DeviceBenchFree PPI calculator
PPI calculator that shows its arithmetic
Give it a pixel grid and a diagonal and it returns pixel density, dot pitch in millimeters, the physical width and height of the glass, and the distance past which 20/20 vision stops separating one pixel from the next. The calculation is Pythagoras on the pixel diagonal followed by a single division, and it is printed next to the answer so you can check it rather than trust it. Nothing here is measured: the pixel counts can be lifted from the display you are on, but the diagonal has to be typed, because no browser on any platform is told how many inches across a screen is.
- 100% free
- No signup
- Dot pitch in mm
- Nine panel presets
- Retina distance
That button fills the two pixel fields only. The diagonal stays yours to type — a tape measure across the glass, corner to corner, ignoring the bezel.
Pixel density
108.79PPI
√(2560² + 1440²) = 2937.21 px ÷ 27.00 in = 108.79 PPI
Two habits make the diagonal wrong more often than the pixel counts do. Measuring across the outer edge of the bezel adds anything from 4 to 20 mm depending on the frame, and on a curved panel a straight tape cuts the corner — manufacturers quote the arc along the glass, so a 1800R screen measures roughly 0.4 inches short if you pull the tape taut. Use the number on the box unless you have reason to doubt it.
How to calculate pixel density
Two numbers you can read off a listing and one you may have to measure.
Put in the pixel grid
Type the horizontal and vertical pixel counts, or press the button that fills them from the display you are reading this on. If the panel is one of the nine in the preset list, picking it fills all three fields at once and is the fastest way to check a monitor you are still deciding about.
Add the diagonal in inches or centimeters
This is the number the calculator cannot obtain for itself, so take it from the product listing, where it is the honest measurement of the glass corner to corner. If you would rather measure, run the tape between opposite corners of the lit area and stop at the edge of the picture, not the edge of the plastic.
Read the density, then the distance under it
The PPI figure appears with its own arithmetic printed underneath, so you can check it rather than take it on faith. The line worth pausing on is the resolving distance: past that many inches your eye stops separating individual pixels, and a panel you sit further away from than that has spent its resolution on nothing.
Technical specifications
| Density formula | √(horizontal² + vertical²) ÷ diagonal in inches, with the working printed beside the result |
|---|---|
| Dot pitch | 25.4 mm ÷ PPI — a 24-inch 1920 × 1080 panel returns 0.2767 mm, matching its datasheet |
| Resolving distance | 3437.75 ÷ PPI inches, the range at which one pixel covers one arcminute of vision |
| Pixels per degree | 0.01745 × viewing distance in inches × PPI; 60 PPD is where added density stops being visible |
| Panel presets | Nine, from a 6.1-inch 2556 × 1179 phone to a 55-inch 3840 × 2160 television |
| Accepted input | 1 to 30,000 pixels per axis; diagonals above 0 and up to 200 inches, typed in inches or centimeters |
| The figure you must supply | The diagonal — pixel counts can be pulled from this display, physical size cannot be read by any browser |
| State carried between visits | None; the form reopens on 2560 × 1440 at 27 inches whatever you last calculated |
Frequently asked questions
Is PPI the same thing as DPI?
They are different units that happen to share arithmetic, and mixing them up is the single most common error in this area. PPI counts the pixels a display emits per inch; DPI counts the ink or toner dots a printer lays down per inch, and printers use several dots of different inks to make one perceived color, which is why a 1200 DPI printer is not four times sharper than a 300 PPI screen. Photoshop compounds the confusion by labeling its image-resolution field DPI when it means PPI. If light comes out of it, the unit is PPI.
What density counts as a retina display?
There is no fixed number, because the claim is about density and distance together. The original threshold was one pixel subtending one arcminute, which is the finest detail 20/20 vision is defined as resolving, and the arithmetic for it is 3437.75 divided by the density: a 326 PPI phone reaches it at 10.5 inches, which is roughly where a phone is held, while a 163 PPI 27-inch 4K monitor reaches it at 21 inches, which is closer than most people sit. That is the honest way to read the word — a distance, not a badge.
My monitor's datasheet gives a dot pitch instead. How do the two relate?
Dot pitch is the width of one pixel, so it is just 25.4 millimeters divided by the density, and the calculator prints both. A 24-inch 1920 × 1080 panel works out at 91.79 PPI and 0.2767 mm, which is exactly the figure such datasheets carry — matching them is a good check that you typed the diagonal correctly. Monitor makers favor dot pitch because it was the standard specification in the CRT era, when it described the shadow-mask spacing rather than a pixel.
Why does a 27-inch 4K monitor need scaling when a 27-inch 1440p one does not?
Because a user interface drawn without scaling has a fixed size in pixels, and those pixels are 1.5 times smaller on the 4K panel. The 1440p panel is 108.79 PPI and the 4K panel at the same diagonal is 163.18 PPI, so an unscaled 12-pixel menu bar physically shrinks by a third when you switch. Setting 150% scaling on the 4K panel puts the interface back to the size it was and spends the extra pixels on sharper edges instead, which is the whole point of buying it.
Where exactly should I measure the diagonal from?
Between opposite corners of the lit area, ignoring the bezel entirely. A frame adds anything from 4 to 20 millimeters depending on the design, and including it inflates the diagonal enough to drag the density figure down by two or three PPI. Curved panels have a second trap: the published diagonal follows the curve of the glass, while a taut tape measure cuts the chord, so an 1800R screen measures roughly 0.4 inches short if you pull the tape straight.
Does more PPI always mean a sharper picture?
Only up to the distance you sit at, which is why the calculator also reports pixels per degree. A 55-inch 4K television is 80 PPI and looks flawless from a sofa; a 24-inch 1080p monitor is 91.79 PPI and looks coarse at arm's length, despite the higher number. Pixels per degree folds the distance in — a desk monitor lands somewhere between 35 and 70 PPD, and past about 60 the extra pixels are being rendered for nobody.
Why can this page not work out the diagonal on its own?
Because no browser interface reports physical size, on any platform, and none is planned. A page can be told how many pixels a screen has and how many device pixels go into a CSS pixel, and there the information stops — the inches exist only on the box and in your tape measure. That is also why the button here fills the two pixel fields and leaves the third one alone.
About pixel density, dot pitch and how far away they stop mattering
Pixel density is the least ambiguous number in display specifications and also the one manufacturers are least keen to print, because it is the number that makes two panels genuinely comparable. Getting it takes one step of geometry: the pixel grid forms a right triangle, so its diagonal is √(w² + h²) pixels, and dividing that by the physical diagonal in inches gives pixels per inch. A 2560 × 1440 grid has a pixel diagonal of 2937.21, which over 27 inches is 108.79 PPI; the same grid on a 32-inch panel drops to 91.79 PPI, the identical density as a 24-inch 1080p monitor. Dot pitch is the same fact inverted — 25.4 millimeters divided by the density, giving the width of one pixel — and it survives on datasheets because it is what CRT specifications quoted, back when it described the spacing of holes in a shadow mask rather than an addressable pixel.
Density on its own is only half an answer, and the missing half is how far away you are. Visual acuity is angular: 20/20 vision is defined as resolving detail one arcminute across, which is 1/60 of a degree, and one arcminute is subtended by a pixel of pitch p at a distance of about 3438 × p. Run that backwards and any density gives a distance beyond which its pixels are invisible — 10.5 inches for a 326 PPI phone, 21 inches for a 27-inch 4K monitor, 43 inches for a 55-inch 4K television. That is why a television with a third of a phone’s density looks perfect and why the marketing word for crossing the threshold was never a fixed number. The distance-aware version of the same idea is pixels per degree, which is what this page reports for the viewing distance you type, and it is the figure worth comparing across devices that are used at wildly different ranges. If you are weighing two panel sizes against each other rather than two densities, the monitor size comparison draws the outlines to scale and leaves density to this page.
The practical consequence of a high density is that the operating system has to scale the interface back up, and that is where the good and bad outcomes separate. A ratio that lands on a whole number is drawn cleanly — a 220 PPI laptop panel at exactly 2× gives every interface element an even number of device pixels. Fractional scaling, 125% or 150% on Windows, has to round element edges onto the pixel grid, and older applications that render at one scale and get resized by the compositor come out soft. None of that shows up in a density figure, which is why a number from this page is worth pairing with your own eyes: the screen test patterns include sharpness targets that make bad scaling obvious in a second, and the dead pixel checker is worth a minute on a panel this dense, where a stuck subpixel is small enough to hide until it annoys you. If the pixel counts you are typing here do not match what your machine reports, the resolution reader explains which of the two grids you are looking at.
Where the numbers you type go
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type, paste or open is uploaded, logged or kept, which is also why the tools carry on working after you disconnect from the network.
This one is a form, so the honest summary is short: the three or four values you enter live in the page’s memory, feed a handful of divisions, and disappear when the tab does. Nothing is stored between visits, which is why the fields come back on their defaults rather than on whatever panel you were pricing last week.