Why is 2160p video called 4K?
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Marketing. Resolutions are typically named after the vertical pixel count, which is the lower number. The jump from 480p (SD) to 720p (HD) was HUGE visually, so huge that small screens like phones and mobile game consoles are still made with 720p screens. AND the numbers of the terminology did look roughly double. However, that’s not quite how it works. You have to multiply the horizontal and vertical pixels. 480p (in 16:9 ratio which wasn’t common at the time) is 848 x 480, or 407,040 pixels. 720 is 1280 x 720 in a standard 16:9 widescreen, or 921,600 pixels.
The jump from 720p to 1080p (FHD) came pretty quickly, and while it wasn’t as big visually, it was still definitely noticeable. It was also still over double the number of pixels. 1080p is 1920 x 1080 in 16:9, or 2,073,600 pixels. The numbers only looked about 400 more again in name, but importantly, it was the baseline for a long time.
Blu-ray in around 2006 allowed for full HD images. Video games struggled to hit 1080p often for that era (PS3/XB360) but PCs could do it, work monitors and TVs often had 1080p panels and are still popular, and it was standard in games by PS4/XBONE in 2013. The PS4 Pro and Xbox One X pushed 4k gaming a bit in 2016, but those were half-gen upgrades, the PS4 Pro didn’t fully render it natively, and that’s still at least a DECADE of 1080p being standard without even having access to anything new. DVDs in 480p were only king in the early 00s, for reference. 720p didn’t have physical media pushing it that took off like DVDs or Blu-ray.
1440p (QHD) started to be a thing for monitors as it typically does first, but wasn’t catching on for TVs. Like at all. 720p had streaming to help it sell budget TVs, 1440p, not so much. It’s STILL not available on most streaming services, and 1080p actually looks worse on a 1440p screen due to video scaling. And like 720p, it had no physical video media or console video games to boost it.
1440p is 2560 x 1440 in 16:9, or 3,686,400 pixels. This is 1.77 times the pixels of 1080p, not ~2.25 times like the previous upgrades. But more importantly, it didn’t SOUND much bigger either from the terminology. The consumer sees it and thinks “what, only 400 more pixels again?”
Visually, I think going from 1080p to 1440p takes you about as far as going from 1080p to 4k, unless the screen is very big of course. Particularly on a computer monitor, you’ll likely not notice the difference between 1440p and 4k even in video games. The only thing you’d see is more aliasing maybe. But it wasn’t really enough for video consumers, or non-PC gamers. Even then, video cards were starting to plateau a bit, until recently it’s been hard to get a PC to run anything new at 1440p with a decent frame rate.
Anyway, 4k (UHD) is 3840 x 2160 in 16:9, or a whopping 8,294,40 pixels. 4x 1080p, and 2.25x 1440p. Normally it would be called 2160p, the vertical pixel count. But for marketing purposes, they decided 3840 (the horizontal pixel count) was close enough to 4000, and 4000 is ~4x 1080p, so they changed it to sound more accurate to what it actually is. Which is even more important, because #1 it sounds like a new technology. To most consumers they know standard definition, HD, and now 4k. And #2 because visually (for the average person and screen size) it’s not all that different than 1080p, and even less different than 1440p, so they needed it to sound more impressive. That, combined with UHD Blu-ray, PS5/XBSeries, and lowering costs of TVs have made 4k a smashing success.
Retroactively, 1440p is sometimes called “2k” now, even though 2560 is further from 2k than 3840 is from 4k. But it is more accurate on the sense that it’s around double 1080p and half of 4k.
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