What DPI do you need for print?
Short answer
What matters is effective DPI: the resolution of an image at the size it will actually print. A 300 dpi image scaled to 400% is a 75 dpi image. Common minimums are 250 to 300 dpi for close-viewed print and 100 to 150 dpi for large format viewed at a distance.
How to calculate it
Divide the pixel dimension by the printed dimension in inches. The number stored in the file is only the resolution at the file’s own nominal size; placing or scaling the image changes it. This is why a file can pass a naive resolution check and still print soft.
Image 1200 × 800 px Placed at 400 × 267 mm = 15.75 × 10.5 in Effective 76 dpi · below 150 minimum
Viewing distance changes the requirement
A banner read from three metres away tolerates far less resolution than a business card. This is why a single minimum across a shop is unhelpful: the same 76 dpi image is unacceptable on a brochure and perfectly adequate on a building wrap. The number belongs in the production profile, per process.
Why upscaling is not a fix
Interpolation, including model-based upscaling, produces plausible pixels. It cannot recover detail that was never captured, and on print it tends to produce soft edges and invented texture that a customer will notice at final size. Where the missing detail matters, the honest answer is to ask for better artwork.
How Prepta handles it
Prepta measures effective DPI at final output size for every placed image, compares it to the minimum in your production profile, and shows the affected area. Below the minimum it escalates with a recommendation rather than upscaling — and it says so plainly in the report.