When you need JPG on the phone itself
Many upload forms in Safari still reject HEIC even though the photo was taken on that iPhone. You need a .jpg before you tap Submit — not after you get to a Windows PC.
Use Safari, not an extra app
Open the HEIC to JPG tool in Safari. Tap the drop zone → Photo Library or Files. Convert. Save the JPG. Safari can decode HEIC natively, so the heavy WebAssembly decoder often never loads. That is easier on battery and memory.
If the tab crashes
A 48 MP ProRAW-adjacent burst or a long Live Photo still can exhaust RAM. Convert one photo, close other tabs, and avoid converting a whole vacation album in one go on an older iPhone. The original stays in Photos.
Apple’s own “Most Compatible” setting
Settings → Camera → Formats → Most Compatible makes new photos JPG. It does not convert the library you already have. A local converter is for the photos that are already HEIC.
Ready? Use the HEIC to JPG converter — files never leave this device.