When you compare two monitors with the same resolution — say, both at 1920x1080 — one might look dramatically sharper than the other. The reason is almost always PPI. Understanding pixels per inch is key to choosing the right display and understanding why some screens look crisp while others look blurry.
PPI (Pixels Per Inch) is the number of pixels packed into one inch of screen area. Higher PPI = more pixels per inch = sharper, more detailed image. Apple's "Retina" displays are simply screens with high enough PPI that individual pixels aren't visible at normal viewing distances.
How to Calculate PPI
PPI is calculated using the screen's resolution and its physical diagonal size. The formula is:
For example, a 1920x1080 display on a 24-inch monitor: the diagonal pixel count is sqrt(1920² + 1080²) = sqrt(3686400 + 1166400) = sqrt(4852800) = approximately 2203 pixels. Divide by 24 inches = ~92 PPI.
You can calculate any display's PPI instantly using our free PPI Calculator.
PPI Ratings by Quality Level
| PPI Range | Quality | Typical Device | Rating |
|---|---|---|---|
| Below 72 | Low — pixels visible at normal distance | Old monitors, large cheap TVs | Low |
| 72 – 100 | Standard — acceptable for desk use | 24" 1080p monitors | Standard |
| 100 – 160 | Good — sharp for most use cases | 27" 1440p, 24" 4K | Good |
| 160 – 220 | Excellent — Retina-class on laptops | MacBook Pro, high-end laptops | Excellent |
| 220 – 400+ | Ultra HD — smartphone-grade clarity | iPhone, Android flagships | Retina |
PPI vs Resolution — What's the Difference?
Resolution is the total pixel count (e.g., 1920x1080 = ~2 million pixels). PPI is the density of those pixels — how tightly packed they are into the physical screen area. The same resolution on a smaller screen = higher PPI = sharper image. The same resolution on a larger screen = lower PPI = softer image.
This is why a 1080p smartphone screen can look dramatically sharper than a 1080p 32-inch monitor. The phone packs the same 2 million pixels into a 6-inch screen vs a 32-inch screen — resulting in over 5x the pixel density.
What Is a Retina Display?
Apple's "Retina" display is a marketing term for displays with high enough PPI that individual pixels aren't distinguishable at typical viewing distances. The exact threshold Apple uses varies by device type:
- iPhone: approximately 300+ PPI (viewed at ~10–12 inches)
- MacBook: approximately 220+ PPI (viewed at ~20 inches)
- iPad: approximately 264+ PPI (viewed at ~15 inches)
The concept is viewing-distance-relative — a display that looks sharp at 20 inches might show pixels if you move it to 6 inches away.