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PPI Calculator

Use this free PPI calculator to find the pixels per inch of any screen. Enter the resolution in pixels and the diagonal screen size in inches, and it gives you the PPI, the dot pitch in millimetres, and the total megapixels — or tap a preset for a common phone, laptop, monitor, or TV to fill the numbers in for you.

How it works

PPI stands for pixels per inch. It tells you how tightly the pixels of a screen are packed together, and it is the single number that best predicts how sharp a screen will look. Two screens can share the same resolution and still look very different: a 1920 by 1080 picture spread across a 55-inch television is soft and visibly pixelated up close, while the same 1920 by 1080 picture on a 13-inch laptop looks crisp, because the laptop squeezes the same number of pixels into a much smaller area.

The calculation uses the diagonal of the screen twice. First, the diagonal resolution in pixels is found with the Pythagorean theorem: take the width in pixels, square it, take the height in pixels, square it, add the two squares together, and take the square root. For a 1920 by 1080 screen, that is the square root of 3,686,400 plus 1,166,400, which is the square root of 4,852,800, or about 2,202.91 pixels along the diagonal. Then divide that diagonal pixel count by the diagonal screen size in inches. Dividing 2,202.91 by 24 inches gives 91.79 PPI.

Dot pitch is the same density expressed the other way around: the distance between the centres of two neighbouring pixels, in millimetres. Since one inch is exactly 25.4 millimetres, dot pitch is 25.4 divided by the PPI. A 91.79 PPI screen has a dot pitch of 25.4 divided by 91.79, which is 0.2767 mm. A phone screen at 460 PPI has a dot pitch of only 0.055 mm, which is why phone text looks so smooth. Total pixels are simply width times height, and megapixels are that total divided by one million: 1920 times 1080 is 2,073,600 pixels, or 2.07 megapixels.

Two practical points matter when you measure. First, always use the diagonal size of the display itself, not the width of the whole device — the bezel around the screen does not count. Second, use the screen's native resolution, the one it was built for, rather than a lower resolution it happens to be set to at the moment. A 4K monitor running at 1920 by 1080 still has the PPI of a 4K monitor; the pixels are simply being grouped together by the display.

Common devices

The table below shows the PPI of screens you probably use every day. Notice that a phone has by far the highest PPI, because it is held close to your eyes, while a television has the lowest, because it is watched from several feet away. A television does not need phone-level PPI — from the sofa, your eyes cannot tell the difference.

As a rule of thumb, a monitor viewed from an arm's length looks sharp from about 100 PPI upward and very crisp above 160 PPI. A laptop viewed from a similar distance benefits from 200 PPI or more because the screen is closer to your face. A phone, held a foot from your eyes, is where the highest numbers pay off: around 300 PPI the pixels become hard to pick out, and beyond 400 PPI the gain is mostly theoretical. For a television, viewing distance does the work instead: a 55-inch 4K television at 80 PPI looks perfectly sharp from a sofa eight feet away, because each pixel covers a tiny angle of your field of view.

Worked example

Take a 24-inch monitor with a Full HD resolution of 1920 by 1080 pixels. Step one: square the width, 1920 times 1920, which is 3,686,400. Step two: square the height, 1080 times 1080, which is 1,166,400. Step three: add them, 3,686,400 plus 1,166,400, which is 4,852,800. Step four: take the square root, which is 2,202.91 pixels along the diagonal. Step five: divide by the diagonal size, 2,202.91 divided by 24, which gives 91.79 PPI. The calculator shows exactly this result when the page loads.

Now compare a phone. Enter 1170 by 2532 pixels and a 6.1-inch diagonal, or tap the phone preset. The diagonal resolution is the square root of 1,368,900 plus 6,411,024, which is the square root of 7,779,924, or about 2,789.25 pixels. Divide by 6.1 and you get 457.25 PPI. The dot pitch is 25.4 divided by 457.25, which is 0.0555 mm, and the total is 1170 times 2532, which is 2,962,440 pixels, or 2.96 megapixels. The phone packs roughly five times as many pixels into each inch as the monitor, which is why its text looks printed rather than drawn.

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