Few image settings cause as much confusion as DPI. A form asks for a 300 DPI photo, your file says 72 DPI, and it seems like the photo is not good enough. In most cases the fix takes a few seconds, because the DPI value on its own says very little about quality. What matters is the number of pixels.
DPI and PPI
Strictly speaking, DPI means dots per inch and describes how many ink dots a printer places in an inch. PPI, pixels per inch, describes how many image pixels go into an inch of the printed result. In everyday use, and in most software, the two terms are mixed up, and the DPI value stored in an image file is really a PPI value. This guide uses DPI in that everyday sense.
One inch is 2.54 cm, or 25.4 mm. Everything else follows from that.
The two formulas you need
- Pixels needed = size in inches x DPI = size in mm / 25.4 x DPI
- Print size = pixels / DPI (in inches), or pixels / DPI x 2.54 (in cm)
Example: a print 10 cm wide at 300 DPI needs 100 / 25.4 x 300, which is about 1181 pixels.
The other direction: a phone photo of 4032 x 3024 pixels printed at 300 DPI comes out at about 34.1 x 25.6 cm. The same photo labeled 72 DPI would nominally print at about 142 x 107 cm. Same pixels, same image, very different print size, and exactly the same sharpness on screen.
What the number in the file means
JPG and PNG files have a small field that stores a resolution value. In a JPG it sits in the JFIF header or the EXIF data; in a PNG it is a chunk called pHYs. It is only a hint that tells printing and layout software how large to make the image by default. Many cameras and phones write 72 there simply out of habit, and some files have no value at all.
Screens ignore this field completely. A web browser displays an image based on its pixels and the size set by the page, never by the DPI value. That is why a 72 DPI image and a 300 DPI image with the same pixels look identical online.
Why changing DPI does not change sharpness
Sharpness depends on how many pixels describe the subject. If you change only the DPI field, the pixels stay exactly the same, so the image is exactly as sharp as before. You have only changed the default print size.
Some programs offer to resample when you change DPI. That is a different operation: the pixel count actually changes. Resampling down reduces detail. Resampling up adds pixels but no new detail, because the program can only guess what lies between existing pixels. An upscaled photo is bigger but not sharper.
How many pixels common prints need
The table shows the pixel dimensions for popular sizes at 150 and 300 DPI. Values are rounded to whole pixels.
| Print size | Size in mm | 150 DPI | 300 DPI |
|---|---|---|---|
| 10 x 15 cm photo | 100 x 150 | 591 x 886 | 1181 x 1772 |
| 13 x 18 cm photo | 130 x 180 | 768 x 1063 | 1535 x 2126 |
| A5 | 148 x 210 | 874 x 1240 | 1748 x 2480 |
| A4 | 210 x 297 | 1240 x 1754 | 2480 x 3508 |
| A3 | 297 x 420 | 1754 x 2480 | 3508 x 4961 |
300 DPI is the usual target for photos and documents that people look at up close. 150 DPI often looks acceptable for posters and large prints that are viewed from a distance. Below that, printed edges and small text may start to look soft.
You do not need to memorize the table. The cm to pixels calculator converts between cm, mm, inches and pixels at any DPI and lists common paper sizes in pixels.
Screen versus print
For screens, think only in pixels. A website column that is 800 pixels wide needs an image about 800 pixels wide, or about 1600 for sharp display on high-density screens. The old values of 72 and 96 DPI come from early screen conventions and do not affect how an image appears on a web page.
For print, think in pixels and physical size together. Ask yourself how many pixels you have, and how large you want the print to be. If the pixels divided by the print size in inches gives at least about 300, you are in good shape.
How to check and set DPI
- Check what you have. The image size checker shows the pixel dimensions, the DPI stored in the file (or not set) and the print size both at that DPI and at 300 DPI.
- If you have enough pixels, change only the field. In the second part of the cm to pixels tool, drop in JPG or PNG files and choose a DPI. The tool rewrites only the resolution field and leaves pixels and quality untouched, then shows the resulting print size.
- If you need an exact print size, use the print size mode of the image resizer. Enter the width or height in cm and a DPI of 72, 150, 300 or 600. The tool calculates the pixels, resizes the image and writes the DPI into the file.
- If you do not have enough pixels, be honest about it. Upscaling makes the file meet a number, but it will not add detail. A smaller print size is often the better solution.
When someone asks for 300 DPI
Forms, print shops and publishers ask for 300 DPI because they want enough pixels for a sharp print at the intended size. Before doing anything, find out the physical size they need, then check the pixels. If your photo has 1181 x 1772 pixels and they want a 10 x 15 cm print, it already qualifies; you only need to set the DPI field to 300. For passport and ID photos, the passport photo maker sets the right pixels and DPI for you, and the guide to ID photo sizes lists the common formats.
Frequently asked questions
Is a 72 DPI image low quality?
Not necessarily. The 72 is just a label. A 4000 pixel wide image marked 72 DPI has exactly the same detail as the same image marked 300 DPI. Check the pixel dimensions instead.
Does changing the DPI make the file smaller?
No. Changing only the DPI field does not touch the pixels, so the file size stays practically the same. To make a file smaller you need fewer pixels or stronger compression.
What DPI should I use for a website?
It does not matter. Browsers ignore the DPI value. Choose pixel dimensions that match the space the image fills, about twice that for high-density screens.
