HEIC files start failing the moment a photo leaves an iPhone. Windows Explorer shows an empty thumbnail, older Android gallery apps refuse to open them, most content management systems reject the upload outright, print kiosks report an unknown format, and email clients offer to download the attachment instead of displaying it. The reason is not corruption. HEIC is a container defined in ISO/IEC 23008-12 that wraps frames of HEVC, a video codec whose licensing position kept it out of most desktop software for years, so a perfectly healthy file reads as garbage anywhere the decoder is missing. Converting to JPEG puts the same pixels into a codec every device has understood since 1994, at the cost of the compression efficiency and the extra headroom that HEIC can hold. Doing that conversion in a browser is unusually appropriate here, because the photos people convert are family pictures, passport scans, receipts and medical results. The problem with them was a missing codec, not a need for a stranger's server.
Why the file says it is broken when it is not
Three things get confused with each other. The container is HEIC, sometimes written with the .heif extension, and inside it are HEVC frames. Renaming the file does nothing at all: the bytes on disk are still HEVC, so a file called photo.jpg will fail exactly as loudly as photo.heic. The second confusion is about which apps can decode it. Apple platforms, and a growing number of third-party apps, include the decoder. Windows does not ship with one; installing a HEIF Image Extension from the Microsoft Store adds a decoder, which is a legitimate alternative if you only need to view the occasional file. Linux has ffmpeg and ImageMagick, both of which read HEIC once built with the right libraries. The third is metadata: HEIC files usually carry an EXIF block with orientation, capture time and often GPS coordinates, so a converted copy should be checked for orientation before it is emailed to anybody.
What the conversion actually costs you
JPEG is a lossy codec, so converting is one generation of loss. The difference is usually invisible at normal viewing size, and it is the only realistic option if the destination needs JPEG. Converting to PNG instead is lossless with respect to the decoded pixels, so the quality is preserved, but the file will usually be much larger and you will not be able to paste it into a form that only accepts JPEG. Two things never survive. The 10-bit depth and the HDR gain maps that modern iPhones record are outside what a JPEG file can express, so tone mapping has to happen somewhere and the result will be flatter than the original in high-contrast scenes. And the HEIF-specific container features, such as image sequences and transparency handled in a non-standard way, are simply not carried over. If a photo matters as a family record, keep the original HEIC and treat every JPEG you make as a copy.
When this converter is the wrong tool
Reach for something else in three situations. If you have thousands of photos to convert on a schedule, a command line tool beats any web page: on macOS, sips -s format jpeg file.heic --out file.jpg does the job with no network at all, and ImageMagick or ffmpeg do the same across platforms and can script cleanly. If your problem is not the format at all, converting will not help - a file reported as corrupt that is actually a valid PNG or a truncated download needs a different diagnosis, and a pixel-level inspection of what the bytes claim to be will tell you which. And if the goal is to keep the higher quality of HEIC rather than escape it, then the right move is installing a decoder or converting to PNG, not to JPEG, since JPEG is the lower-fidelity of the two.
Frequently asked questions
Will converting HEIC to JPG reduce the image quality?
Slightly, yes. Converting to JPEG re-encodes the picture with a lossy codec, which is one generation of loss. The degradation is normally invisible at normal viewing sizes, but if the photo matters as a record, keep the HEIC original and treat the JPEG as a convenience copy. Converting to PNG instead preserves the pixels exactly, at the price of a much bigger file.
Does the converted file keep my location data?
That depends on the converter, and it is worth checking rather than assuming. Phone photos usually carry GPS coordinates in their EXIF metadata, and a good privacy habit is to strip location data before sharing. After you download a converted file, check its properties: if a location field is present, remove it with your operating system's sharing options or with an EXIF editor before sending the photo to anyone.
Why is the converted photo sideways?
Phone cameras usually record a landscape sensor image plus an orientation flag in the EXIF block, and the flag tells the viewer to rotate it. If your converted copy comes out rotated, the converter did not carry the flag over. Rotate it once in a viewer and save again, and check whether the same problem happens with other photos from the same phone.
Can I convert a whole folder of HEIC photos at once?
This page links to a separate converter, so its batch size and any daily limits are set by that site - read its own notes before uploading a large folder. If you need to convert thousands of files repeatedly, a local command line tool such as sips on macOS, or ImageMagick on Linux and Windows, will be faster and will not send your photos anywhere.