How to convert images without uploading them anywhere
Your computer has been able to do this on its own for years. Most web converters upload anyway, out of habit rather than necessity.
Last reviewed 2026-09-08
Why most converters upload when they no longer need to
The upload-process-download pattern is a holdover. In 2010 a browser genuinely could not decode a
photo, re-encode it and hand back a file, so the work had to happen on a server. Browsers gained those
abilities years ago — the File API for reading local files, createImageBitmap and canvas
for decoding and re-encoding, WebAssembly for formats the browser does not natively support — and most
converters never revisited the architecture.
There is also an incentive worth naming. A server-side flow puts you through a queue page, a progress page and a download page: three ad impressions instead of one. A local conversion finishes in the same view you started in. That is not a conspiracy, but it does explain why the slower design has been so durable.
Method 1 — What your operating system already does
The zero-install answer, and the one most people forget:
- macOS. Open in Preview → File → Export, then pick JPEG, PNG, HEIC or TIFF with a quality slider. For a batch: select the files in Finder, right-click → Quick Actions → Convert Image.
- Windows. Open in Photos → … → Save as, and choose the format. Paint will also do it via File → Save as. Both are limited to one file at a time.
- Linux. ImageMagick is usually already installed:
magick input.png output.jpg, ormagick mogrify -format jpg *.pngfor a whole folder.
Limitation: none of these handle formats the OS itself does not understand. Windows without the HEVC codec cannot open HEIC in Photos either — see the HEIC on Windows guide for that specific case.
Method 2 — A desktop application
For volume, native software wins and it is not close:
- XnConvert (Windows, macOS, Linux, free for personal use) — batch conversion with a processing chain: resize, watermark, colour adjustments, then export.
- IrfanView (Windows, free) — the veteran. File → Batch Conversion/Rename chews through thousands of files.
- ImageMagick (everywhere, free) — the right tool the moment the job needs to be repeatable or scripted.
The cost is installation and, for ImageMagick, a command line. For a one-off job on a work machine where you cannot install software, that is a real barrier — which is where the third method fits.
Method 3 — A browser tool that runs locally
A web page can now do the whole job in the tab. The sequence is: the File API hands the page your
file as bytes, createImageBitmap decodes it, a canvas re-encodes it to the target format,
and toBlob produces the file you download. No part of that requires a network round trip.
Formats the browser cannot decode natively — HEIC being the common one — are handled by a WebAssembly
decoder, which means the page fetches code, not your file.
This is how the tools on this site work, and the honest accounting looks like this:
| You gain | You give up |
|---|---|
| Nothing to install | Your device's memory is the ceiling |
| The file never leaves the tab | Large batches are slower than native code |
| No queue, no round trip | Exotic formats (RAW, PSD, AI) are out of reach |
| Works offline once loaded | A browser tab can be closed mid-job |
A side effect worth knowing either way: canvas re-encoding discards EXIF metadata, including GPS coordinates. Good if you are about to post the photo publicly, inconvenient if you wanted the capture date preserved.
How to tell what any given tool is doing
Applies to this site as much as to anyone else's:
- Network tab. F12 → Network → convert a file. An upload shows as a request roughly the size of your file. Local conversion shows only page assets.
- Airplane mode. Load the page, disconnect, convert. Server-side tools fail instantly. Local tools carry on — and unlike the first test, this one cannot be faked.
Picking between them
One or two files, any machine, nothing installed: a local browser tool. Hundreds of files, or the same job every week: a desktop batch converter. A format your OS already understands and a single file: you probably do not need a tool at all — Preview or Photos will do it in four clicks.
Common questions
Is a browser converter actually as private as a desktop app?
For the file itself, yes — with a local tool the bytes never leave the tab, same as a desktop app. The difference is that a web page is loaded fresh from a server each visit, so you are trusting that the code you receive today still behaves the way it did last week. The offline test settles it in ten seconds whenever you want to re-check.
Why do some browser tools still need an internet connection the first time?
Formats the browser cannot decode natively — HEIC most commonly — need a WebAssembly decoder, which is fetched on demand. That download is program code and happens once; your file is not part of it. After it has loaded, conversion works with the network off.
What is the largest file a browser tool can handle?
There is no fixed limit, only your device's available memory. A decoded image occupies width × height × 4 bytes uncompressed, so a 50-megapixel photo needs about 200 MB just for the bitmap. Phones typically manage batches of 50 or so ordinary photos; desktops handle several hundred.
Does converting locally change image quality compared with an online service?
No meaningfully. Both decode and re-encode with comparable encoders, and quality is governed by the setting you choose, not by where the work happens. What differs is the metadata: canvas-based conversion drops EXIF, while most server-side converters preserve it.