Anyone who has tried to print a photograph has met the number 300. "Make sure your file is 300 DPI." It is repeated everywhere, and it is only half understood. This post explains what resolution is, how it relates to the size of a print, and what matters when you buy one.
Pixels
A digital image is a grid of tiny squares of colour, called pixels. An image described as 6000 × 9000 has 6,000 pixels across and 9,000 down. Multiply them and you get 54 million pixels, or 54 megapixels.
The pixel dimensions are the real measure of how much detail a file holds. Everything else follows from them.
PPI: pixels per inch
A file has no physical size until you print it. When you do, its pixels are spread across the paper. PPI is the number of pixels that fall in each inch of the print.
It is simple division. A file 3,000 pixels wide, printed 10 inches wide, has 300 pixels in each inch: 300 PPI. The same file printed 30 inches wide has 100 PPI. Nothing in the file has changed. The pixels are larger on the paper.
So PPI isn't a property of the file. It is the relation between the file and the size you print it.
DPI: dots per inch
DPI belongs to the printer. An inkjet printer makes each colour by laying down many tiny dots of a few inks. DPI is how many of those dots it can place in an inch. Printer figures such as 1440 or 2880 DPI describe the machine, and they are much higher than the PPI of the image, because many ink dots go to make one pixel's worth of colour.
In everyday speech, people say DPI when they mean PPI. "A 300 DPI file" nearly always means an image that will print at 300 pixels per inch. This post uses PPI for the image, to keep the two apart.
Why 300?
At a normal reading distance of about 12 inches, a person with good eyesight can't make out individual pixels much finer than about 300 to the inch. A print at 300 PPI looks perfectly smooth when held in the hand. That is why 300 became the standard for books, magazines and photographs.
The important words are "held in the hand".
Viewing distance changes everything
Nobody looks at a 40 × 60 inch canvas from 12 inches away. You stand back to see it. The further away you are, the less fine detail your eye can resolve, and the fewer pixels per inch the print needs.
| Viewed from | Typical use | PPI that looks sharp |
|---|---|---|
| 1 foot | A small print, a book, a pin | 300 |
| 2 to 3 feet | A poster by a desk | 150 to 200 |
| 5 to 6 feet | A canvas above a sofa | 100 to 150 |
| 10 feet and more | A very large canvas or tapestry | 50 to 100 |
A roadside billboard is printed at a tiny fraction of 300 PPI, and looks sharp from the road. A rule of thumb is that the usual viewing distance is about one and a half to two times the diagonal of the picture.
So a large print doesn't need a file six times bigger than a small one. It needs enough pixels for the distance at which it will be seen.
How many pixels for each size
These are the pixel dimensions for our common sizes at 300 PPI, which is the ideal for close viewing, and at 150 PPI, which looks good at normal wall distance.
| Print size (inches) | At 300 PPI | At 150 PPI |
|---|---|---|
| 8 × 10 | 2400 × 3000 | 1200 × 1500 |
| 12 × 18 | 3600 × 5400 | 1800 × 2700 |
| 18 × 24 | 5400 × 7200 | 2700 × 3600 |
| 24 × 36 | 7200 × 10800 | 3600 × 5400 |
| 40 × 60 | 12000 × 18000 | 6000 × 9000 |
To work it out for any size, multiply the inches by the PPI.
What low resolution looks like
- Pixelation. Visible squares, and stepped, jagged edges on curves and diagonals.
- Softness. A general blur, as if slightly out of focus.
- Compression marks. Blocky patches and halos round edges. These come from a heavily compressed file, and they aren't the same as low resolution, though the two often go together.
You can't add detail that isn't there
Changing a file's setting from 72 to 300 does nothing by itself. The pixels are the same. Enlarging an image in software, called upscaling or resampling, invents new pixels between the old ones. Modern upscaling tools are good at this and can produce a clean result, and they are estimating, not recovering, detail.
This is why an image saved from a website or a phone screenshot makes a poor large print. It was sized for a screen.
The material matters too
- Canvas has a woven texture. The weave hides the very finest detail, so canvas is forgiving of lower resolution, and it softens a razor-sharp file a little.
- Matte paper shows detail well.
- Fabric, as on tapestries and desk mats, is softer again. The fibres spread the ink slightly. A tapestry is seen from across a room, so this suits it.
- Small items such as pins and keychains are seen close, and they are tiny, so even a modest file has plenty of pixels for them.
See canvas vs poster vs framed poster.
What this means when you buy a print
- You don't supply the file. With our prints, the artwork is prepared by us at a size suited to each product. There is nothing for you to check or set.
- Judge a large print from where you will stand. Any big print looks softer with your nose against it than from the sofa. That is how large prints work.
- Texture is not blur. Many of our pieces have grain, scan lines, glitch effects or soft, misty edges on purpose. Those are part of the artwork. See CRT and glitch art explained.
- A product photo on a screen can't show print resolution. Your screen has its own limits.
If a print arrives with a real fault, such as visible pixel blocks, banding or streaks, contact us within 30 days with photographs. See the inspection checklist.
Screens, for comparison
A screen has a fixed number of pixels. A 27-inch monitor at 2560 × 1440 has about 109 pixels per inch. A modern phone has 400 or more, because it is held close. The old figure of 72, often quoted for screens, comes from early computer displays and has no meaning now. It is the reason people still speak of "72 DPI web images".
Common myths
- "Every print must be 300 DPI." Only prints seen at arm's length. Large wall art needs less.
- "A higher DPI setting makes a better file." The setting is a label. The pixel count is what matters.
- "More megapixels always means a better print." Beyond what the size and distance call for, extra pixels can't be seen.
- "DPI and PPI are the same." One is the printer's dots. The other is the image's pixels.
The short version
- Pixels are what a file contains.
- PPI is pixels divided by print size.
- DPI is the printer's ink dots.
- 300 PPI is for close viewing. Wall art seen from across a room looks sharp at far less.
For more on how prints are made, see what is a giclée print and why colours look different on screen and in print. Browse the poster prints collection.