To convert HEIC to JPG, add the HEIC photo, keep JPG as the output format and download the result. The JPG opens on Windows, in older apps and on upload forms, but it is compressed again with an older codec, so it is usually larger than the HEIC and holds one still frame only.
Transfers use HTTPS, and files converted on the free tier are deleted within 24 hours, so the result is not a long-term archive. Quality depends on the source file, so open the JPG file once before you share it.
FromHEICLossyTransparency
HEIC (High Efficiency Image Container)
The still-image format iPhones have used by default since iOS 11. It wraps HEVC-compressed pictures in the HEIF container, usually smaller than a JPG of the same picture, and can also hold deeper colour, depth maps and image sequences.
ToJPGLossy
JPEG (Joint Photographic Experts Group)
The most widely readable photo format, standardised in 1992. Phones, browsers, operating systems, print kiosks and upload forms all accept it, at the cost of 8-bit colour, lossy compression and no room for depth or motion data.