Background · DPI
What is DPI in an image?
How many pixels an image puts in each inch of paper. It sits in a small record inside the file, sets the print size, and leaves the pixel data alone.
01
What DPI means
A number in the file header that turns pixels into inches of paper.
DPI stands for dots per inch. Inside an image file it is a print instruction: 300 DPI means 300 pixels across every inch of finished paper. A 3000 pixel wide photo at 300 DPI fills 10 inches; the same file at 150 DPI fills 20 inches from the same pixels.
The value sits in the file header: the JFIF or EXIF block for JPEG, the pHYs chunk for PNG, the header fields for BMP, TIFF and WebP. Nothing shows it on screen, so you have to open the file.
02
DPI, PPI and resolution
DPI vs PPI vs resolution: what each term measures.
| Term | Measures | Where you meet it |
|---|---|---|
| DPI | Pixels per inch of printed paper | Print submissions, scanner settings, print shops |
| PPI | Pixels per inch of a screen or a scan | Displays, cameras, image editors |
| Resolution | Pixel dimensions, such as 1200 x 800 | Print dialogs, camera menus, file properties |
Strictly speaking a screen has pixels and a printer has dots, so screens are PPI and paper is DPI. Most software ignores the distinction. When an editor shows 72 PPI beside a photo, it is reading the same number the DPI checker shows as 72.
03
Where 72 and 96 come from
Both numbers come from screen hardware.
72 DPI dates back to the first Macintosh screens, which drew 72 pixels per inch so that one pixel matched one point of type. Windows assumed 96 for much the same reason. Cameras, phones and editors still write one of the two when nobody picks a value, which is why so many photos arrive tagged 72 DPI.
Neither number says anything about quality. A 4000 px wide photo tagged 72 DPI holds more detail than a 900 px wide one tagged 300 DPI, and it prints larger.
04
How many pixels a print needs
Multiply each side in inches by the DPI. The result is the pixel count the file has to hold.
A 10 x 11 inch print at 300 DPI needs 3000 x 3300 pixels, and the same print at 150 DPI needs 1500 x 1650. A 3.5 inch wide photo at 300 DPI needs 1050 pixels across. When a file holds fewer pixels than the row says, raising the DPI only makes the print smaller.
| Print size | At 150 DPI | At 300 DPI | At 600 DPI |
|---|---|---|---|
| 3.5 x 4.5 in, ID photo | 525 x 675 px | 1050 x 1350 px | 2100 x 2700 px |
| 4 x 6 in | 600 x 900 px | 1200 x 1800 px | 2400 x 3600 px |
| 5 x 7 in | 750 x 1050 px | 1500 x 2100 px | 3000 x 4200 px |
| 8 x 10 in | 1200 x 1500 px | 2400 x 3000 px | 4800 x 6000 px |
| 10 x 11 in | 1500 x 1650 px | 3000 x 3300 px | 6000 x 6600 px |
| A4, 210 x 297 mm | 1240 x 1754 px | 2480 x 3508 px | 4961 x 7016 px |
For a size that is not in this table, the print size calculator takes any width, height and DPI and returns the pixels. It runs the other way too, turning a pixel count into inches.
05
DPI for printing: why 300 keeps coming up
At normal reading distance, paper hides the single pixels at that density.
Print shops, passport offices and print-on-demand services ask for 300 DPI because the printed page stops showing individual pixels at that density. A 300 DPI tag on a 900 px photo still prints 3 inches wide, and a 6000 px photo at 72 DPI can be relabelled 300 and print 20 inches.
Use 300 DPI for photos, ID uploads and books, 150 for posters and banners read from a distance, 600 for line art and scans where the edges show. When a provider publishes its own number, that number wins.
06
What changing the DPI changes
A 3000 pixel file prints 10 inches wide at 300 DPI and 20 inches at 150.
Rewriting the tag leaves every pixel in place. The print size changes, the picture does not, and the file size barely moves because only a few bytes of metadata are replaced.
Adding pixels is a separate operation. Resampling to a higher pixel count smooths an enlargement, and it cannot recover detail the camera never recorded.
The DPI checker reads what a file already carries, and the DPI converter rewrites the tag on JPEG, PNG, BMP and TIFF without recompressing the picture.
DPI questions people ask
What does DPI stand for?
Dots per inch. A dot is one printed pixel, so 300 DPI means 300 pixels in every inch of the print. The same acronym on a mouse counts something else entirely.
Is DPI the same as resolution?
Resolution usually means the pixel dimensions, such as 1200 x 800, while DPI is the print density. Software blurs the two, so one dialog calls the number PPI and the next calls the same number DPI.
What does 72 DPI mean in pixels?
72 pixels per inch. A 1200 x 800 photo at 72 DPI prints 16.67 x 11.1 inches, and the same file tagged 300 DPI prints 4 x 2.67 inches.
What is 300 DPI?
A file tagged 300 DPI prints 300 pixels per inch of paper. It says nothing about how many pixels exist: a 1200 x 800 photo at 300 DPI prints 4 x 2.67 inches, and a 6000 x 4000 one prints 20 x 13.3 inches.
Is 600 DPI better than 300 DPI?
For photos, rarely. 600 DPI doubles the pixel count a print needs, and a camera file only looks sharper if it actually holds those pixels. For line art, text and scans, 600 does show more.
Does DPI affect image quality?
No. Quality comes from the pixels and the compression, and the tag touches neither. A low tag only asks the printer for a larger sheet than the file has detail for.
What DPI should I use for printing?
300 for photos, ID uploads and books, 150 for banners read from a distance, 600 for line art. A print provider that publishes its own requirement overrides these.