What is an AVIF file?

Why AVIF is so much smaller

AVIF stores a single still frame using the AV1 video codec's intra-frame tools, and video codecs have had two extra decades of research poured into them. Where JPEG works in fixed 8×8 blocks, AV1 partitions the image adaptively — large blocks across a smooth sky, tiny ones across detailed foliage — and has a far richer set of prediction modes for guessing a block's contents from its neighbours.

In practice this means an AVIF at the same perceived quality as a JPEG is typically 40–50% smaller, and roughly 20–30% smaller than the equivalent WebP. The gap is widest on smooth gradients, where JPEG's banding is most obvious and AV1's directional prediction is most effective.

HDR, wide gamut, and 12-bit colour

AVIF is the first widely supported still format to handle high dynamic range properly. It stores up to 12 bits per channel and carries the colour primaries and transfer characteristics as metadata, so a display capable of HDR can show the full range the camera captured, while an ordinary display tone-maps it down sensibly.

For anyone working with modern phone photography this matters more than it sounds. iPhone and recent Android cameras capture HDR by default; saving that to 8-bit JPEG throws away the highlight detail that made the shot look good on the phone screen in the first place.

The cost: encoding time

AVIF's compression advantage comes from searching a very large space of possible encodings, and that search is expensive. Encoding a large AVIF can take several seconds where a JPEG takes milliseconds — noticeably so when you are converting a batch in a browser tab. Decoding, thankfully, is fast, so the cost falls on whoever creates the file rather than on the reader.

This is the practical reason AVIF has not simply replaced everything: it is excellent for images you encode once and serve many times, and a poor fit for anything that needs to be generated on demand.

Converting AVIF files

If a program refuses to open an AVIF, converting to JPG is the pragmatic fix — accepting that you are re-encoding lossy data and should use a high quality setting. Convert to PNG instead when the image has transparency or when you intend to edit it further.

Going the other way, converting an existing JPEG library to AVIF will shrink it substantially but cannot undo the artefacts already baked in. The real gains come from encoding to AVIF from a lossless master.