of shrinking the file.

Why a BMP is so much bigger than it needs to be

BMP stores a picture the most literal way possible: a list of every pixel, one after another, with no compression. That is the whole format. It is why it survives, because it is trivially simple to read and write, and it is why the files are the size they are.

The arithmetic is straightforward. A 3000 by 2000 image is six million pixels. At three bytes each for red, green and blue that is 18 million bytes, about 17 MB. That figure does not depend on the content at all: a photograph of a busy street and a completely blank white page are exactly the same size.

Every other format exploits the fact that real pictures are not random. Neighbouring pixels are usually similar, large areas are often identical, and the eye cannot see fine colour detail well. BMP exploits none of it. So a BMP that becomes a 400 KB PDF page has not lost 97 percent of its information, it has stopped storing the same information wastefully.

How much you actually save, and what it costs

The saving comes almost entirely from JPEG compression applied as the image goes into the PDF, so the quality slider is the lever that matters.

At the default quality a photographic BMP typically lands somewhere between two and five percent of its original size with no visible difference. That is not a trick; it is the difference between storing every pixel and storing a description of what the picture looks like.

The exception is content JPEG handles badly. Screenshots, scanned line drawings, diagrams and anything with sharp black on white edges will show compression artefacts around those edges at default quality. For that material push the slider above ninety, accept a larger file, and it will still be a small fraction of the BMP you started with.

Worth saying plainly: this is a lossy conversion and there is no undo. If the BMP is an archival original of something that cannot be recaptured, keep it as well as the PDF rather than instead of it.

A worked check you can repeat

Take a 3000 by 2000 pixel image onto A4 portrait with a 10 mm margin, set to Whole image.

A4 is 595.28 by 841.89 points. A 10 mm margin is 10 times 72 divided by 25.4, which is 28.35 points a side, leaving 538.59 by 785.20. The image aspect ratio is 1.5 against a box of 0.686, so width is the limit and the scale is 538.59 divided by 3000, or 0.17953.

The picture lands 538.59 wide by 359.06 tall, filling the width and centred vertically with 213.07 points above and below. Effective resolution is 3000 pixels across 7.48 inches, about 401 dpi. As a BMP that image was around 17 MB; in the PDF it will typically be a few hundred kilobytes.

Every geometric figure above was checked against an independent implementation before publishing, along with the page presets against their real dimensions, orientation switching, the fill and stretch modes and margins larger than the paper.

Mistakes that catch people out

Converting scanned line drawings at default quality. JPEG smears sharp edges, which is exactly what a line drawing is. Push the quality above ninety for anything technical.

Deleting the original straight away. This is a lossy conversion. If the BMP is the only copy of something that cannot be recaptured, keep it.

Assuming a large BMP means a high resolution image. It does not. File size tells you the pixel dimensions and nothing about quality. Check the dpi badge on the thumbnail instead.

Loading dozens of large BMPs at once. They are uncompressed, so the browser holds all of it in memory. Work in smaller batches if the tab starts struggling.

Expecting Whole page to keep the whole image. It covers the page, so anything that does not fit is cut off. Use Whole image unless you want edge to edge.

Common questions

Is my BMP uploaded anywhere?

No. The images are read by your browser, the PDF is assembled here on the page, and the download comes from your own machine. Nothing crosses the network, so there is nothing on a server to store or delete.

Why is my BMP file so enormous?

Because BMP stores every pixel individually with no compression at all. A 3000 by 2000 image is six million pixels at three bytes each, which is about 18 MB regardless of what the picture actually shows. A blank white BMP is the same size as a detailed photograph.

How much smaller will the PDF be?

Usually dramatically. The pixels are compressed as JPEG on the way in, and typical reductions are ninety percent or more. The exact figure depends entirely on the content, since detailed images compress less than flat ones.

Will it look worse?

Slightly, in the sense that JPEG compression is lossy. At the default quality the difference is invisible on photographic content. On flat colour, screenshots or line art, push the quality above ninety.

Where do BMP files even come from?

Scanners, older Windows software, some measurement and instrument equipment, and screen capture tools that have not been updated in years. It is rarely a format anyone chooses, and usually one they have been handed.

Can I mix BMP with other formats?

Yes. Drop in JPG, PNG, WebP and GIF alongside them and they all become pages in the same document, in whatever order you arrange the thumbnails.

Related PDF tools and reference

Reference: ISO 216, the standard that defines the A series paper sizes used here.