What HEIC is and why your iPhone uses it
HEIC is Apple's default photo format since iOS 11. It stores the same picture as JPG in about half the space, which lets an iPhone hold twice as many photos before storage fills up. The format is based on the HEVC video codec and can also keep extras like Live Photo data and depth information.
The downside is compatibility. Older Windows versions need a paid extension to show HEIC, many web forms reject it, and some email clients and printing kiosks cannot open it. JPG is the format everything understands.
Converting without losing much quality
Both HEIC and JPG are lossy, so a conversion decodes the photo and encodes it again. At a high quality setting, such as 90 percent, the result is visually the same as the original for everyday use. The JPG will be larger than the HEIC, often around double.
If the converted file is too big for an upload form, lower the quality a notch or resize the photo. For a strict limit like 100 KB, run the JPG through the image compressor afterward.
Changing the iPhone setting instead
You can also stop the problem at the source. Open Settings, then Camera, then Formats, and choose Most Compatible. New photos will be saved as JPG. This does not fix photos you have already taken, which is where a converter helps.
When you share a photo from the Photos app through AirDrop to a Mac or through email, iOS sometimes converts it for you. Uploading straight from the library to a website does not always do that, which is why the file arrives as HEIC.
What gets kept and what gets dropped
Orientation is respected, so photos should not come out sideways. Location and camera metadata are removed during conversion, which is good for privacy but means the JPG will not show where it was taken. Live Photo motion is not kept; you get the still frame.
Opening HEIC files on other devices
Recent versions of Windows can show HEIC when the right codec is installed, and Android phones differ by brand. Rather than hunting for add-ons, converting once to JPG gives you a file that opens on every computer, phone and printer.
If you send photos to a relative with an older laptop, convert them first. The same goes for school or government sites that list JPG and PNG as the only accepted types. A HEIC file will often just fail with a vague error message and no hint about why.
Moving a whole trip's worth of pictures is where HEIC causes the most grief. Add up to 20 at a time, convert, and download the ZIP. Repeat in groups if you have more. Keep an eye on the file sizes shown, since JPGs from a 48 megapixel sensor can be large.