Aspect ratios for screens and video: 16:9, 4:3, 21:9 and 9:16 explained

Every screen you own is a rectangle, and every rectangle has a shape. The aspect ratio is simply the name of that shape: the width of the picture compared with its height, written as two numbers with a colon. When I say a TV is 16:9, I am saying that for every 16 units of width it has 9 units of height — and that one little ratio quietly decides whether your film fills the screen or floats in a sea of black bars. In this guide I will show you how to read any screen aspect ratio, work out the ratio of any resolution in seconds, and predict exactly how big the black bars will be before you press play.

An aspect ratio is just a simplified fraction

Here is the part most explanations skip: a resolution like 1920 × 1080 is a ratio — 1920:1080 — it just has not been simplified yet. Divide both numbers by their greatest common divisor, 120, and you get 16:9. That is the entire method. 1280 × 720 divides by 80 to give 16:9 as well, and an old 1024 × 768 monitor divides by 256 to give 4:3. If the dividing does not come out clean, that itself is information, as you will see with laptop panels below.

I built our ratio calculator to do exactly this simplification, plus the reverse: give it one side and a target ratio and it works out the missing side. Everything in this article is that same arithmetic, applied to screens.

The reference table: resolutions and their ratios

Every ratio above simplified by dividing width and height by their greatest common divisor. Two rows deserve a closer look — the ultrawides — and I come back to them below.

The four shapes that run your life

4:3 — the old television shape

For most of the twentieth century, pictures were 4:3 (1.33 as a decimal). It is nearly square, which made it a poor fit for human peripheral vision but a fine fit for the cathode-ray tubes of the time. You still meet it in classic TV reruns, older security cameras, iPads (which kept 4:3 for reading) and retro gaming.

16:9 — the shape that won

The 16:9 aspect ratio (1.78 as a decimal) is now the default shape of television, streaming, YouTube and most monitors. It won partly because it is a geometric compromise: it can show both old 4:3 content and wide cinema films with the least wasted space, and partly because it sits close to the shape of a cinema frame while remaining practical to manufacture. When a spec sheet just says HD, Full HD or 4K, it almost always means a 16:9 panel underneath.

21:9 — the ultrawide, with an asterisk

Ultrawide monitors promise a cinema-like 21:9 (2.33), and for films shot at 2.39:1 they genuinely deliver a near-full-bleed picture. But look at the table again: a 2560 × 1080 panel simplifies to 64:27 (2.37), and a 3440 × 1440 panel to 43:18 (2.39). Neither is exactly 21:9 — the label is marketing shorthand for a family of ratios around 2.37–2.39. The panels are honestly wide; the name is approximate.

9:16 — the phone shape

Turn 16:9 on its side and you get 9:16 (0.5625), the shape of vertical phone video. It is not a new ratio, just the old one held upright — which is precisely why vertical clips look so awkward on a television, as the next section shows in pixels.

Black bars are just leftover ratio

When picture and screen shapes disagree, something has to give. Players refuse to stretch or crop, so they pad the difference with black — and I can tell you exactly how much padding, because it is pure ratio arithmetic on a 1920 × 1080 screen:

• 4:3 content (pillarbox): at the screen’s full height of 1,080 px, a 4:3 picture is 1,080 × 4 ÷ 3 = 1,440 px wide. The leftover 480 px becomes two bars of 240 px on each side.
• Ultrawide film at 2.39:1 (letterbox): at the full width of 1,920 px, the picture is 1,920 ÷ 2.39 ≈ 803 px tall. The leftover becomes bars of about 138 px top and bottom.
• 9:16 vertical video: at full height, the picture is only 1,080 × 9 ÷ 16 = 607.5 px wide, leaving bars of about 656 px on each side — more bar than picture, which is why broadcasters blur a blown-up copy of the video behind it instead.

None of this is a defect in your screen. It is the visible signature of two ratios that do not match, and once you can do this arithmetic you will never be surprised by it again. The same method also works in reverse when you are exporting video: pick the target ratio first, then compute the pixel dimensions, never the other way round.

Black bars at 4K: the same aspect ratio fractions, bigger bars

Upgrade the screen to 3840 × 2160 and every bar grows in pixels — but the share of the screen you lose does not move at all, because that share is fixed by the two ratios alone. Here is the same content on a 4K screen:

Compare the last column with the 1080p numbers in the previous section: 4:3 content uses 75.0% of a 16:9 screen at any resolution, and a 2.39:1 film uses 74.4% of its height. Resolution changes the pixel counts; the ratio decides the fractions.

Two of these rows surprise people. The 1.85:1 “flat” cinema format — the shape of many dramas and comedies — nearly fills a 16:9 screen, leaving bars of only about 42 px that many viewers never notice. And vertical video is the real casualty of the big-screen era: a 9:16 clip on a 4K television uses less than a third of the panel, which is why news channels fill the sides with a blurred blow-up of the same footage rather than leave them black.

Resizing without stretching: keep the ratio locked

The golden rule of resizing an image or video is that width and height must scale by the same factor. Take 1280 × 720 up to a width of 1,920: the factor is 1,920 ÷ 1,280 = 1.5, so the height must become 720 × 1.5 = 1,080. If you change one side without the other, faces widen and circles become eggs — the picture’s internal ratios are broken even though the file still opens.

One honest wrinkle: not every panel is perfectly on-ratio. The common laptop resolution 1366 × 768 simplifies not to 16:9 but to 683:384 (1.7786 instead of 1.7778), because panel makers rounded the height to an even number. The distortion is far too small to see, but it is a good reminder that a screen aspect ratio on a spec sheet is sometimes a very close approximation rather than an exact fraction.

The same locked-ratio rule runs the crops you make for other platforms. Start from a 1920 × 1080 master and cut the other shapes out of it: a 9:16 vertical version at full height is only 1080 × 9 ÷ 16 = 607.5 px wide, so you keep just 31.6% of the frame’s width — which is why vertical edits are shot with the subject dead centre. A 4:5 portrait post keeps 1080 × 4 ÷ 5 = 864 px of width, and a 1:1 square keeps the middle 1080 × 1080. One master, three crops, and every one of them is the same multiplication: pick the side that stays fixed, and let the ratio decide the other.

One diagonal, many shapes: what your screen aspect ratio hides

Screen sizes are sold by the diagonal, but the diagonal says nothing about shape — and shape decides both the width you can fit on a desk and the area you actually look at. Here are the real dimensions behind common advertised sizes, computed from the diagonal and the ratio (width = diagonal × ratio share, by Pythagoras):

Ultrawide rows use the real 64:27 panel ratio rather than the nominal 21:9 label. Areas are the visible picture area, before bezels.

Read the two 27-inch rows against each other, because they expose the diagonal trick: the ultrawide is 1.4 in wider but 2.7 in shorter, and it gives you about 16% less picture area than the plain 16:9 screen with the same advertised size. A 34-inch ultrawide, meanwhile, has almost exactly the height of a 27-inch 16:9 monitor (13.2 in both) — which is the honest way to think about it: an ultrawide is a standard screen stretched sideways, not a bigger screen. When I help someone choose, I ignore the diagonal entirely and compare height first, width second, area third.

Sources: ratios simplified from the stated resolutions by greatest-common-divisor division and verified arithmetically for this article; cinema DCI dimensions follow the Digital Cinema Initiatives specification (2048 × 1080 / 4096 × 2160).