What is PPI?
PPI (pixels per inch) measures how densely packed a screen's pixels are — the higher the number, the sharper the image, since more pixels are being used to render the same physical space. Two screens can share the exact same resolution (say, 1920×1080) and still look completely different in sharpness, because PPI depends on both resolution and physical size: cramming that resolution into a small phone screen produces a far higher PPI than stretching it across a large TV.
PPI = √(width² + height²) ÷ diagonal size The numerator is the resolution's diagonal in pixels, found the same way as a physical screen's diagonal — via the Pythagorean theorem — then divided by the actual diagonal size of the screen to get pixels per inch.
Worked examples
The iPhone 4 had a 3.5-inch display with a 640×960 resolution and was specified at 326 PPI. Using the stated resolution and 3.5-inch diagonal in the standard PPI formula gives about 329.7 PPI, illustrating that published PPI specifications can differ slightly from values calculated from rounded screen dimensions.
Typical PPI ranges
| Device Type | Typical PPI | Notes |
|---|---|---|
| Older LCD monitors | 70–100 PPI | Common for 1080p at 22–27" |
| Modern laptops | 110–220 PPI | Wide range depending on resolution |
| 4K monitors (27–32") | 140–165 PPI | Sharp enough that most people can't see individual pixels at normal desk distance |
| Modern smartphones | 400–500+ PPI | Viewed much closer to the eye, so higher PPI is needed to avoid visible pixels |
Frequently asked questions
What is a good PPI for a monitor?
For a desktop monitor viewed at arm's length, 100-165 PPI is typically sharp enough that individual pixels aren't visible. Phones need much higher PPI (400+) because they're held much closer to the eyes.
Does a higher resolution always mean higher PPI?
No. PPI depends on both resolution and physical screen size. A 4K TV and a 4K monitor share the same resolution, but the monitor has a much higher PPI simply because the same number of pixels is packed into a smaller physical space.
What's the difference between PPI and DPI?
PPI (pixels per inch) describes a digital display's pixel density. DPI (dots per inch) is a print term describing how many ink dots a printer places per inch. The terms are often used interchangeably online, but strictly speaking they describe different things.