Screen Size Calculator

Last updated: 2026-05-18

What Is PPI and Why Does It Matter?

PPI — pixels per inch — is the single most useful number for comparing screen sharpness. It tells you how many pixels are packed into each inch of your display's diagonal. A 1080p panel that measures 24 inches has very different sharpness from one that measures 55 inches, even though both have the same number of pixels. PPI captures that difference immediately.

Higher PPI generally means crisper text, tighter icons, and smoother curves. Below about 80 PPI, individual pixels become noticeable at arm's length. Above 200 PPI at a typical laptop distance, most people cannot resolve the pixel grid at all — which is why Apple coined the term "Retina" for that threshold.

How PPI Is Calculated

The calculation has two steps.

Step 1 — diagonal pixel count: Use the Pythagorean theorem on the pixel grid:

diagonal_px = √(width_px² + height_px²)

For 1920 × 1080: $\sqrt{1920^2 + 1080^2} = \sqrt{4{,}852{,}800} \approx 2{,}202.9$ pixels

Step 2 — divide by diagonal in inches:

PPI = diagonal_px ÷ diagonal_inches

For a 15-inch screen: PPI = 2,202.9 ÷ 15 ≈ 146.9 PPI

Physical dimensions follow directly. The screen's proportions match the pixel grid, so:

physical_width  = (width_px  / diagonal_px) × diagonal_inches × 25.4 mm
physical_height = (height_px / diagonal_px) × diagonal_inches × 25.4 mm

Pixel pitch — the gap between pixel centres — is just the inverse of PPI expressed in millimetres:

pixel_pitch (mm) = 25.4 / PPI

Common Screen PPI Reference

A 27-inch 4K display at ~163 PPI is widely regarded as the sweet spot for close-up desktop work. Most people cannot distinguish further improvement beyond 200 PPI at typical monitor distances (~60 cm).

What "Retina Display" Actually Means

Apple defines "Retina" as a display whose PPI is high enough that the human eye cannot resolve individual pixels at the device's normal viewing distance. The threshold varies by use case:

• iPhone (held ~25 cm away): ≥300 PPI
• iPad (held ~40 cm away): ≥264 PPI
• MacBook (used ~50 cm away): ≥220 PPI
• iMac / Pro Display XDR (~65 cm away): ≥218 PPI

The key insight is that viewing distance matters as much as pixel density. A 92 PPI TV at 3 metres can appear perfectly sharp, while the same PPI on a phone held at 25 cm looks pixelated. The question is not "what is the PPI?" but "is the PPI high enough for the expected viewing distance?"

How to Measure Your Screen Size

Spec sheets give the diagonal of the active area, but if you want to confirm or measure an unknown display:

1. Use a cloth tape measure or a rigid ruler.
2. Place one end at a corner of the visible image area — not the bezel, not the case.
3. Stretch diagonally to the opposite corner.
4. Read the measurement in inches (for US-market specs) or centimetres (divide by 2.54 for inches).

Never include the plastic or metal frame (bezel) in the measurement. A 27-inch panel might sit inside a case that adds 1–2 cm on each side, making the whole unit wider, but the rated screen size refers only to the glass.