PPI / DPI Calculator (Screen Pixel Density)

PPI (pixels per inch) and DPI (dots per inch) both measure how tightly packed pixels are on a display. PPI is the correct term for screens; DPI strictly refers to printers. In practice, the two get used interchangeably.

The formula uses the Pythagorean theorem to get the diagonal in pixels, then divides by the diagonal in inches:

diagonal_pixels = sqrt(width^2 + height^2) PPI = diagonal_pixels / diagonal_inches

A 27 inch monitor at 1920 x 1080 has a diagonal of about 2202 pixels divided by 27 = 81.6 PPI. The same screen at 3840 x 2160 (4K) gives 163.2 PPI.

Sharpness ratings, based on what people actually notice at typical viewing distance:

• Below 80 PPI: visibly pixelated. Old TVs, large budget monitors. You can count pixels at arms length.
• 80 to 110 PPI: standard desktop monitors. Acceptable for most work.
• 110 to 150 PPI: sharp. Most premium laptops sit here.
• 150 to 220 PPI: very sharp. iPad, MacBook Retina, high-end monitors.
• 220+ PPI: phone-grade. The eye cannot resolve individual pixels at normal hold distance.

Why this matters in practice:

• Designers care about target PPI when sizing graphics. A 100 px button is twice as physically large on an 80 PPI monitor as on a 160 PPI one.
• Print and screen are separate. Print DPI of 300 means 300 dots per inch of paper; screen PPI works in pixels.
• Apple’s “Retina” branding starts around 220 PPI for laptops and 300+ for phones.

Operating systems use scaling to compensate. macOS at 2x scale on a Retina display draws each logical pixel as a 2x2 block, so the UI is the right physical size while text and images use the extra detail. Windows handles this through DPI scaling at 100% / 125% / 150% / etc.

When buying a monitor: above 27 inches, 1080p starts to look soft. 1440p is a good middle ground at 27 to 32 inches. 4K shines at 27 inches and up.