How do you calculate PPI?

Measure the screen in pixels across its diagonal, then divide by the diagonal in inches. For 1920 x 1080 the pixel diagonal is the square root of 1920 squared plus 1080 squared, which is 2202.907 pixels; on a 24 in panel that is 2202.907 divided by 24, or 91.79 PPI. The same panel at 27 in is 81.59 PPI, because the pixels are spread over more glass. Pixel pitch is the same answer turned around: 25.4 mm divided by 91.79 gives 0.2767 mm between the centres of neighbouring pixels.

Is PPI the same as DPI?

Not on a screen. PPI here is a physical property of the panel: how many of its pixels fall in an inch of glass. The DPI number stored in a JPEG, PNG or TIFF header is print metadata, a note to a printer about how big to lay the image out on paper, and editing it changes nothing about how the file looks on any display. A 3000 pixel wide photo tagged 72 DPI and the same photo tagged 300 DPI are identical files on screen. DPI in its original sense also means printer dots, and a printer lays several dots down per pixel, so a 600 DPI printer is not a 600 PPI screen.

What is the PPI of common monitor sizes?

At 16:9 the usual ones come out like this: 1920 x 1080 at 24 in is 91.79 PPI, the same resolution at 27 in is 81.59; 2560 x 1440 at 27 in is 108.79; 3840 x 2160 at 27 in is 163.18 and at 32 in is 137.68. Laptops sit higher, since the same pixels are packed into a smaller diagonal: 2560 x 1600 at 13.3 in is 226.98 PPI. Type any of these into the fields to see the pitch and the panel size that go with them.

Why does my answer differ from the PPI the manufacturer quotes?

Almost always because the diagonal is rounded. A monitor sold as 24 in is often 23.8 in of actual glass, and 1920 x 1080 across 23.8 in is 92.56 PPI rather than 91.79. Phones are rounded the same way: 2532 x 1170 at 6.1 in reads 457.25 PPI, and a spec sheet quoting a slightly higher number is measuring a slightly shorter diagonal. Enter the real measured diagonal if you have it, and the answer moves with it.

What is pixel pitch, and why would I want it?

Pixel pitch is the distance from the centre of one pixel to the centre of the next, and it is what LED wall and industrial display specifications quote instead of PPI. It is 25.4 divided by the PPI, in millimetres: 0.2767 mm at 91.79 PPI, 0.1557 mm at 163.18 PPI. A smaller pitch means the pixels sit closer together, which is why pitch is quoted for panels that are viewed from a fixed distance.

Does a higher PPI always mean a sharper picture?

Not on its own. Sharpness as the eye sees it depends on viewing distance as well as density, which is why a phone at 457.25 PPI and a desk monitor at 91.79 PPI can both look clean. Subpixel layout matters too, and this page does not model it: an RGB stripe panel, a PenTile panel and a WOLED panel at the same PPI do not resolve fine text equally. The number here is geometry, not a verdict on a screen.

Can it handle centimetres, ultrawide panels and odd resolutions?

Yes. The unit buttons take the diagonal in inches, centimetres or millimetres, so 60.96 cm and 24 in give the same 91.79 PPI, and a unit typed into the field is accepted when it agrees with the buttons and refused with a message when it does not. Shapes are reduced exactly rather than rounded to a marketing ratio: 3440 x 1440 is reported as 43:18, and 1366 x 768 as 683:384. A diagonal of 0 is refused, because PPI is undefined there.

PPI Calculator

Pixels per inch is one division: the diagonal measured in pixels, which is the square root of the width squared plus the height squared, over the diagonal measured in inches. This page does that division and then keeps the numbers that follow from it apart instead of folding them into one figure, so a 1920 x 1080 panel sold as 24 in reads 91.79 PPI, a pixel pitch of 0.2767 mm and a panel 20.92 by 11.77 in. The working is printed under the fields, the unit buttons take a diagonal in inches, centimetres or millimetres, and anything that cannot be read is refused with a sentence saying why rather than shown as a zero.

Pixels per inch91.791920 x 1080 at 24 in · pixel diagonal 2202.907 · pitch 0.2767 mm

PPI: 91.79
Pixel pitch: 0.2767 mm
Panel width: 20.92 in
Panel height: 11.77 in

The screen

The resolution the panel runs at, and the diagonal it is sold as, measured corner to corner. A number that cannot be read gets a line above saying why, never a silent zero.

Pixel widthPixel heightScreen diagonalDiagonal measured in

sqrt(1920^2 + 1080^2) = 2202.907 px across the diagonal, divided by 24 in = 91.79 PPI
1920 x 1080 is 2,073,600 pixels, 2.07 megapixels, in the shape 16:9.
Square pixels, so the panel itself measures 20.92 in by 11.77 in.
Pixel pitch 0.2767 mm, or 276.7 micrometres, centre to centre between neighbouring pixels.
91.79 PPI is 36.14 pixels per centimetre.
This is the display's own pixel density. The DPI tag stored inside a JPEG or PNG is print metadata: editing it changes nothing on this panel.

Worked cases

Each one is run through the same function the fields above are, refusal included.

1920 x 1080 at 24 in16:9 panel
91.79 PPI
3840 x 2160 at 27 insame shape, 4K
163.18 PPI
Diagonal 0any resolution
refused, PPI is undefined

One flat rectangle of square pixels. A curved panel, rounded corners or a notch make the real density vary a little across the screen.
Makers round the diagonal. The same panel measured at 23.8 in reads 92.56 PPI instead of 91.79, so a spec sheet that says 24 in and means 23.8 in moves the answer.
Subpixels are not counted. An RGB stripe, a PenTile layout and a WOLED panel at the same PPI do not resolve the same detail.

Accuracy. Exact arithmetic: the pixel diagonal, which is the square root of width squared plus height squared, divided by the physical diagonal you enter. Panel width, height and pitch follow from it and assume square pixels in a plain rectangle. This is a display's own pixel density; it is not the DPI number stored in an image file, and the page says so.

Common questions

How do you calculate PPI?: Measure the screen in pixels across its diagonal, then divide by the diagonal in inches. For 1920 x 1080 the pixel diagonal is the square root of 1920 squared plus 1080 squared, which is 2202.907 pixels; on a 24 in panel that is 2202.907 divided by 24, or 91.79 PPI. The same panel at 27 in is 81.59 PPI, because the pixels are spread over more glass. Pixel pitch is the same answer turned around: 25.4 mm divided by 91.79 gives 0.2767 mm between the centres of neighbouring pixels.
Is PPI the same as DPI?: Not on a screen. PPI here is a physical property of the panel: how many of its pixels fall in an inch of glass. The DPI number stored in a JPEG, PNG or TIFF header is print metadata, a note to a printer about how big to lay the image out on paper, and editing it changes nothing about how the file looks on any display. A 3000 pixel wide photo tagged 72 DPI and the same photo tagged 300 DPI are identical files on screen. DPI in its original sense also means printer dots, and a printer lays several dots down per pixel, so a 600 DPI printer is not a 600 PPI screen.
What is the PPI of common monitor sizes?: At 16:9 the usual ones come out like this: 1920 x 1080 at 24 in is 91.79 PPI, the same resolution at 27 in is 81.59; 2560 x 1440 at 27 in is 108.79; 3840 x 2160 at 27 in is 163.18 and at 32 in is 137.68. Laptops sit higher, since the same pixels are packed into a smaller diagonal: 2560 x 1600 at 13.3 in is 226.98 PPI. Type any of these into the fields to see the pitch and the panel size that go with them.
Why does my answer differ from the PPI the manufacturer quotes?: Almost always because the diagonal is rounded. A monitor sold as 24 in is often 23.8 in of actual glass, and 1920 x 1080 across 23.8 in is 92.56 PPI rather than 91.79. Phones are rounded the same way: 2532 x 1170 at 6.1 in reads 457.25 PPI, and a spec sheet quoting a slightly higher number is measuring a slightly shorter diagonal. Enter the real measured diagonal if you have it, and the answer moves with it.
What is pixel pitch, and why would I want it?: Pixel pitch is the distance from the centre of one pixel to the centre of the next, and it is what LED wall and industrial display specifications quote instead of PPI. It is 25.4 divided by the PPI, in millimetres: 0.2767 mm at 91.79 PPI, 0.1557 mm at 163.18 PPI. A smaller pitch means the pixels sit closer together, which is why pitch is quoted for panels that are viewed from a fixed distance.
Does a higher PPI always mean a sharper picture?: Not on its own. Sharpness as the eye sees it depends on viewing distance as well as density, which is why a phone at 457.25 PPI and a desk monitor at 91.79 PPI can both look clean. Subpixel layout matters too, and this page does not model it: an RGB stripe panel, a PenTile panel and a WOLED panel at the same PPI do not resolve fine text equally. The number here is geometry, not a verdict on a screen.
Can it handle centimetres, ultrawide panels and odd resolutions?: Yes. The unit buttons take the diagonal in inches, centimetres or millimetres, so 60.96 cm and 24 in give the same 91.79 PPI, and a unit typed into the field is accepted when it agrees with the buttons and refused with a message when it does not. Shapes are reduced exactly rather than rounded to a marketing ratio: 3440 x 1440 is reported as 43:18, and 1366 x 768 as 683:384. A diagonal of 0 is refused, because PPI is undefined there.

Exact arithmetic: the pixel diagonal, which is the square root of width squared plus height squared, divided by the physical diagonal you enter. Panel width, height and pitch follow from it and assume square pixels in a plain rectangle. This is a display's own pixel density; it is not the DPI number stored in an image file, and the page says so.