Why iCloud isn't always the answer
Apple's standard advice for HEIC compatibility is to use iCloud Photos with “Optimize iPhone Storage” enabled. When you access photos on a Windows PC through iCloud for Windows, Apple serves a JPEG version automatically. This works — but it requires iCloud Photos to be enabled, sufficient iCloud storage (free tier is 5 GB, which fills quickly), and the iCloud for Windows app installed.
Many people run into HEIC problems precisely because they don't use iCloud: they transferred photos via USB, AirDrop to a Windows PC, or copied them from an SD card. In all these cases, iCloud is not involved, and you end up with HEIC files that nothing on Windows can open.
How browser-based conversion works
LocalJPG runs a compiled version of libheif — the reference HEIC decoder — directly in your browser via WebAssembly. The library was compiled from C++ to a format browsers can execute natively, with no server involved. Your HEIC file is read by the decoder on your own device, then re-encoded as JPEG using MozJPEG at quality 85.
The entire process happens locally. No Apple ID, no iCloud account, no internet connection required after the page loads. You can disconnect from Wi-Fi mid-conversion and it will still complete.
AirDrop and USB transfers
AirDrop from iPhone to Mac automatically converts HEIC to JPEG (since macOS Mojave). AirDrop from iPhone to Windows does not — you receive the raw HEIC file. USB transfers via File Explorer also give you the original HEIC. In both cases, dropping the file into the converter above is the fastest path to a JPEG that opens everywhere.
Related: HEIC to JPG converter · Convert HEIC without uploading · How to open HEIC on Windows