One image is a quick fix. Three hundred images — a client's asset folder, an iPhone export, a camera dump, a product catalog — is a different problem. Opening each file in an editor and re-saving is hours of clicking; most "free batch converter" sites either cap you at 5 files, watermark results, or demand your files be uploaded to their servers first.
Here's the workflow that handles a whole folder in one pass, entirely on your own machine.
The tool stack
All three batch tools share the same interface: drop files, set options, process, download ZIP. None of them upload anything — the work is distributed across your browser's own decoders and WebAssembly codecs.
- Bulk Image Converter — format conversion: JPG ⇄ PNG ⇄ WebP ⇄ AVIF ⇄ BMP, plus GIF and ICO. Handles iPhone HEIC too.
- Bulk Image Compressor — same files, smaller bytes: quality targeting or exact size targets.
- Bulk Image Resizer — same files, new dimensions, with cover/contain/fill modes.
Step by step: converting a folder
1. Select all and drop
In File Explorer (or Finder), select the files — Ctrl+A in the folder, or drag a box around them — and drop the selection onto the converter. Mixed input formats are fine: JPG, PNG, WebP, HEIC, BMP and more are detected by content, not just extension.
2. Choose the output format
| Convert to | When |
|---|---|
| JPG | Photos destined for forms, email, CMS — universal acceptance, small files |
| PNG | Graphics, screenshots, anything with transparency or sharp text |
| WebP | Website assets — 25–35% smaller than JPG at equal quality |
| AVIF | Modern web targets where maximum compression matters |
| BMP | Legacy Windows tooling and niche industrial software |
| ICO | Windows application icons |
3. Set quality once
JPG/WebP quality 85–90 is the sweet spot: visually transparent for photos, with 3–10× size reduction from camera originals. Convert, glance at the per-file results, adjust if a particular image needs more budget.
4. Download the ZIP
Every output carries the original filename with the new extension. Grab the ZIP, unpack, done — 300 files in the time a single online conversion takes.
Real-world pipelines
- iPhone HEIC dump → usable JPGs: export from the phone, drop the .HEIC files into the converter, output JPG. No iTunes gymnastics, no cloud sync.
- Etsy/eBay/Amazon listings: convert to JPG → resize to marketplace specs → compress under platform limits. Three batch passes, one folder.
- Website image optimization: convert to WebP, resize to max 1600px, compress — the standard pre-deploy checklist, all local.
- Scan cleanup: convert BMP/TIFF-era scans to PNG or JPG and compress for archiving.
Why local beats upload-based for batches
Upload-based converters scale badly: 300 files means 300 uploads (or a ZIP upload plus their unzip queue), file-count caps, and gigabytes of personal photos transiting someone else's storage. Local processing inverts the math: your CPU does the work at full speed, the network does nothing, and the only queue is your own RAM. If you're on a plane or a hotel connection, it doesn't matter at all. The architecture behind this is explained in Why Offline Image Tools Beat Cloud Uploaders.
FAQ
What's the maximum batch size?
There's no artificial cap — memory is the only limit. For very large batches (1000+ heavy RAW-derived files), process in chunks of a few hundred to keep the tab responsive.
Can I convert HEIC photos without installing anything?
Yes — HEIC decoding runs in the browser via WebAssembly. This is the fastest route from iPhone photos to standard JPG/PNG on Windows, where HEIC isn't natively supported.
Will conversion change my originals?
No. Originals are read-only inputs; outputs are new files with the same names and new extensions.
Which format is best for my website images?
WebP for photos and general assets; PNG for transparency-heavy graphics; AVIF when you control the whole stack. A deeper comparison is in Pick the Right Image Format.
Convert your first folder in the Bulk Image Converter — then keep the compressor and resizer bookmarked for the rest of the pipeline.