PPI Calculator
This free PPI calculator finds pixels per inch, physical width and height, and simplified aspect ratio from a screen's resolution and diagonal size. Useful for comparing screen sharpness, choosing a monitor, or matching content to a display's shape. Nothing you enter is sent anywhere or stored.
How to Use It
- Enter the screen's horizontal and vertical resolution in pixels.
- Enter the screen's diagonal size in inches.
- The PPI, physical width and height, and aspect ratio update instantly, no submit button.
How It Works
This calculator first finds the diagonal resolution in pixels using the Pythagorean theorem on the horizontal and vertical pixel counts, then divides that by the diagonal screen size to get PPI. Dividing each resolution dimension by the PPI gives the screen's actual physical width and height, and reducing the resolution to its simplest whole-number ratio gives the aspect ratio.
Formulas
diagonal pixels = √(horizontal² + vertical²)
PPI = diagonal pixels ÷ diagonal size (in)
width (in) = horizontal ÷ PPI, height (in) = vertical ÷ PPI
A worked example: a 1920 by 1080 resolution on a 24 inch monitor has a diagonal resolution of about 2,202.9 pixels, giving about 91.8 PPI and a physical size of about 20.92 by 11.77 inches. A 55 inch 4K TV (3840 by 2160, exactly double the resolution in each direction) works out to about 80.1 PPI, lower than the 24 inch monitor above despite the much higher resolution, since the screen itself is so much larger.
Everything runs client-side in your browser. Nothing you enter is sent anywhere or stored.
Frequently Asked Questions
What does PPI actually measure?
PPI (pixels per inch) measures how densely packed a screen's pixels are, not the resolution or screen size on their own. Two screens can share the exact same resolution and still look very different: the same pixel count spread over a smaller screen packs those pixels closer together, giving a higher PPI and a sharper-looking image at normal viewing distance.
What counts as a "good" or "high" PPI?
It depends on how close you sit to the screen. Phones are held close to the eyes, so they commonly exceed 400 PPI to stay sharp at that distance. Desktop monitors, viewed from farther away, are commonly 90 to 110 PPI and still look sharp at normal desk distance. A lower PPI on a screen meant to be viewed from farther away is not automatically worse.
Why do the same resolution and screen size sometimes give a different PPI than expected?
PPI is calculated from the diagonal pixel count divided by the diagonal screen size, not simply the horizontal resolution divided by the physical width. Since the diagonal uses the Pythagorean theorem on both resolution dimensions, small differences in aspect ratio between two screens with the same nominal resolution and size can shift the result slightly.
Why does aspect ratio matter?
Aspect ratio describes the shape of the screen or image, like 16:9 for most modern TVs and monitors or 4:3 for older screens. Matching content to the intended aspect ratio avoids black bars or a stretched, distorted image, and it matters just as much when choosing dimensions for a video, image, or design file as it does when buying a screen.
Can you give a real-world example?
A 1920 by 1080 resolution on a 24 inch monitor works out to about 91.8 PPI, a fairly typical desktop density. The same 1920 by 1080 resolution stretched across a 55 inch TV would work out to well under half that PPI, since the same pixel count is spread over far more physical screen area.