Bulk Convert Images Without Uploading: The Routes Compared

Can you bulk convert images without uploading them?

Yes, and you probably already own a tool that does it. Converting an image is arithmetic on its pixels. Nothing about that arithmetic needs a server, and it is fast: in the test further down, one laptop converted forty photos in about a second.

There are four places the work can happen on your own machine:

• the tools your operating system ships with,
• a desktop batch converter,
• the command line,
• a browser page or extension that converts locally.

They differ in how many files they take, which formats they read, and what you have to install. The short version: a Mac needs nothing installed, Windows needs one free app, and a browser-based converter is the quick route for a few dozen files in the formats the web uses.

An online converter that uploads is the fifth option, and for a folder it is the worst fit on practical grounds alone: every file crosses your connection twice. The privacy side of that trade is its own subject, covered in are online image converters safe.

How can you tell a converter is not uploading your files?

“No upload” is a claim, and with a browser-based tool it is a claim you can check in a minute. A web page is allowed to read a file you hand it. The File API lets web content ask you to select local files, through a file picker or drag and drop, and then read their contents inside the page.[1] A converter built this way decodes and re-encodes the file in your browser’s memory. A converter built the other way sends the bytes to a server first. Both show you the same drop zone.

Two checks separate them:

1. Turn the connection off. Load the page, disconnect from Wi-Fi, then convert a file. A local converter still works. An uploading one fails or hangs.
2. Watch the Network tab. Open DevTools with F12, pick Network, and convert a file. An upload shows up as a request about the size of your file. This is the stricter check, because it also catches a page that converts locally and reports something home afterwards.

Desktop apps and command-line tools do not need the first check. A program running on your machine has the file already. Whether it also phones home is a question for its privacy policy, and for the tools named below the conversion itself is local.

What can your computer already do without installing anything?

A Mac converts a folder out of the box. Windows does not, and the Microsoft tool people reach for says so in its own documentation.

macOS: three built-in routes

Finder. Select the files, Control-click, choose Quick Actions, then Convert Image, and pick a format and a size.[3] Apple’s own example is turning an .heic photo from an iPhone into a .jpeg or .png.[3] This is the least effort of anything in this article.

Preview. Open the files in one window, select them in the sidebar, and export. Preview writes HEIC, JPEG, JPEG-2000, OpenEXR, PDF, PNG or TIFF.[2] WebP and AVIF are not on that list as outputs, so Preview gets you out of a modern format and not into one.

sips in Terminal. macOS ships a command-line image tool, and one line converts a folder:

mkdir converted
sips -s format jpeg -s formatOptions 85 *.webp --out converted

The originals stay where they are and the JPGs land in converted. On macOS 26.2 sips read both WebP and AVIF files in our test below.

Windows: nothing built in for a batch

Paint and Photos each save one image at a time. The tool that looks like the answer is PowerToys Image Resizer, Microsoft’s free shell extension for “bulk image-resizing”.[4] It resizes a selection of files in one pass. It does not convert them: the only place it asks for an output format is a fallback encoder for files it cannot write back in their original format, and the documentation states that “this isn’t a file type conversion tool”.[4]

So on Windows a batch conversion means installing something, or using the browser you already have. Both are below.

Which desktop apps convert a whole folder?

Three cover almost every case, and they suit different people.

XnConvert is the general answer. It runs on Windows, Mac and Linux, reads more than 500 formats and writes about 70.[6] With that range, HEIC from a phone, TIFF from a scanner, camera RAW files and WebP all go in the same queue. It is free for private or educational use, and a company has to buy a licence.[6] If you have several hundred files, mixed formats, or subfolders, this is the tool, and nothing browser-based competes with it.

IrfanView is the Windows veteran. It is a viewer first, with batch conversion among its listed features, and it is freeware for private, non-commercial use only.[7] It is small and fast, and its batch dialog is dense. People who already use it need nothing else.

ImageMagick is the command-line route, on every platform. The batch command is mogrify, and it carries one trap to know about before you run it. mogrify works like ImageMagick’s main command “except that the original image file is overwritten”, unless you change the file suffix with -format.[5] A format change is therefore safe by construction, because the output has a new extension. A resize with no format change replaces your originals. Sending the output to a separate folder removes the question:

mkdir converted
magick mogrify -path converted -format jpg -quality 85 *.webp

The same “85” is not the same file

We ran the two commands above on the same batch. The test: 40 WebP photos at 1600 x 900 pixels, 3.6 MB in total, converted to JPG on an Apple M3 Pro running macOS 26.2, on 11 October 2026.

Speed is a non-issue at this scale. The size column is the finding. Both tools were asked for quality 85 and one produced files 1.7 times the size of the other, because a JPEG quality number is a setting inside one encoder and means nothing outside it. If the folder is headed for an upload limit or an email attachment, convert three files first and look at the sizes before running the rest. Why a converted image comes out bigger than the original goes through the causes.

Both sets of JPGs were also larger than the WebP originals, 3.6 MB in and at least 7.7 MB out. That is the normal direction when leaving a newer format for an older one.

What about a browser tool that converts locally?

A browser already contains decoders for every format the web uses and encoders for several. A page or an extension that puts those to work converts files with nothing to install beyond the browser. The trade is range: a browser reads what browsers read, and that excludes most of what a camera or a scanner produces.

Squoosh, from Google’s Chrome team, is the reference example of a local web converter. Its README states that it “does not send your image to a server” and that all compression “processes locally”, and it lists what its analytics collects: basic visitor data and the before and after file sizes.[8] It gives you fine control over one image at a time, with a live preview, so a folder means repeating the steps for every file.

Our extension’s file converter is the batch version of that idea, and this is the part of the article where we are describing our own product. Click the extension’s toolbar icon and press the Open the file converter button in the popup, or open it from Settings. Drop files on the page, pick them with the file dialog, or paste an image from the clipboard. Pick JPG, PNG, WebP, AVIF or PDF, and press Convert and save. One file downloads directly. Several arrive as a single ZIP.

The files are read by the page and converted in the browser. No image and no filename is sent anywhere, and both checks from earlier in this article pass: it works offline, and the Network tab shows no upload. What the Chrome and Edge builds do send, unless you turn it off in Settings, is an anonymous usage event per converted file that records the output format. The Firefox build sends nothing at all.

Where the extension’s converter stops

• 150 files per run. Drop more and the rest are refused with a message. Run it again for the next 150, or use a desktop tool.
• Web formats only. It reads JPEG, PNG, GIF, BMP, WebP, AVIF and SVG. A HEIC file is listed as unsupported, and TIFF and camera RAW are not recognised.
• No folders. Subfolders are not walked and no directory structure is kept. You select the files themselves.
• Output is a ZIP in your downloads folder, not converted files beside the originals.
• Settings are global. Quality, resizing, the filename template and the save folder come from the extension’s Settings. There is no per-run quality slider on the converter page.
• Animations flatten. An animated GIF or WebP converts to its first frame.
• Metadata is dropped. The files are redrawn through a canvas, so EXIF data such as camera model and GPS position does not survive. That is a privacy benefit for photos you are about to share and a loss for a photo archive.

The same extension also handles the other half of the batch problem, saving every image from a web page in one format, which no desktop converter does.

Which route fits which job?

The last row cuts both ways. If the folder is going to a marketplace listing or a forum, losing the GPS coordinates is the outcome you want.

One habit applies to every row: convert into a new folder and keep the originals until you have opened a few results. Every lossy conversion throws detail away, and a JPG made from a WebP cannot be turned back into that WebP. If the files that would not open were WebP in the first place, the WebP problem, end to end covers why they exist and every way out, and AVIF to JPG does the same for the newer format.