Manatee

How to convert BMP to PNG on a Mac

· 5 min read

Preview opens BMP files and saves them as PNG through File, Export, and the Finder’s Convert Image quick action does the same without opening anything. For a folder of them, Manatee converts the lot in one drop, and a one-line sips command in Terminal works too. PNG is lossless, so the picture does not change; the file simply gets much smaller and far more welcome on the web.

Why bother converting

BMP is an old Windows format that stores every pixel uncompressed: three or four bytes per pixel, so a 1920x1080 screenshot is about 6 MB no matter what is in it. PNG compresses losslessly, and for screenshots, diagrams, logos, and anything with flat color it typically lands at a fraction of that size. PNG also supports transparency, which BMP generally does not, and it is accepted everywhere BMP is not: web pages, Markdown, chat apps, upload forms, and most document tools.

Because both formats are lossless, the conversion is exact. Every pixel in the PNG has the same color value it had in the BMP. This is different from converting to JPEG, which would shrink the file further but soften edges and introduce artifacts around text.

Convert one file with Preview

  1. Double-click the BMP. It opens in Preview (if another app has claimed .bmp files, right-click and choose Open With, Preview).
  2. Choose File, Export.
  3. In the Format menu, choose PNG. If you do not see it, hold the Option key while clicking the Format menu to reveal the full list.
  4. Leave Alpha checked if you want a transparency channel preserved; for a plain BMP it makes no difference.
  5. Pick a location and click Save. The BMP stays where it was.

Preview also lets you resize before exporting (Tools, Adjust Size) if the BMP is a giant scan and you only need a web-sized copy.

Convert several with the Finder quick action

On macOS Monterey and later, Finder can convert images in place without opening an app.

  1. Select the BMP files in Finder. Shift-click or Command-click to pick several.
  2. Right-click the selection and choose Quick Actions, then Convert Image.
  3. Set Format to PNG and Image Size to Actual Size. Tick Preserve Metadata if you want any embedded metadata carried across.
  4. Click Convert to PNG. New PNG files appear beside the originals.

If Convert Image is missing or the result is blank for a particular file, macOS could not decode that BMP variant. Preview may still open it, and Manatee’s bundled engine handles the variants that macOS does not.

Convert a folder with Manatee

Manatee is the least fiddly route when there are dozens of files or when some of them are the odd BMPs that macOS refuses.

  1. Open Manatee. Every feature is free for 24 hours with no account; afterward it is $9 once.
  2. Drag the files, or the whole folder, onto the menu bar icon or into the window’s drop zone.
  3. Choose Convert and pick PNG.
  4. Each file becomes a job in the list, and each PNG is written beside its BMP. The originals are never modified.

Manatee accepts bmp among its image inputs, along with tga, pbm, ppm and similar older formats, and modern ones like webp and avif, and it can write JPEG, HEIC, TIFF, and GIF as well as PNG. So if the real goal is a photo for the web rather than a screenshot, choose JPEG here instead and get a much smaller file.

Manatee does not edit images. It will not crop, resize, or touch colors; it changes the format and nothing else.

Convert from Terminal with sips

macOS includes a command-line image tool called sips. It is useful when you want the conversion in a script or simply prefer the keyboard.

For one file:

sips -s format png picture.bmp --out picture.png

For every BMP in the current folder:

for f in *.bmp; do sips -s format png "$f" --out "${f%.bmp}.png"; done

Open Terminal, use cd to move into the folder, and paste the command. The originals are left alone. sips uses the same decoder as Preview, so it fails on the same unusual BMPs.

BMPs that refuse to open

The BMP format has accumulated variants over the decades, and Apple’s decoder does not handle all of them. The usual offenders are files with run-length compression, 16-bit color depths, top-down row order, or headers from very old OS/2 tools. Symptoms: Preview shows a blank window, Quick Look shows a generic icon, or Convert Image produces nothing.

Try Manatee first, since its engine is independent of macOS and reads the variants the system skips. If that also fails, the file is probably damaged rather than unusual. A file that was interrupted during download or copy will show the top portion of the image and grey below, or nothing at all; why files get corrupted and what can actually be recovered explains what is and is not salvageable, and PNG file is invalid or unsupported on Mac covers the same diagnosis on the output side.

Questions

Will the PNG look any different from the BMP?

No. Both formats are lossless, so the pixel values are identical. Open both at 100 percent in Preview and they will match exactly.

The PNG is still huge. Why?

Because the picture is a photograph or has gradients and noise, which PNG cannot compress well. PNG is the right choice for screenshots and graphics; for photos, convert to JPEG (or HEIC, if everyone involved is on Apple devices) and the file will drop dramatically.

Does converting keep the image’s metadata?

BMP files rarely carry any. If yours does, the Finder quick action offers a Preserve Metadata checkbox; Preview and sips generally carry basic fields across. If you want metadata removed before sharing, that is what Manatee’s Clean mode is for.

Can I convert PNG back to BMP?

Preview can: File, Export, and choose BMP from the full format list (hold Option on the Format menu). Manatee does not write BMP; it accepts it as input only.