Image Format Converter — PNG, JPG, WebP

A screenshot saves as PNG. A stock library hands back WebP. The upload form in front of you takes JPEG and nothing else. Image Format Converter re-encodes whatever this browser can decode into PNG, JPEG, WebP or BMP, and it works through twenty files in one pass rather than making you repeat yourself twenty times.

BMP is the odd one out of the four. No browser ships a BMP encoder, so canvas cannot produce one and this tool writes the bytes by hand: a 14-byte file header, a 40-byte BITMAPINFOHEADER, then rows of BGR triplets stored bottom-up. That is why a bitmap of a holiday photo comes back many times heavier than the JPEG it started as, and also why it remains the format an old Windows viewer or a plotter driver will never argue with.

Transparency gets settled before the encoder sees a single pixel. Every decoded image is sampled on a scratch canvas capped at 512 px a side to find out whether anything in it is less than fully opaque. If something is, and your target is JPEG or BMP, the matte swatch wakes up and paints those pixels the colour you choose rather than letting them fall to black. Nothing is uploaded at any point: files are read from local object URLs and handed back as downloads, so a passport scan or an unreleased product shot is re-encoded on the machine it already sits on.

Getting a Queue of Screenshots Converted

1. Drop one file or twenty onto the panel, or press Load Sample Image to queue alpha-badge-512.png, a 512 × 512 gradient disc captioned PNG above alpha demo on a fully transparent background.
2. Choose a target from the four cards. PNG and WebP carry an alpha channel through; JPEG and BMP cannot, and the queue line reads transparency detected when that difference is about to cost you something.
3. Drag Quality for JPEG or WebP. It greys out on PNG and BMP, both of which record every pixel exactly.
4. Press the terracotta convert button. It turns into a live counter reading Converting 3 of 12…, because encoding happens one canvas at a time on the main thread.
5. Collect the output row by row with the small download button beside each thumbnail, or take the lot with Download all, which spaces the saves 150 ms apart so the browser does not throttle the burst.
6. Tap the copy icon in the Converted files header before you leave. It writes a plain-text report of every before and after size with a queue total at the bottom, and it is the control people scroll straight past.

Supported Formats, In and Out

• Reads whatever the browser decodes, which covers JPEG, PNG, WebP, GIF, AVIF and BMP. An animated GIF gives up its first frame and nothing else.
• Writes four. Where a browser has no WebP encoder, that card is disabled and captioned Not supported here, so a format that would fail cannot be selected in the first place.
• Non-images are dropped rather than argued about. Queue two PDFs alongside six photos and the amber bar answers 2 files skipped — not a readable image.
• Targeting the format a file already uses still re-encodes it, which is how an oversized JPEG gets smaller without losing a pixel of its dimensions. It is equally how a lossless PNG throws data away if you aim it at JPEG.
• Output keeps the original stem and swaps the extension, turning invoice-scan.png into invoice-scan.jpg.

How Big a 24-Bit BMP Gets

• Three bytes a pixel, no compression, and every scanline padded up to a multiple of 4 bytes. A bitmap three pixels wide still spends 12 bytes on each row.
• Any file projecting past 48 MB is refused on its own row instead of locking the tab. A 4000 × 3000 photograph works out at roughly 34 MB and converts; 5000 × 4000 reaches 57 MB and the row reports that figure back to you.
• The 20-file cap is about the main thread, not about storage. Hand it thirty files and the warning reads The queue holds 20 images. Files past that were left out.
• Anything heavier than 30 MB raises a notice before conversion starts, since mobile Safari tends to return a blank canvas around that weight rather than a proper error.
• The transparency probe never inspects more than 512 px a side. One stray soft edge in a 6000 px image can fall between samples and leave the matte swatch greyed out, so send that file to PNG or WebP, where alpha survives either way.