Image Size Converter

Image size depends on pixels and DPI (dots per inch).

The formula:

• Size in inches = Pixels / DPI
• Size in cm = Pixels / DPI × 2.54
• Pixels needed = Size in inches × DPI

Common DPI values:

• 72 DPI: The old Macintosh screen standard, still the default tag on a lot of web images
• 96 DPI: The modern reference for screens, and what CSS actually assumes for one inch
• 150 DPI: Draft quality print
• 300 DPI: High quality print (photos, brochures)
• 600 DPI: Professional/fine art print

The 72 figure is a leftover. It came from the original 1984 Mac, whose screen genuinely put 72 pixels in an inch so that a point on screen matched a typographic point on paper. Nothing has been built to 72 for decades, and CSS settled on 96 as its reference pixel density, but the number survives in image-editor defaults and in a lot of advice that has not been rechecked since. If you are tagging an image for the web, the tag does not matter at all. If you are matching a screen measurement, 96 is the more honest starting point.

Example (1920 px wide image), and how big it prints:

• At 72 DPI: 26.67 inches (67.7 cm): far too coarse to hold in your hand
• At 150 DPI: 12.8 inches (32.5 cm): decent print
• At 300 DPI: 6.4 inches (16.3 cm): high quality print

Note that those are print verdicts. On a screen the same 1920 px file is a perfectly good full-width web image, because a screen has no dots per inch to honour: it simply maps one image pixel to one screen pixel and the DPI tag is ignored entirely.

Standard print sizes:

• 4×6 in (10×15 cm) at 300 DPI needs: 1200×1800 px
• 8×10 in (20×25 cm) at 300 DPI needs: 2400×3000 px
• A4 (21×29.7 cm) at 300 DPI needs: 2480×3508 px

The point that confuses most people is that DPI isn’t baked into the pixels, it’s just a label telling the printer how tightly to pack them. A 1200×1800 photo is the same file whether you tag it 72 or 300 DPI; changing the number only changes how big it prints, not how much detail it holds. So “make this 300 DPI” without resampling means nothing more than choosing a print size.

That’s why the famous 300 DPI rule has an asterisk: it’s the right target for something held in your hand, like a photo print or a magazine page. The farther away an image will be viewed, the lower the DPI you can get away with, which is how a billboard printed near 20 DPI still looks crisp from across the street. The hard limit runs the other way. You can always print fewer dots per inch, but you can’t invent detail that was never captured, so upscaling a small image to hit a pixel target just gives you a bigger blurry picture, not a sharper one.