Two formats from the same family
HEIC and AVIF are siblings. Both are built on the HEIF container, and the difference is the video codec used for the still image inside: HEIC holds an HEVC (H.265) frame, AVIF holds an AV1 frame. Apple devices produce HEIC by default and Apple software opens it readily, but Android, Windows and web browsers mostly do not, and HEVC is patent encumbered. AVIF is royalty free and is decoded by every major current browser, which makes it the format to reach for when an iPhone photo has to be published on a website at the smallest possible size.
What happens to the photo along the way
The browser cannot read HEIC itself, so the file is first decoded with heic2any, a WebAssembly build of libheif, into a lossless PNG in memory. That PNG is handed to a Web Worker, which turns it into raw pixels and encodes AVIF with the jSquash build of the AV1 encoder at the quality you set. The decode is the slow half on most machines; the AV1 encode at the effort level this tool uses is the slow half on very large images. A 12 megapixel photo typically takes five to fifteen seconds end to end on a laptop and longer on a phone. Nothing is uploaded; the delay is your own processor working.
Choosing a quality setting
The slider runs from 20 to 95 and defaults to 80. AVIF degrades gracefully, so 60 to 70 is usually enough for web photos where the viewer sees a scaled-down version, and 80 to 85 is a sensible choice for anything that will be viewed at full size. Going above 90 inflates the file quickly for little visible gain. Because the source is a lossy HEIC, the pipeline is lossy twice; keep the quality at 75 or higher if the AVIF is meant to replace the original rather than be a web copy.
A worked example
A 4032 by 3024 iPhone photo of 2.1 MB as HEIC typically becomes about 700 to 1100 KB as AVIF at quality 80, around 400 to 600 KB at quality 65, and 250 to 350 KB at quality 50. Compared with converting the same photo to JPG at similar visual quality, AVIF tends to be 30 to 50 percent smaller. Images with large smooth areas such as sky compress much better than fine textures such as foliage, so treat these as typical ranges, not guarantees.
What is lost in the conversion
The HEIC to PNG decode flattens everything to 8 bits per channel in sRGB. iPhone photos are captured in Display P3, and newer models record 10 bit HDR with a gain map; that extra colour range and the HDR presentation are gone in the output, even though AVIF itself is capable of storing both. EXIF metadata including GPS location, capture time and lens data is discarded when the image passes through the canvas, which is often desirable before publishing. Live Photo motion and depth data are not carried over. If any of those matter, keep the HEIC as your master and treat the AVIF as a derivative.
When AVIF is the wrong choice
AVIF is a web delivery format. Many desktop applications, older phones and some content management systems still reject it, and older browser versions cannot display it at all; Safari only added support in 2022. If the photo is going into a document, being sent to a stranger or printed, HEIC to JPG is safer. If you want a modern format with broader tool support and faster encoding at slightly larger sizes, HEIC to WebP is the middle ground. For a fuller comparison see WebP vs AVIF vs JPEG.
Limits
Very large HEIC files, such as 48 megapixel exports, may exceed the memory available to a browser tab on a phone. Multi-image HEIC containers contribute their primary image only. Colour profiles other than P3 and sRGB may render with a slight shift. Each converted file downloads individually.