“DPI” is used loosely across the print industry; the technically correct term for an image file is PPI (pixels per inch). DPI describes how many dots a printer lays down per inch, which is a property of the press, not of your file. In everyday use the two get swapped freely, and the calculation everyone means is the same:
pixels = inches × dpi and inches = pixels ÷ dpi
An A4 page is 210 × 297 mm, which is 8.27 × 11.69 inches. At 300 dpi that comes to 2480 × 3508 pixels — the figure quoted everywhere as “A4 at 300dpi”, and worth recognising on sight.
| Print job | Typical target | Why |
|---|---|---|
| Photographs, magazine, fine art | 300 dpi | The standard for continuous-tone images held in the hand |
| Newspaper, high-volume mono text | 150–200 dpi | Absorbent paper spreads ink; more resolution is wasted |
| Poster viewed at arm’s length | 150–200 dpi | Still resolving fine detail at that distance |
| Large-format banner | 50–150 dpi | Seen from metres away; see the viewing-distance tool |
| Billboard | 10–30 dpi | Seen from tens of metres |
You have a 4000 × 3000 pixel photograph and need an A3 print (297 × 420 mm, or 11.69 × 16.54 inches). What dpi will it actually print at?
Is that enough? For a poster read at arm’s length, yes — 242 dpi is comfortably above the 200 dpi where most people stop noticing. For a fine-art print inspected closely, it is slightly under the 300 dpi ideal, and you would want a larger source file.
Upscaling does not add detail. If your file works out to 90 dpi at the size you need, resampling it to 300 dpi in an image editor produces a much larger file containing exactly the same information, plus interpolation blur. Printers can spot it immediately. The honest options are to print smaller, source a higher-resolution original, or accept the lower resolution and check a proof.
No — it is the right target for a specific case: a continuous-tone image viewed close up. Text is resolution-independent (it is vector outlines, not pixels) and large-format work is governed by viewing distance, where 300 dpi would produce an unnecessarily enormous file.
Because the image and the page rarely share an exact aspect ratio, so the two axes are scaled differently. The lower of the two is the true resolution, and it is the one to quote.
Yes, but understand what you are getting. Web images are often tagged at 72 dpi, which describes how they are displayed on screen, not how much information they hold. What matters for print is the pixel dimensions and the physical size you are printing them at.