Why choose PNG for a phone photo
Most people converting HEIC want a JPG, so it is worth being clear about when PNG is the better answer. PNG is lossless. Once the HEIC has been decoded, nothing further is discarded, which makes PNG the right intermediate when you plan to edit a photo heavily, composite it, run it through OCR, or feed it to software that produces artefacts from JPG compression. It is also the format to pick for HEIC files that are not really photographs: diagrams, whiteboard captures, documents and app screenshots that some third-party camera apps save as HEIC. Those have flat areas and sharp text, and PNG preserves them exactly.
What PNG is not good at is size. A lossless 12 megapixel photograph is enormous, and if the goal is simply to share or upload, HEIC to JPG will serve you better.
The decoding chain
Browsers have no built-in HEIC support, so this page loads heic2any, a JavaScript build of the libheif decoder, the first time you press convert. The library is a few megabytes and takes a moment to fetch and start; afterwards it stays cached. It decodes the HEVC image data and produces a PNG in memory. Because the target here is also PNG, that intermediate is passed to a Web Worker, which decodes it once more and re-encodes it with the WebAssembly PNG encoder from jSquash. There is no quality slider on this page, since PNG has no lossy mode. The whole process runs on your device.
Only the primary image inside the HEIC is used. A Live Photo's video, the extra frames of a burst and any depth map are dropped. Colour comes out as 8-bit sRGB rather than the 10-bit Display P3 of the source, HDR gain maps are discarded, and all EXIF metadata is stripped because the pixels pass through a canvas. Capture time, location and camera details are gone from the PNG.
File size and time: a worked example
A 4032x3024 iPhone photo that is 1.9 MB as HEIC typically becomes a PNG of 15 to 25 MB. That is not a fault; it is what lossless storage of 12 million noisy pixels costs. A 1170x2532 app screenshot saved as HEIC, around 400 KB, usually lands at 300 KB to 1 MB as PNG, and a whiteboard photo of a few hundred kilobytes in HEIC tends to reach 5 to 8 MB. Encoding a 12 megapixel PNG takes a few seconds on a laptop, plus the HEIC decode, so expect the total to be well over five seconds for a full-size photo. Treat all of these as typical, not guaranteed.
Memory limits on phones
The decoded image is held as RGBA, four bytes per pixel, which is about 48 MB for a 12 megapixel frame, and there are two copies in flight plus the PNG being built. Desktop browsers handle this without trouble. Mobile Safari and low-memory Android phones sometimes fail on full-size HEIC files, especially several in a row. If that happens, convert on a computer, or convert to JPG first and only go lossless where you genuinely need it. See file size limits for what each kind of device can realistically process.
Keep the original
The HEIC remains the highest-quality version you own: wider gamut, more bit depth, and the metadata intact. Convert an exported copy, do your editing on the PNG, and archive the HEIC. If you only need to read the metadata rather than convert anything, the EXIF tool shows it without changing the file. Background on the format is in What is HEIC.