DPI is mostly a label: what really decides how big you can print
It happens all the time. A print shop, a publisher or an online form says images must be 300 DPI. You check your photo's properties and it says 72 DPI. It feels like the photo is a quarter as good as it needs to be.
It almost certainly is not. The DPI number stored in an image file is a label. It does not change a single pixel.
Pixels are the real resolution
A digital image is a grid of pixels, say 4032 across and 3024 down, which is a standard 12 megapixel phone photo. That grid is everything the file knows about the picture. How sharp it can look depends on how many of those pixels are spread over how much paper.
The DPI value (strictly, PPI, pixels per inch, but everyone says DPI) is a suggestion for that spreading. At 300 DPI, our phone photo would print 4032 ÷ 300 = 13.4 inches wide. At 72 DPI, the same file would print 56 inches wide. Same file, same pixels, different suggestion.
That is why you can change the number without touching the image: tools like the DPI changer rewrite just the header, and the picture data is copied unchanged. It is also why web browsers ignore the value entirely. On a screen, only the pixel dimensions matter.
Where 72 came from
The number 72 is a leftover. Early Macintosh screens displayed roughly 72 pixels per inch, which lined up neatly with the 72 points in an inch used in typography. Software began stamping 72 into files as a default, and many programs still do. It says nothing about the quality of your image.
Why printers ask for 300
There is real reasoning behind 300. A person with normal eyesight can separate details down to about one arcminute, a sixtieth of a degree. At a reading distance of about 12 inches, that works out to roughly 290 pixels per inch. Print at that density and the eye cannot see individual pixels; below it, and edges start to look soft or stepped.
A useful rule follows from the same geometry: the pixels per inch you need is about 3438 divided by the viewing distance in inches. It explains why posters and billboards are printed at far lower densities without anyone complaining.
| Viewed from | PPI needed | Typical example |
|---|---|---|
| 30 cm (12 in) | ~290 | Photo prints, magazines, books |
| 60 cm (24 in) | ~145 | Desk calendars, framed prints on a shelf |
| 1.5 m (5 ft) | ~57 | Posters on a wall |
| 10 m (33 ft) | ~9 | Billboards, building banners |
Working out what you can print
Divide the pixel dimensions by the DPI you are aiming for. That is the print size in inches. Or go the other way: multiply the print size by the DPI to see how many pixels you need.
| Print size | Pixels needed at 300 DPI | At 150 DPI |
|---|---|---|
| 6 × 4 in (15 × 10 cm) | 1800 × 1200 | 900 × 600 |
| 7 × 5 in (18 × 13 cm) | 2100 × 1500 | 1050 × 750 |
| 10 × 8 in (25 × 20 cm) | 3000 × 2400 | 1500 × 1200 |
| A4 (21 × 29.7 cm) | 2480 × 3508 | 1240 × 1754 |
| A3 (29.7 × 42 cm) | 3508 × 4961 | 1754 × 2480 |
A 12 megapixel phone photo covers A4 comfortably at 300 DPI, and A3 at about 240 DPI, which looks excellent on a wall. If you would rather not do the division yourself, StatGarden's print resolution calculator works it out in either direction.
So what should you do when someone asks for 300 DPI?
First, check the pixels. If your image has at least 300 pixels per inch of the requested print size, it is good enough. Change the label to 300 so the form or the software stops complaining, and send it.
If it falls a bit short, say 220 to 250 DPI at the requested size, it will usually still print well, especially for photos, which have few hard edges. Tell the printer, or accept it.
If it falls well short, changing the label will not help, and neither will enlarging the image. Resizing up gives the printer more pixels but no more detail; the result looks soft rather than blocky, which is sometimes an improvement, but it is not a fix. The honest options are a smaller print, a larger original, or a crop that makes the most of what you have.
One trap to avoid
Some image editors offer to change DPI and resample at the same time. With resampling on, changing 72 to 300 multiplies the pixel dimensions by about four, making the file sixteen times bigger with no new detail. With it off, only the label changes. For satisfying a 300 DPI requirement, you almost always want resampling off.