The resolution formulas
Three quick formulas describe any display from its resolution and diagonal:
total pixels = width × height
megapixels = total pixels ÷ 1,000,000
PPI = √( width² + height² ) ÷ diagonal inches
PPI uses the Pythagorean theorem to turn the width and height into a diagonal pixel count, then divides by the physical diagonal in inches. The aspect ratio is the width and height reduced by their greatest common divisor.
Worked example
A 24-inch 1920×1080 monitor:
Total pixels: 1920 × 1080 = 2,073,600 (2.07 MP).
Diagonal pixels: √(1920² + 1080²) = 2202.9.
PPI: 2202.9 ÷ 24 ≈ 91.8; ratio 16:9.
Sharpness rule of thumb: phones often top 400 PPI, laptops 130–230, and large desktop monitors 90–160. Higher PPI means smaller, less visible pixels and crisper text.
Why pixel density matters
Two screens can share a resolution yet look very different: a 1080p phone is razor sharp while a 1080p 32-inch TV shows visible pixels up close, because density falls as the same pixels are spread over a larger area. PPI captures that, which is why it is the best single measure of perceived sharpness.