Converting a batch of product photos or an iPhone HEIC export shouldn't require installing anything, but "free online image converter" search results mix three genuinely different architectures — pure browser Canvas, in-browser WebAssembly, and old-fashioned server upload — that behave very differently once you throw real files at them. I compared 5 tools specifically on what they actually support versus what their marketing claims.
1. ToolTiny — clean single-file conversion, with two claims worth verifying yourself
ToolTiny's Image Converter uses HTML5 Canvas to convert between JPG, PNG, WebP, and GIF entirely in your browser — nothing uploads. It handles the classic transparency problem correctly: converting a transparent PNG to JPG (which doesn't support alpha channels) fills the transparent area with white instead of rendering it as broken black blocks, which is a real, useful detail a lot of naive converters get wrong.
Two things worth testing yourself before relying on them, since the page's FAQ claims go further than what the interface itself appears built for: it states you can convert multiple images at once, but the upload flow is a single dropzone with one preview and one download — I'd verify with 2-3 files before planning a batch workflow around it. It also claims HEIC (iPhone photo format) support, which is technically notable because plain HTML5 Canvas can't decode HEIC without an additional decoder library — most tools that genuinely support HEIC either use WebAssembly specifically for it or rely on Safari's partial native support. Worth a quick test with an actual iPhone photo before counting on it.
2. Squoosh — Google's own tool, and proof single-file-only isn't a ToolTiny-specific limitation
Squoosh runs entirely in-browser via WebAssembly, open source, with the best encoding control of anything on this list — a side-by-side before/after preview and fine-grained parameters for each format, including AVIF. It's explicitly single-file, no batch conversion, same as ToolTiny's actual (not claimed) behavior. Worth knowing this specifically: if Google's own well-regarded tool doesn't do batch conversion client-side, that's a genuine technical trade-off in this category, not a corner someone cut.
3. BulkPicTools — genuinely local, genuinely batch, HEIC via WebAssembly
Runs every conversion — HEIC to JPG, WebP to JPG, PNG to JPG, and more — locally via WebAssembly, with batch processing built into the core workflow rather than added on top. If HEIC support specifically is why you're looking for a converter, this is the one built around it rather than mentioning it in passing.
4. Convertify — the fastest broad-format option, with a clear privacy trade-off
Built on Rust and libvips, supporting 20+ formats including HEIC, AVIF, and TIFF with batch conversion up to 10 files and a quality control slider. The trade-off: it's server-based (files are deleted within 6 hours rather than never uploaded), and capped at 20MB per file. Reasonable for non-sensitive images where format breadth matters more than the on-device guarantee.
5. PicTechAI — 30+ formats, single and batch, browser-based
Covers an unusually wide format range — JPG, PNG, WebP, HEIC, GIF, SVG, PSD, TIFF — with both single-image and batch workflows, downloading batch results as a ZIP. Useful specifically if your files aren't all the same format to begin with, since it doesn't require picking one input type before starting.
Quick comparison
| Tool | Fully local | Batch conversion | HEIC support |
|---|---|---|---|
| ToolTiny | ✅ | Claimed, verify yourself | Claimed, verify yourself |
| Squoosh | ✅ | ❌ (single file, by design) | ❌ |
| BulkPicTools | ✅ | ✅ | ✅ (WebAssembly) |
| Convertify | ❌ (server-based, deleted in 6h) | ✅ (up to 10 files) | ✅ |
| PicTechAI | ✅ | ✅ | ✅ |
Which one for which situation
- One image, quick and private, standard formats? ToolTiny or Squoosh — Squoosh if you want more encoding control, ToolTiny if you want a simpler flow.
- A folder of iPhone HEIC photos to convert at once? BulkPicTools — built specifically around this case, fully local.
- Mixed formats you need converted together, don't mind a ZIP download? PicTechAI.
- Broadest format support and don't mind server-side processing? Convertify.
The honest note
FAQ sections across this entire category tend to claim more than the actual upload interface supports — "batch conversion" and "HEIC support" show up in marketing copy more often than they show up in a dropzone that actually accepts more than one file or correctly decodes an .heic extension. Before building a workflow around any converter's claimed feature list, test it directly with the exact file type and quantity you actually need — two or three real files, not the tool's own demo image. It takes thirty seconds and it's the only way to know whether the FAQ describes the product or describes the product someone wishes they'd built.
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