DPI, PPI and Print Size Explained
DPI is one of the most misunderstood numbers in photography. This guide explains it with a single formula, a table of print sizes and answers to the questions people ask most.
Updated 2026-10-23
DPI and PPI: the same idea with two names
A digital picture is a grid of pixels. DPI (dots per inch) and PPI (pixels per inch) describe how densely those pixels are packed when the picture is printed or displayed at a physical size. Strictly speaking, DPI is a printer’s measure of ink dots and PPI is the right term for pixels, but in everyday use, and in the settings of most programs and forms, people say DPI for both.
The one formula you need
Print size in inches = pixels ÷ DPI. A photo that is 3000 pixels wide prints 10 inches wide at 300 DPI, 20 inches wide at 150 DPI and 41.7 inches wide at 72 DPI. To find the pixels you need for a print, rearrange it: pixels = inches × DPI. In millimetres, pixels = mm ÷ 25.4 × DPI.
Pixels needed for common sizes
| Print size | At 300 DPI (photo quality) | At 200 DPI (good) | At 150 DPI (draft, large posters) |
|---|---|---|---|
| 35 × 45 mm passport photo | 413 × 531 | 276 × 354 | 207 × 266 |
| 2 × 2 in passport photo | 600 × 600 | 400 × 400 | 300 × 300 |
| 4 × 6 in (10 × 15 cm) | 1200 × 1800 | 800 × 1200 | 600 × 900 |
| 5 × 7 in (13 × 18 cm) | 1500 × 2100 | 1000 × 1400 | 750 × 1050 |
| 8 × 10 in (20 × 25 cm) | 2400 × 3000 | 1600 × 2000 | 1200 × 1500 |
| A4 (210 × 297 mm) | 2480 × 3508 | 1654 × 2339 | 1240 × 1754 |
| A3 (297 × 420 mm) | 3508 × 4961 | 2339 × 3307 | 1754 × 2480 |
Even a modest phone photo of 12 megapixels (4032 × 3024) is enough for a sharp 13 × 10 inch print at 300 DPI, which is why resolution is rarely the limit for photographs. It is the limit for pictures downloaded from the web, which are often only 800 or 1200 pixels wide.
The DPI tag inside a file
JPEG and PNG files can store a DPI value. It is only a suggestion to the program that opens or prints the picture: it does not change a single pixel. That has surprising consequences:
- A picture saved at “72 DPI” and the same picture at “300 DPI” look exactly alike on a screen, because browsers and monitors use pixels only.
- Changing the tag from 72 to 300 makes the picture print at a quarter of the size, not sharper.
- Forms that ask for “300 DPI” often only read this tag, or check the pixel size, or both. Writing the tag with the DPI tool is then enough.
Tag only or resample?
There are two different jobs. To satisfy a program or form that reads the number, change only the tag. To print at an exact physical size, the pixels must match: a picture of 800 × 600 pixels at 300 DPI is only 2.7 × 2 inches, so printing it at 8 × 6 inches means either a lower DPI (100 here, and visibly soft) or more pixels. Resampling creates those extra pixels by interpolation, but it cannot invent detail. If the original is too small for the size you want, the honest answer is to go back to a larger source.
Choosing a DPI for the job
| Purpose | DPI |
|---|---|
| Websites, email and screens | Irrelevant: use pixels |
| Photo prints at a shop or at home | 300 |
| Magazines and brochures (offset printing) | 300 |
| Large posters seen from a distance | 100–150 |
| Scanning documents for reading | 200–300 |
| Scanning text for OCR or small print | 300–600 |
| Passport and visa photos | 300, matching the size in millimetres |
How to check the DPI of a picture
On Windows, right-click the file, choose Properties, then Details, and look for the horizontal and vertical resolution. On a Mac, open the file in Preview and choose Tools, then Adjust Size. In Photoshop and similar programs, open the Image Size dialog. If the field is empty, the file has no tag and programs assume 72 or 96.
Passport photos and DPI
Passport photos are the place where DPI matters most, because the print must be an exact size in millimetres. A 35 × 45 mm photo needs 413 × 531 pixels at 300 DPI, and a 2 × 2 inch photo needs 600 × 600. The Passport Photo Maker outputs those pixel sizes with the 300 DPI tag, and the print sheet places them on 4 × 6 in, A4 or Letter paper so that they print at the right size when you choose “actual size” in the print dialog.
Frequently asked questions
Is 72 DPI or 300 DPI better?
Neither for screens, which use pixels only. For printing, 300 DPI is the standard for photographs, and you need enough pixels to reach it at the print size.
Does increasing DPI improve image quality?
Changing only the tag does not. Resampling adds pixels by interpolation, which does not add detail either. Only a higher-resolution source gives a sharper print.
How many pixels do I need for an A4 print?
About 2480 × 3508 at 300 DPI, or 1240 × 1754 at 150 DPI.