Effective PPI / printer DPI
How many image pixels land in each printed inch. Scale a photo up in the layout and its effective PPI drops, so it prints soft.
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What it is
PPI is how many image pixels land in each printed inch. DPI is how many ink dots the device sprays per inch, a property of the machine, not your file. People say DPI for both, which is fine in conversation and confusing in a spec. Effective PPI is the number that matters: the image's native PPI divided by how far you scaled it in the layout. Place a 300 PPI photo at 200% and it is now a 150 PPI photo.
Rules of thumb: 300 effective PPI for anything held in the hand (cards, brochures, books), around 150 for posters seen from a few feet, and 30 to 100 for banners and billboards read from across a street. InDesign's Links panel shows actual PPI next to effective PPI, and that second column is the one to audit before you send.
Gotcha: you cannot upsample your way to detail. Resampling adds pixels, not information, and it prints as soft mush (ML upscalers help, but they invent texture rather than recover it). Also the PPI tag stored in a file is just metadata: an 800px-wide web JPEG marked '300 dpi' still only prints about 2.6 inches wide. Pixel count and final size are the only real inputs.
Ask AI for it
Audit every placed image in this layout for print resolution. For each one, report pixel dimensions, placed size in inches, scale percentage, and effective PPI (native PPI divided by scale). Flag anything under 300 effective PPI for hand-held pieces or under 150 for posters, and for each flagged image tell me the largest size it can print at 300 PPI instead of upscaling it. Return the result as a table sorted worst first.