1366 × 768 is not 16:9, no “21:9” monitor is 21:9, and the same DVD frame is two different shapes. The round number on the box is usually not the ratio.
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Ratio683:384
As cinema writes it1.78:1 · 1.778646
1.05 megapixels, landscape. Close to 16:9, but not equal to it.
This is not 16:9. It reduces to 683:384 and sits 0.049% wider than 16:9. Small, and not nothing: content authored at 16:9 is scaled slightly to fit, and a layout that assumes the round number will be out by that much.
What common resolutions actually are
1280 × 720 (720p)16:9 · 1.7778
1366 × 768683:384 · 1.7786
1440 × 9008:5 · 1.6000
1920 × 1080 (1080p)16:9 · 1.7778
1440 × 10804:3 · 1.3333
2560 × 108064:27 · 2.3704
2560 × 1440 (1440p)16:9 · 1.7778
3440 × 144043:18 · 2.3889
3840 × 2160 (UHD 4K)16:9 · 1.7778
4096 × 2160 (DCI 4K)256:135 · 1.8963
3840 × 108032:9 · 3.5556
1080 × 19209:16 · 0.5625
What this converter covers
Exact integer reduction, a missing dimension from a ratio, what letterboxing costs against what cropping costs, and what non-square pixels do to a stored resolution.
Any resolution reduced to its exact integer ratio
Whether it really is the named ratio, decided on integers not a tolerance
The missing dimension for a given ratio and one side
Letterbox bars, wasted screen, and the equal cost of cropping instead
Anamorphic formats where the stored resolution is not the shape
Exact reduction Near-misses flagged Anamorphic handled Fit and crop compared
“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.
Related calculators
Other places a stated number is not the measurement:
Data StorageBytes, KB, MB, GB and TB alongside the binary KiB, MiB, GiB and TiB — and why a 1 TB drive shows as 931 GB.
AreaSquare feet, square metres, acres and hectares, with the factors squared for you — a square metre is 10.76 sq ft, not 3.28.
Numeral BaseBinary, octal, decimal and hex — showing the 4-bit grouping that makes hex work, and handling negatives at a width you choose.
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.
ITU-R BT.709 — Parameter values for the HDTV standardInternational Telecommunication Union · verified 2026-09-07 · The 16:9 picture aspect ratio and square-pixel sampling that define HD, against which the near-miss resolutions on this page are measured
Two limits worth naming. First, this page describes the shape of a frame, not what a display physically does with it. Overscan on televisions, non-uniform scaling modes, a player's own zoom setting, and the difference between a container's aspect ratio and the active picture inside it can all mean the shape reaching your eye is not the one computed here — if something looks stretched, the setting is usually in the player or the display rather than in the file. Second, rounding is unavoidable: a ratio and a pixel grid do not always agree, and where a computed dimension is not a whole number it has to be rounded, which puts the result very slightly off the ratio you asked for. For video encoding specifically, many codecs require dimensions divisible by two, four or sixteen, so the practical size is often not the mathematically exact one; encode to the nearest permitted size rather than the exact figure, and let the container carry the intended display ratio.
Published the Aspect Ratio Calculator with four modes: reduce a resolution exactly, solve for a missing dimension, fit one ratio inside another, and read a format with non-square pixels.
Decides exactness by reducing both ratios to integers rather than by a percentage tolerance. 1366x768 is 683:384 and sits 0.049 percent from 16:9 -- any tolerance loose enough to be useful would call it exactly 16:9, which is precisely the claim worth contradicting.
Shows that no 21:9 monitor is 21:9. The two common ultrawides are 64:27 and 43:18 against 21:9s 2.333, and they are not the same shape as each other.
Gives the letterbox and crop costs together, because they are always the same number: both equal one minus the narrower ratio over the wider one. The choice is never whether to lose that share, only whether to lose it from the screen or the picture.
Handles anamorphic formats, where the same stored 720x480 DVD frame is 16:9 or 4:3 depending only on the pixel aspect ratio -- so the resolution alone cannot tell you the shape.
Separates UHD 4K (3840x2160, 16:9) from DCI 4K (4096x2160, 256:135), which are different shapes sold under one name.
Verified by 67 automated cases, including a 45-combination sweep proving the letterbox-crop identity from an independently derived figure.
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