A resolution is not a ratio
“1080p” is a count of horizontal lines. It says nothing about the shape: 1920 × 1080 is 16:9 and 1440 × 1080 is 4:3, and both are 1080p. HDV cameras recorded the second and displayed it as the first, which is exactly why the number alone was never enough.
The more surprising case is the one most people have looked at for years. 1366 × 768, the most-shipped laptop panel in history, reduces to 683:384. That is 1.7786 against 16:9’s 1.7778 — 0.049% wider. The reason is mundane: 768 × 16/9 is 1365.33, and a panel cannot have a third of a pixel, so the manufacturers rounded up to 1366 and the ratio stopped being 16:9.
Half a thousandth is not going to ruin anything. It is worth knowing for two reasons. It means a 16:9 image on that screen is very slightly scaled rather than mapped pixel for pixel, which is a real if invisible softening. And it is a useful reminder that these numbers are engineering compromises rather than clean mathematics — which matters much more in the next case, where the gap is fifty times larger.
This is why the calculator above decides exactness by reducing both ratios to integers and comparing them, rather than by asking whether they are within some percentage. A tolerance loose enough to be useful would call 1366 × 768 exactly 16:9, and that is the one thing worth saying about it.
No 21:9 monitor is 21:9
Ultrawide monitors are sold as 21:9. None of them are. 21:9 is 2.333, and the two resolutions that carry the label are 2.370 and 2.389.
What the two common “21:9” ultrawide resolutions actually are| Resolution | True ratio | Decimal | Off 21:9 by |
|---|
| 21:9 as marketed | 21:9 | 2.3333 | — |
|---|
| 2560 × 1080 | 64:27 | 2.3704 | 1.59% |
|---|
| 3440 × 1440 | 43:18 | 2.3889 | 2.38% |
|---|
So “21:9” is a marketing category rather than a measurement, and — the part that actually bites — the two are not the same shape as each other. Content mastered to fill one leaves a visible sliver on the other. Two monitors with the same badge have genuinely different proportions.
There is a reason for at least one of the numbers. 64:27 is the third term in a geometric series that starts at 4:3 and multiplies by 4:3 each time — 4:3, then 16:9, then 64:27 — which is why it turns up in the standards rather than being invented for a product line. 43:18, meanwhile, lands within 0.05% of the 2.39:1 cinema scope ratio, which makes a 3440 × 1440 monitor very close to a perfect fit for a scope film: an accident, and a genuinely useful one.
Letterbox or crop — the same cost either way
When content of one ratio meets a screen of another there are exactly two honest options, and a fact about them that is not obvious.
Fit it, and the whole picture is shown with bars — black stripes top and bottom for content wider than the screen (letterbox), or at the sides for content that is narrower (pillarbox). A 2.39:1 film on a 1920 × 1080 screen renders at 1920 × 803, with 138-pixel bars, leaving 25.6% of the screen unused.
Fill it, and the picture is scaled until it covers the screen and the overflow is cut off. For that same film, filling a 16:9 screen means losing 25.6% of the picture — the sides.
Those two numbers are identical, and always are. Both equal one minus the narrower ratio divided by the wider one, so the proportion you lose is fixed by the mismatch itself. The choice is never whether to lose that share; only whether to lose it from the screen or from the image. Which is why “fill screen” is not a free upgrade over “fit”, and why a film shot for scope really does want the black bars.
When the pixels are not square
Everything above assumes a pixel is as wide as it is tall. On modern computers, phones and cameras that is true. On broadcast and DVD-era video it is not, and the consequence is that the stored resolution genuinely cannot tell you the shape of the picture.
A widescreen NTSC DVD stores 720 × 480. That is 3:2. It displays as 16:9, because each pixel is stretched to 32/27 of its height on playback — an anamorphic encoding, which packs a wide picture into a narrow frame and unpacks it at the other end. The identical 720 × 480 frame with a pixel ratio of 8:9 instead displays as 4:3. Same file size, same pixel count, different picture.
PAL does the same thing with different numbers, storing 720 × 576 and using 64:45 or 16:15 to reach the same two shapes. HDV recorded 1440 × 1080 with 4:3 pixels to make 16:9. Anamorphic cinema lenses do it optically, squeezing the image onto the film and unsqueezing it in projection.
The practical upshot is that a video file carries two ratios — the storage one and the display one — and software that reads only the first shows the picture stretched or squashed. If a ripped DVD looks tall and thin, this is why: the pixel aspect ratio was dropped somewhere in the chain, and the fix is to tell the player the intended display ratio rather than to rescale the file.
Which 4K did you mean
“4K” names two different resolutions with two different shapes, and which one is meant depends on whether you are talking to a television or a cinema.
UHD 4K is 3840 × 2160. It is exactly 16:9, exactly four times 1920 × 1080, and it is what every consumer television and streaming service means. It is not actually four thousand pixels wide, which is where the name stops making sense.
DCI 4K is 4096 × 2160, the digital cinema container. That is 256:135, or 1.90:1 — wider than 16:9, and genuinely close to four thousand pixels. Cinema then works inside that container: a 1.85:1 film uses the full width and less height, a 2.39:1 scope film uses the full width and less again, so the projected image is smaller than the container in both cases.
The same ambiguity runs through the other names. “2K” is 2048 × 1080 in cinema and usually means 1920 × 1080 elsewhere. “8K” is 7680 × 4320 for television. The consumer names count roughly-thousands of horizontal pixels and the older names counted lines vertically, which is why 1080p and 4K are measured along different axes and cannot be compared without stopping to think.
Sources and methodology
The arithmetic here is a greatest common divisor and needs no authority. What the sources settle is which shapes the standards actually define — that HD and UHD are 16:9 with square pixels, that the same 720-sample television line is displayed as either 4:3 or 16:9 through its pixel aspect ratio, and that digital cinema’s 4K container is a different shape from the 4K sold on televisions. Those are the points where a confident answer can be confidently wrong.