PPI Calculator (Pixels Per Inch)

How sharp is that screen?

Two screens with the same resolution can look very different. A 1920 by 1080 picture spread over a 24 inch monitor is softer than the same pixels packed into a 14 inch laptop. Pixels per inch, or PPI, puts a number on that sharpness, so you can compare screens of any size fairly.

This PPI calculator works out pixel density from a resolution and a diagonal size, along with the dot pitch and the physical width and height of the screen. Switch to Print size to see how big an image will come out on paper at a chosen PPI.

How to use it

1. Choose Screen PPI or Print size.
2. For a screen, enter the resolution in pixels and the diagonal size in inches, as printed in the specs.
3. For a print, enter the image size in pixels and the PPI you will print at.
4. Read the PPI or the print size, plus the dot pitch, screen size and total pixels.

The PPI formula

diagonal in pixels = √(width² + height²)
PPI = diagonal in pixels ÷ diagonal in inches
dot pitch in mm = 25.4 ÷ PPI
print size in inches = pixels ÷ PPI

Screen sizes are measured corner to corner, so the pixel count has to be measured the same way. Pythagoras gives the number of pixels along the diagonal, and dividing by the diagonal in inches gives the density. Dot pitch is the same idea turned round: the distance from one pixel to the next.

Worked example: a 24 inch full HD monitor

• Diagonal in pixels: √(1,920² + 1,080²) = 2,202.91
• PPI: 2,202.91 ÷ 24 = 91.79 PPI
• Total pixels: 1,920 × 1,080 = 2,073,600

A 24 inch full HD monitor has 91.79 pixels per inch. A 27 inch monitor at 2560 by 1440 comes to about 108.79 PPI, which is why text looks noticeably crisper on it even though the screen is bigger.

For print, a 3000 by 2000 photo at 300 PPI comes out at 10 by 6.67 inches. Print the same file at 150 PPI and it doubles to 20 by 13.33 inches, but each pixel is twice as large and the result looks softer up close.

Typical pixel densities

How sharp a screen looks also depends on how far away you sit. A TV across the room can have a low PPI and still look sharp, while a phone held close needs a very high one.

Common mix-ups

• Treating PPI and DPI as the same thing. PPI is pixels in an image or on a screen; DPI is ink dots from a printer. Printers use several dots to make one pixel.
• Using the wrong diagonal. Some screens are sold by a rounded size; use the exact figure from the spec sheet.
• Changing the PPI setting in an image editor and expecting more detail. It only changes the print size; the pixels stay the same.
• Comparing screens by resolution alone. The aspect ratio calculator tells you the shape; PPI tells you the sharpness.

Frequently asked questions

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