DPI, PPI and resolution for print

PPI: how many pixels per inch

PPI (pixels per inch) says how many image pixels are spread across one inch (25.4 mm) of paper when printing. A file has no physical size of its own, only a number of pixels. Only once you set the PPI do you get the size of the print:

size in cm = number of pixels ÷ PPI × 2.54

A photograph 6000 px wide at 300 PPI: 6000 ÷ 300 = 20 inches; 20 × 2.54 = 50.8 cm. A height of 4000 px gives 33.87 cm, or 33.9 cm.

DPI: how many droplets the printer makes

DPI (dots per inch) is a printer specification: how many ink droplets it can place per inch. Figures such as 2400 × 1200 dpi or 2880 × 1440 dpi describe how fine the dot pattern is, not how many pixels you need. The printer builds a single image pixel from many droplets to mix its shade.

When a lab says “send the file at 300 DPI”, it actually means 300 PPI. The resolution value stored in the file is only a note. If you change it without resampling (image editors usually have a Resample option), the number of pixels, and therefore the detail, stays the same. Only the suggested print size changes.

How much PPI you really need

300 PPI is the established threshold for prints that people look at in their hands. It comes from how finely the human eye can resolve detail. A resolving power of about one arcminute is commonly assumed. That gives a simple rule of thumb: the distance beyond which you can no longer make out individual pixels is roughly

distance in cm ≈ 8733 ÷ PPI

PPI	Pixels merge from about	Typical use
300	29 cm	print held in the hand, album, portfolio
240	36 cm	smaller sizes on the wall, desk
180	49 cm	medium sizes on the wall
150	58 cm	large sizes above furniture
100	87 cm	posters and large-format printing

It is a rule of thumb, not a law. How sharp a print looks also depends on the contrast of the subject, noise, sharpening, the paper surface, and whether people step up close to a large picture. People do look at large photographs from close up as well, so a higher PPI pays off there.

Table: how many pixels a print size needs

Pixel counts are rounded up so that the size is fully covered.

Print size	At 300 PPI	MP	At 150 PPI	MP
10 × 15 cm	1182 × 1772	2.1	591 × 886	0.5
20 × 30 cm	2363 × 3544	8.4	1182 × 1772	2.1
A4 (21 × 29.7 cm)	2481 × 3508	8.7	1241 × 1754	2.2
30 × 40 cm	3544 × 4725	16.7	1772 × 2363	4.2
A3 (29.7 × 42 cm)	3508 × 4961	17.4	1754 × 2481	4.4
40 × 60 cm	4725 × 7087	33.5	2363 × 3544	8.4
50 × 70 cm	5906 × 8268	48.8	2953 × 4134	12.2
70 × 100 cm	8268 × 11,812	97.7	4134 × 5906	24.4

A 24-megapixel camera (6000 × 4000 px) therefore manages 50.8 × 33.9 cm at 300 PPI and 101.6 × 67.7 cm at 150 PPI. A 12-megapixel phone (4032 × 3024 px) gives 34.1 × 25.6 cm at 300 PPI.

Cropping costs pixels

Every crop reduces the number of pixels you print with. Crop a 6000 × 4000 px frame to half its width and you are left with 3000 × 2000 px, which at 300 PPI is only 25.4 × 16.9 cm. A print size with a different aspect ratio (30 × 40 cm, for example, which is 4 : 3) will also require cropping an edge. The aspect ratios of common sizes are covered in Photo print sizes.

Upscaling: what interpolation can do

Resampling upwards (interpolation) adds pixels by calculating them from their neighbours. It does not create detail that the image never captured. In practice:

• modest enlargement (from 180 to 300 PPI, say) with a good algorithm followed by gentle sharpening usually improves the result, especially on glossy paper,
• large enlargement adds smooth, “plastic” areas; it is often better to accept a lower PPI and a greater viewing distance,
• AI-based tools invent detail. On a landscape that can look convincing; on faces in old family photographs it can produce features the person never had. Treat them with caution when restoring an archive, and always keep the original scan.

Quick method

1. Find the file’s dimensions in pixels (in the file properties or the image information in your editor).
2. Divide by the PPI and multiply by 2.54. Or use the calculator.
3. Decide by viewing distance whether 300, 240, 180 or 150 PPI is enough.
4. If you need a larger size than the file can carry at an acceptable PPI, choose a smaller print with a wider mount instead.