Every screen is just a grid of coloured dots, but the marketing around those grids has turned into an alphabet soup. HD, FHD, QHD, UHD, 4K, 5K, 8K — they all describe the same property in different ways. This guide walks through the numbers, the aspect ratios behind them, and what actually matters for buying decisions.
Common Resolutions at 16:9
| Name | Pixels | Total | Common use |
|---|---|---|---|
| SD (480p) | 854 × 480 | 0.41 MP | Old streaming, low-end webcams |
| HD (720p) | 1280 × 720 | 0.92 MP | Entry-level streaming, basic monitors |
| Full HD (1080p) | 1920 × 1080 | 2.07 MP | Standard TVs, laptops, gaming |
| QHD / 1440p | 2560 × 1440 | 3.69 MP | High-end monitors, gaming |
| 4K UHD | 3840 × 2160 | 8.29 MP | Premium TVs, professional monitors |
| 8K UHD | 7680 × 4320 | 33.18 MP | Flagship TVs, broadcast production |
Aspect Ratios
16:9 dominates almost every consumer screen: TVs, most monitors, laptops, phones in landscape, YouTube videos. 4:3 is the classic TV and old-computer ratio — still common in tablets and presentation slides. 21:9 (or 21.5:9) is the ultrawide format used for cinematic monitors; it matches the 2.39:1 cinema aspect ratio almost exactly. 3:2 is the photographic ratio of a 35 mm full-frame sensor, used in some premium laptops. 1:1 turns up in Instagram squares and a handful of niche displays.
Pixel Density Matters More Than Pixel Count
A 1080p phone screen and a 1080p television have wildly different sharpness because their pixels per inch differ by an order of magnitude. The right way to think about resolution is in PPI, not raw pixel count. Beyond about 300 PPI at a typical viewing distance, the human eye cannot resolve individual pixels — that is the rough definition of a "retina" display. Once you cross that line, more pixels deliver diminishing returns and just consume more GPU power.
Worked Example: Calculating PPI Yourself
PPI is the diagonal pixel count divided by the diagonal size in inches. To get the diagonal pixel count, use Pythagoras on the width and height in pixels:
- 27-inch 4K monitor (3840 × 2160): diagonal pixels = √(3840² + 2160²) ≈ 4406. Divide by 27 → about 163 PPI — crisp at desk distance.
- 6.1-inch phone (1170 × 2532): diagonal pixels ≈ 2789. Divide by 6.1 → about 457 PPI — far past the retina threshold, which is why phone text looks razor sharp.
- 55-inch 4K TV: same 4406 diagonal pixels ÷ 55 → about 80 PPI — low, but fine because you sit metres away.
The takeaway: the same 4K resolution can be sharp or soft depending on screen size and viewing distance, so judge a display by PPI in context, not by the headline resolution alone.
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