What is a HEIC file?

HEIF is a container: a structure of boxes taken from the ISO base media file format (ISO/IEC 14496-12) that holds, in the IANA registration's words, "one or more image items", and, as image/heif-sequence or image/heic-sequence, image sequences. It does not by itself say how the pixels are compressed. HEIC is the name for the common case where they are compressed with HEVC, the codec standardised as H.265.

The IANA registration draws the line through brands, the four-letter codes in the file's first box. A file whose brands include 'heic', 'heix', 'heim' or 'heis' and that meets those brands' requirements is image/heic and conventionally ends in .heic. A HEIF file meeting the general 'mif1' brand is image/heif and ends in .heif. The .hif extension is allowed for both. AVIF, for comparison, is the same container with AV1-coded images.

Start by checking that the file really is HEIC. Open it in a hex viewer: bytes five to eight read "ftyp" and the next four name the main brand, "heic" for the sample measured on this page. If you see FF D8 at the very start instead, it is a JPEG with a .heic name and any image viewer will open it once renamed.

A genuine HEIC needs software with an HEVC image decoder. Browsers are the usual gap: MDN's format guide does not list HEIC, and the Chromium build measured here refused to decode it. AltFTool's HEIC to JPG Converter gets around this by bringing its own decoder, the heic2any library (libheif compiled to JavaScript), so the file is decoded and re-encoded as JPG or PNG inside your browser tab.

Choose PNG if the HEIC has transparency or you plan to edit the image further; choose JPG for sharing and upload forms. Expect the JPG to be larger than the HEIC was, as the measurement below shows: you are trading a compact file for a format everything can read.

Because HEIF holds image items rather than a single bitmap, one file can carry more than one picture. The registration's optional itemtypes parameter describes each item and refers to "the primary item of the file", the picture a reader is meant to treat as the image. Multi-layer items get their own parameter (lhevcptl) describing which combinations of layers a decoder must handle.

In practice this matters when a converter gives you a different number of pictures than you expected. If a HEIC turns into more than one JPG, or a burst turns into a single frame, the file held several items and the tool chose which to decode. Check the result before deleting the original.