Screen scale tool

Screen PPI calculator

Calculate pixels per inch (PPI) and pixels per millimeter from the resolution and physical diagonal you enter. The result is a useful screen-density estimate for comparing displays or starting ruler calibration; confirm an actual-size ruler with a physical reference on the display you are using.

Calculate screen scale

Result

Calculated pixels per inch (PPI)

Enter a diagonal

Calculated pixels per millimeter

-

Your browser screen size can be filled automatically. Enter the physical diagonal from your device specifications.

How the calculation works

Online rulers usually need CSS pixels per inch, not the raw hardware PPI printed in a device spec. Device pixel ratio and browser scaling can make those numbers different.

Screen diagonal in pixels

sqrt(width^2 + height^2)

Pixels per inch (PPI)

diagonal pixels / physical screen diagonal inches

Pixels per millimeter

pixels per inch / 25.4

When to use a PPI estimate

Estimating actual-size ruler scale from a monitor or laptop diagonal.

Checking whether browser CSS pixels differ from hardware pixel density.

Finding a starting calibration value before confirming with a credit card.

Use the rulerRead calibration guide

Common screen PPI values

These are approximate CSS PPI values for popular device categories. Your exact PPI depends on model, resolution mode, and OS scaling settings.

DeviceTypical resolutionDiagonalCSS PPI
MacBook Air 13"1470x956 (scaled)13.6"~128
MacBook Pro 16"1728x1117 (scaled)16.2"~127
27" 4K monitor1920x1080 (4K at 2x)27"~82
24" 1080p monitor1920x108024"~92
15.6" 1080p laptop1920x108015.6"~141
iPad Pro 12.9"1024x1366 (CSS)12.9"~132
iPhone 15 Pro393x852 (CSS)6.1"~154

CSS PPI differs from hardware PPI because of device pixel ratio scaling. Use the calculator above with your actual screen diagonal for an exact value.

Screen PPI FAQ

What is screen PPI and how is it calculated?+

PPI (pixels per inch) describes the pixel density of a resolution at a physical screen size. Calculate it by dividing the screen diagonal in pixels (the square root of width² plus height²) by the diagonal in inches.

Why do I need to know my screen PPI for an online ruler?+

PPI can provide a starting estimate for mapping pixels to real-world units. An actual-size online ruler still needs a physical reference check because the browser may render a different CSS scale from the display's hardware pixel density.

Is browser CSS PPI the same as hardware PPI?+

Not always. Device pixel ratio, operating-system display scaling, and browser zoom can make the browser's rendered CSS-pixel scale differ from the hardware resolution. Use a physical reference to verify the rendered ruler.

What PPI does a typical laptop screen have?+

It varies with the panel's resolution and physical diagonal. Enter the values for a specific display rather than relying on a category average; the calculation will show the PPI for those inputs.

Can I use PPI to calibrate a screen ruler automatically?+

PPI is a useful starting point, not a complete verification. Confirm the ruler with a flat physical reference of known size, such as the 85.60 mm width of a standard ID-1 card, on the display and zoom setting you will use.

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