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DPI and PPI, explained plainly
DPI and PPI are often used as if they meant the same thing, and for preparing image files they nearly do. PPI means pixels per inch: how many image pixels fall in each inch of the printed picture. DPI means dots per inch: how many dots of ink the printer places in each inch. A printer lays down many ink dots to reproduce one image pixel, so a printer’s DPI rating is much higher than the image’s PPI, but when someone says “give me a 300 DPI image” they almost always mean a 300 PPI image. The number that matters for your file is pixels per inch, because it decides how many pixels the image needs.
This converter answers both directions of the question. Enter a print size and a resolution, and it tells you how many pixels the image needs. Or enter an image’s pixel width and a print width, and it tells you the effective resolution at that size. Both directions use the same simple rule: pixels equal inches times resolution.
How to use the converter
- To find pixels needed: enter the print size in inches and the resolution in PPI, then read the pixel count.
- To find effective resolution: enter the image width in pixels and the print width in inches, then read the PPI.
- Compare the result with the print quality checker on this site, which judges whether that resolution is sharp at your viewing distance.
A worked example
Suppose you want a 10 inch wide print at 300 PPI. Multiply 10 by 300 and you need 3000 pixels across. An image that is 3000 pixels wide prints at exactly 300 PPI at 10 inches. Print the same image at 20 inches wide and the effective resolution halves to 150 PPI, because the same pixels now stretch over twice the distance. Print it at 5 inches wide and the resolution doubles to 600 PPI, which is more than the eye can use at reading distance but harmless. Resolution is not a property of the file alone; it is a property of the file at a size.
Screen resolution works the same way but with lower numbers. A typical screen shows about 100 to 200 pixels per inch, so a 1000 pixel image looks large on screen but prints only about 3.3 inches wide at 300 PPI. This is why images downloaded from the web often disappoint in print: they were made for screens, where 1000 pixels is plenty, not for paper, where 3000 pixels is the norm. Always check the pixel count against the print size before promising a print.
Resolution guide
- 300 PPI: standard for photo books, brochures and quality prints viewed up close.
- 150 to 200 PPI: acceptable for posters and large prints viewed at arm’s length or more.
- 72 to 100 PPI: screen use and very large displays viewed from metres away.
- 600 PPI and above: fine-art reproduction and line art, where crisp edges matter more than file size.
Higher is not always better. Doubling the resolution quadruples the pixel count and the file size, while the eye gains nothing beyond what it can resolve at the viewing distance. A billboard at 300 PPI would be an absurdly large file for no visible gain, because nobody reads a billboard from 30 centimetres away. The right resolution is the lowest one that still looks sharp at the distance people will actually view the print, and the print quality checker on this site works that number out from the viewing distance.
Checking a file before print
Before sending a file to print, open the image properties and read the pixel dimensions, then divide by the print size in inches. If the answer is below 150 PPI for a close-viewed print, find a larger original or print smaller. Upscaling software can add pixels, but it cannot add detail that was never in the photograph, so an upscaled image looks smoother, not sharper. The honest fix for a low-resolution file is a better original or a smaller print, and this converter shows which one you need in seconds.
Related calculators
- Spanish version
- Paper Thickness Caliper Calculator – Sheet Thickness from GSM and Density
- Print Quality Checker – Sharp, Acceptable or Too-Low Verdict for Your Print Size