No lossy conversion is perfectly lossless, but you can get to a point where you cannot see any difference. Here is how.
Why there is always some loss
Both HEIC and JPG are lossy formats. Turning one into the other means decoding the HEIC and compressing the pixels again as JPG. Every lossy pass discards a little detail. The goal is a setting where the discarded detail is below what the eye can notice.
Use quality 90 to 95
At around 92 on a 100-point scale, JPG output is visually very close to its source for normal viewing, printing at regular sizes and light editing. Going up to 100 mainly inflates the file size without a visible benefit. Dropping to around 80 shrinks files substantially and still looks clean for screens and email.
Do not resize unless you must
Resizing throws pixels away. Keep "Largest side" on Original size for photos you want to preserve. Reduce it only for web use or a file-size limit.
Convert once, from the original
Quality loss accumulates with every re-save. Always convert from the HEIC original rather than from an earlier JPG, and avoid repeated editing and re-saving of the same JPG.
If you need lossless
Choose PNG (in the browser) or TIFF. These store the decoded pixels without adding new compression loss. They produce very large files.
What cannot be preserved
- HEIC’s greater colour depth is reduced to JPG’s 8 bits, which is imperceptible in nearly every case.
- Capture date and location tags are not copied by in-browser conversion. For that, export from the Photos app. See how.
- Live Photo motion and depth information are not part of a JPG.