There is no JPEG quality number that guarantees a particular file size or an unchanged picture. Quality is an encoder setting, not a percentage of original detail retained. Set the intended dimensions first, compare a few exports, and choose the smallest one that still preserves the details your audience needs to see.
Why the same number can produce different results
JPEG normally uses lossy compression. Different encoders can interpret their quality scales differently and make different decisions about color detail and compression. A quality setting of 80 in one application is not a promise that it matches 80 in another. Even a high-quality JPEG export is not proof of pixel-identical preservation.
Run a small comparison using the original
- →Keep the original file unchanged.
- →Choose the final pixel dimensions for the intended placement.
- →As an illustrative comparison, export versions at 70, 82 and 90 in the same application. These are trial settings, not universal recommendations.
- →Compare each export with the original at the delivery size, then inspect critical detail at full size.
- →Choose a higher setting if visible defects affect the image purpose.
- →If all versions are larger than necessary, reassess dimensions rather than lowering quality indefinitely.
Look for the defects that matter
Check block patterns in flat areas, halos around sharp edges, smeared fabric texture and steps in smooth gradients. Small text and diagrams are particularly demanding: a screenshot can look worse as a JPEG even when a photograph looks acceptable at the same setting. Compare PNG or another suitable lossless export when crisp text matters more than photographic compression.
Do not repeatedly compress the compressed version
Generate each candidate from the same original. Reopening a lossy export and saving it again can accumulate degradation, and increasing its quality setting cannot restore the discarded source detail. Keep the archival original separate from the smaller delivery copies.
If a receiving form imposes a byte limit, check the actual exported file size. Reducing width and height may be more effective than pushing quality lower, but never resize below a stated minimum. For transparent artwork, JPEG also needs a chosen background because it cannot preserve an alpha channel. Inspect those edges after export.