Everything else follows. An A4 page at 300 DPI is 8.27 × 11.69 in × 300 = 2480 × 3508 px. There is no other calculation — the difficulty is that "DPI" is used for two unrelated things.
The 72 DPI myth
"Web is 72 DPI, print is 300 DPI" is repeated everywhere and is not true in any useful sense. 72 was the physical pixel density of a 1984 Macintosh screen. Nothing has been 72 PPI for forty years — the phone in your pocket is between 300 and 500.
A digital image has no DPI. It has pixel dimensions. DPI is a number stored in the file's metadata that says how large to print those pixels, and it changes nothing about the image itself. Setting a web export to 300 DPI does not improve it; setting it to 72 does not degrade it.
The number only becomes real when the file meets paper.
Two different DPIs
| Term | Means | Belongs to |
|---|---|---|
| PPI — pixels per inch | How many image pixels land in one printed inch, or how densely a display packs its pixels. | Your file, and the screen. |
| DPI — dots per inch | How many ink dots the press lays down per inch. A 300 PPI image on a 2400 DPI press is normal — several ink dots build one pixel. | The printing hardware. |
| LPI — lines per inch | Halftone screen frequency. Rule of thumb: image PPI ≈ 2 × LPI, which is where 300 comes from (150 LPI offset). | The prepress setup. |
In everyday use people say DPI when they mean PPI, and mostly no harm comes of it. It matters when a printer asks for 300 DPI and you have a 300 DPI desktop inkjet output — those are different requests.
What resolution you actually need
| Output | Resolution | Reasoning |
|---|---|---|
| Magazine, brochure, business card | 300 PPI | Held at reading distance. The standard for offset and digital press work. |
| Newspaper | 200 PPI | Coarser halftone screen and absorbent stock — more resolution is thrown away. |
| Poster, viewed from 1–2 m | 150 PPI | Viewing distance does the work. 300 PPI on an A0 poster is a 6-gigapixel file for no visible gain. |
| Billboard | 20–50 PPI | Genuinely. At 20 m the eye cannot resolve more. |
| Large-format banner | 100–150 PPI | At final size, not at some scaled-down proxy size. |
Bleed, trim and safe margin
Paper shifts as it is cut. Three zones handle it:
- Bleed — 3mm past the trim edge on every side. Any element meant to run off the page must extend into it, or a white sliver appears when the cut lands 1mm off.
- Trim — the finished size. A4 is 210 × 297mm.
- Safe margin — 3–5mm inside the trim. Nothing you need to read goes outside it.
At 300 PPI, 3mm of bleed is 35 px. An A4 page with bleed is therefore 2551 × 3579 px rather than 2480 × 3508 — a difference worth getting right before you start rather than after.
The habits to unlearn
- RGB. Print is CMYK, and the conversion loses the most saturated blues and greens. Check the colours you care about before the file is final, not after the proof.
- Pure black text.
#000000becomes 100% K, which on press looks thin and grey. Rich black (roughly 60/40/40/100) is for large areas; keep small text at 100% K only, or it will misregister. - Hairlines. A 1px border scales to nothing. Below 0.25pt a line may not print at all.
- Fonts. Outline or embed them. A missing font at the press substitutes silently.
Every common paper format worked out in pixels at 72, 96, 150, 200, 300 and 600 DPI is on print sizes in pixels.
Questions
What size is A4 in pixels?
It depends entirely on the DPI. At 300 DPI, A4 (210 × 297 mm) is 2480 × 3508 px. At 150 DPI it is 1240 × 1754 px. At 72 DPI it is 595 × 842 px, which is also why PDF page dimensions are stated in 72nds of an inch. See A4 in pixels.
Can I convert a 72 DPI image to 300 DPI?
You can change the metadata, which does nothing. You can resample it, which invents pixels that were never captured and looks soft. If the pixel dimensions are not there, the resolution is not there — go back to the source.
How do I know if an image is high enough resolution for print?
Divide the pixel dimensions by the printed size in inches. A 1200 px wide image printed 6 inches wide is 200 PPI — fine for a poster, short of the 300 a brochure wants.
Do I need bleed on a digital-only PDF?
No. Bleed exists because paper gets cut. A PDF that will only ever be read on screen needs no bleed and no crop marks.