DPI vs PPI: the difference, and why it rarely matters
PPI is the pixels in an inch of an image or a screen. DPI is the ink dots a printer lays in an inch. What each means, and which number to use when.
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PPI, pixels per inch, counts the pixels of an image, or of a screen, in one inch. DPI, dots per inch, counts the dots of ink a printer puts down in one inch. An image has PPI; a printer has DPI.
In practice nearly everyone says DPI for both. When a print shop asks for “a 300 DPI file” it means 300 pixels per inch at the printed size. So to convert DPI to PPI for an image, there is nothing to convert: a 300 DPI image is a 300 PPI image.
The three numbers
| Term | Counts | Belongs to | Typical values |
|---|---|---|---|
| PPI (image) | Pixels per inch of print | An image at a chosen print size | 150 for posters, 300 for print held in the hand |
| PPI (screen) | Pixels per inch of glass | A display | About 92 for a 24 inch 1080p monitor, 163 for a 27 inch 4K, 460 for a recent iPhone |
| DPI (printer) | Ink dots per inch | A printer or imagesetter | 600 or 1200 for lasers, 1440 and up for inkjets |
| LPI | Halftone lines per inch | A printing press | About 85 for newspapers, 150 for magazines |
PPI: the image
An image is a grid of pixels with no size of its own. PPI gives it one. Pixels divided by PPI is inches: a 3000 × 2400 pixel photo is 10 × 8 inches at 300 PPI and 20 × 16 inches at 150 PPI. The pixels are the same; they are only spread wider.
That is why the PPI stored in a file (which software labels “DPI” or “resolution”) says nothing about quality. It is a note about print size, and changing it changes no pixels. The DPI converter rewrites that note without touching the picture.
A screen has PPI too: its pixel count divided by its size. A 27 inch 4K monitor is 3840 × 2160 pixels across a 27 inch diagonal, which works out to about 163 PPI.
DPI: the printer
A printer cannot make a pixel of any colour it likes. An inkjet has a few inks and can only place a dot of one of them, or not. To print one orange pixel it lays down a small cluster of yellow and magenta dots that the eye blends. So a printer needs many dots for each pixel, and its DPI is much higher than the PPI of the image it prints: a 1440 DPI inkjet printing a 300 PPI photo has room for nearly five dots across every pixel.
That is the real difference between the two. You choose the PPI of your image. The DPI belongs to the machine, and you rarely need to think about it beyond picking a quality setting.
LPI: the press
Commercial presses print tone through a halftone screen, rows of dots that vary in size, and the fineness of that screen is measured in lines per inch. The usual rule is to supply an image with about twice as many pixels per inch as the screen has lines: 300 PPI for a 150 LPI magazine. That rule is where the 300 figure comes from.
Which number to use
- Printing a photo or a document: 300 PPI at the printed size. Work out the pixels with the DPI calculator.
- A poster or canvas seen from a few steps back: 150 PPI is usually enough. See 150 vs 300 DPI.
- The web and screens: PPI is irrelevant. Only the pixel dimensions matter; “72 DPI for web” is a habit, not a requirement. See 72 vs 300 DPI.
- Scanning: the scanner’s “DPI” is samples per inch of the original. Scan at 300 to reprint at the same size, 600 to print at double size.
- Laser engraving: here DPI is the spacing of the lines the laser scans, and the right value comes from the width of the beam. See what DPI for laser engraving photos.
Why software says DPI
Windows shows “horizontal resolution: 300 dpi” in a file’s properties. Scanner software says DPI. Most web forms say DPI. Photoshop is the careful one: its Image Size box says “Pixels/Inch”. They all mean the same stored number, the pixels per inch the image is meant to be printed at. The DPI spelling won decades ago and there is no harm in using it, as long as you know that an image’s “DPI” and a printer’s DPI are different things.
Questions
Is 300 DPI the same as 300 PPI? For an image file, yes. It means 300 of its pixels are printed in each inch.
How do I convert DPI to PPI? You don’t need to. The number is the same. What you may need to convert is a print size into pixels: inches multiplied by PPI.
Does a higher printer DPI need a higher PPI image? No. Past about 300 PPI a photo print does not get visibly sharper, whatever the printer’s DPI.
What PPI is my image? Whatever you print it at. To see the value stored in the file, use the image DPI checker.