What a HEIC file actually is
HEIC is the file extension Apple uses for images stored in the HEIF container with HEVC (H.265) compression. iPhones have shot in it by default since iOS 11 in 2017, and some Samsung and Canon devices offer it as an option. It is a genuinely better format than JPG at the pixel level: roughly half the file size for the same quality, 10-bit colour, the wide Display P3 gamut, and room in the container for extras like the short video of a Live Photo, burst sequences and depth maps. The problem is everything outside the Apple ecosystem. Windows needs a paid codec, many web forms reject the extension, and older software simply cannot open it. That is why HEIC to JPG is the most requested conversion on this site.
How the decode works, and why it is slower here
Browsers cannot decode HEIC natively, so this page uses heic2any, a JavaScript port of the libheif decoder, loaded on demand the first time you convert. That library is several megabytes, so the first conversion in a session pauses for a moment while it downloads and initialises. After that it is cached. The decoder produces a lossless PNG in memory, which is then passed to a Web Worker that re-decodes it and encodes the final JPG with mozjpeg. Two decode passes and a lossless intermediate make this noticeably slower than converting between ordinary formats: expect two to six seconds for a 12 megapixel photo on a laptop and longer on a phone. The file itself never leaves your device.
What changes and what is lost
Only the primary image is kept. The Live Photo video, other frames of a burst, and any depth or portrait map are not carried over. Colour goes from 10-bit P3 to 8-bit sRGB as interpreted by the decoder, and any HDR gain map is dropped, so very bright highlights in a scene may look flatter than they do on the iPhone screen. All metadata is removed: the pixels pass through a canvas, which strips EXIF entirely, including the capture date, camera model, lens data and GPS position. For sharing that is a privacy benefit. If you rely on dates for sorting, keep the original alongside the JPG. To inspect what is in a photo before you convert it, use the EXIF viewer.
Picking a quality and what size to expect
The slider runs from 20 to 95 and defaults to 80. For photos going to family, social media or a listing, 80 to 85 is visually indistinguishable from the source on a normal screen. Use 90 or above if the JPG will be edited and saved again later, since each lossy save costs a little. Values around 70 are fine for images that will only be seen small.
Here is a worked example. A 4032x3024 iPhone photo of about 1.9 MB as HEIC typically becomes a JPG of 1.5 to 2.5 MB at quality 80, and 3 to 4 MB at quality 90. It is normal for the JPG to be larger than the HEIC: JPG is a 1992 codec and needs more bytes to hold the same detail. A 2 MB screenshot-style HEIC with flat areas usually shrinks instead, to a few hundred kilobytes. These figures are typical, not guaranteed, and a noisy night shot will be bigger.
When not to convert, and alternatives
Do not convert the copy in your Photos library; export a copy and convert that, so the higher-quality original stays intact. If you regularly need JPGs, the iPhone can produce them directly: Settings, Camera, Formats, Most Compatible. For web use where file size matters more than universal compatibility, HEIC to WebP gets files closer to the HEIC size. And for background on the format itself, read What is HEIC. Several files can be dropped at once; they are converted one after another, each downloading separately.