A digital image contains pixels; a printer places physical dots. PPI describes how many image pixels are assigned to one inch of the intended print, while DPI usually describes a device’s dot placement. The values can appear together in a print workflow, but they answer different questions. PaperSize therefore uses PPI for pixel dimensions and explains familiar DPI wording without pretending the terms are identical.
Pixels describe the file; dots describe the device
A 1200 × 1800 image always has that pixel count. Assigning it to a 4 × 6 inch print gives 300 pixels per inch in both directions. A printer may use several ink or toner dots to reproduce one image pixel, so a printer’s advertised DPI does not change the file’s width and height. PaperSize never multiplies physical size by printer DPI when the requested result is an image canvas.
Use inches × PPI, then round once
Pixel width equals physical width in inches multiplied by PPI; height follows the same rule. Metric dimensions first divide by the exact conversion 25.4 millimetres per inch. A4 width at 300 PPI is 210 ÷ 25.4 × 300 = 2480.3149… pixels, so the nearest whole-pixel display is 2480. PaperSize retains the decimal calculation and rounds only the final pixel count.
Pixel count is not a universal quality verdict
Viewing distance, image detail, halftoning, printer technology, paper, sharpening, and the print provider’s workflow all affect the result. A selected PPI is an input, not a promise that every printer will produce the same quality. The calculation also says nothing about unprintable margins, bleed, cropping, colour profiles, or file compression. Use the printer or provider specification for those decisions.