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Aspect Ratios Explained: 16:9, 9:16, 1:1, and 4:3

An aspect ratio is a reduced fraction of width to height, and every question about it is answered by arithmetic. This guide derives the common ratios, works through a conversion, and shows what letterboxing costs.

Published 2026-08-11 · Updated date pending

The short answer

An aspect ratio is width divided by height, reduced to its smallest whole-number terms. 3840x2160 has a greatest common divisor of 240, so it reduces to 16:9. That is the whole mechanism.

16:9 is the standard landscape frame — YouTube states plainly that “The standard aspect ratio for YouTube on a computer is 16:9.” 9:16 is the same frame turned vertical. 1:1 is square. 4:3 is the older, taller landscape frame. Converting between them means either adding bars or cutting picture, and the Aspect Ratio Calculator will tell you exactly how much of either.

How a ratio is derived from pixels

g       = gcd(width, height)        // Euclidean algorithm
ratio   = (width / g) : (height / g)
decimal = width / height

Worked on the two most common frames:

gcd(3840, 2160) = 240  ->  3840/240 : 2160/240  = 16:9
gcd(1080, 1920) = 120  ->  1080/120 : 1920/120  =  9:16

The decimal form of 16:9 is 3840 / 2160 = 1.7778. It is worth carrying, because some frames do not reduce to friendly terms and the decimal is the only readable description of them.

Both reductions reproduce in the Aspect Ratio Calculator in its dimensions-to-ratio mode.

The four ratios in practice

16:9 is the landscape standard. YouTube states it is the standard aspect ratio for its computer player and lists the 16:9 dimensions it recommends: 7680x4320 for 4320p, 3840x2160 for 2160p, 2560x1440 for 1440p, 1920x1080 for 1080p, 1280x720 for 720p, 854x480 for 480p, 640x360 for 360p and 426x240 for 240p. The HDTV parameter values behind this family of formats are specified by the ITU-R in Recommendation BT.709, “Parameter values for the HDTV standards for production and international programme exchange”, currently at BT.709-6.

Note that 854x480 does not reduce to 16:9 exactly — 854 / 480 is 1.7792, not 1.7778 — because 480 x 16/9 is 853.33, and pixel dimensions must be whole numbers. Rounding to an even width is why the entry reads 854.

9:16 is 16:9 rotated. A 1080x1920 frame has a gcd of 120 and reduces to 9:16. YouTube states that where a video’s aspect ratio differs from the player’s, “the player will automatically change to the ideal size to match your video and the viewer’s device”, and that for vertical video on computer browsers it “may add more padding for optimal viewing”, with padding that is white by default and dark grey under the dark theme.

1:1 is square: width equals height, gcd equals the dimension, and the reduction is trivially 1:1. Its decimal form is 1.0000, and the orientation is neither landscape nor portrait.

4:3 is the older landscape frame, decimal 1.3333. A 1440x1080 frame has a gcd of 360 and reduces to 4:3. Sony documents AVCHD HQ and LP modes recording at 1,440 x 1,080 pixels, which is a 4:3 pixel grid presented as 16:9 — a reminder that stored pixel dimensions and displayed shape are not always the same thing.

Worked example: fitting a 16:9 frame into 9:16

This is the conversion most often needed, and the one most often done by eye. The arithmetic gives two exact answers depending on whether you refuse to lose picture or refuse to add bars.

Source 3840x2160, target ratio 9:16.

Contain — keep the whole picture, add bars:

sourceRatio = 3840 / 2160 = 1.7778
targetRatio = 9 / 16      = 0.5625
sourceRatio > targetRatio, so width is kept:
  fitW = 3840
  fitH = 3840 / 0.5625 = 6826.666... px
  bars = (6826.666... - 2160) / 2 = 2333.333... px top and bottom

Cover — fill the frame, crop the sides:

cropH = 2160
cropW = 2160 x 0.5625 = 1215 px
crop  = (3840 - 1215) / 2 = 1312.5 px from each side

Cover discards 68.4% of the original width. That is the real cost of turning a landscape edit vertical, and it is worth knowing before an edit rather than after. Both results reproduce in the Aspect Ratio Calculator in its fit mode.

Solving for a missing dimension

The third common question is: at this ratio, what is the other side?

heightFromWidth = width  x (ratioH / ratioW)
widthFromHeight = height x (ratioW / ratioH)
16:9 at width 1920  ->  1920 x (9/16) = 1080 px   exact
16:9 at width 1280  ->  1280 x (9/16) =  720 px   exact
16:9 at width 1919  ->  1919 x (9/16) = 1079.4375 px

The third line is the interesting one. The exact answer is not a whole number of pixels, and many codecs require even dimensions. The calculator shows both the unrounded value and the rounded one, and offers an even-pixel snap that would give 1080 here.

This is worth doing deliberately rather than letting an application round for you. Rounding a single side of a frame changes its actual ratio: 1919x1079 has a decimal ratio of 1.7785 rather than 1.7778, and if both sides get rounded independently across a chain of tools the drift accumulates. Deciding the exact target dimensions once, at the start, avoids a frame that is almost but not quite the shape you intended.

The reduced ratio is also the least useful answer surprisingly often. A 1998x1080 frame has a greatest common divisor of 54 and reduces to 37:20, whose terms are small enough to look like a finished answer. But 37 divided by 20 is exactly 1.85, and 1.85:1 is how that frame is normally described. The reduction is correct and unrecognisable at the same time. This is why the calculator checks a table of known ratios first and reports the exact reduction beneath it, rather than presenting the reduction alone.

Aspect ratio and file size are unrelated

It is worth stating explicitly, because the two get conflated. Aspect ratio is a shape. File size follows from bitrate and duration.

A 3840x2160 frame holds 8,294,400 pixels. Cropping it to 1215x2160 leaves 2,624,400 — under a third. But if you export both at 35 Mbps for ten minutes, both files are 2.65 GB, because the file size formula has no pixel-count term. What changes is how many bits each remaining pixel gets, not how many bits there are.

What this does not tell you

  • Pixel aspect ratio is not covered. Everything above assumes square pixels. Formats with non-square pixels store a frame whose stored dimensions and displayed shape differ.
  • It does not tell you where to crop. The arithmetic gives the size of the crop, not which part of the frame to keep.
  • Platform delivery rules change. The YouTube and Vimeo statements above carry an access date. Vimeo, for example, publishes maximum playback dimensions per rendition — 4K at up to 2160px high and 4096px wide, 1080p at up to 1080px high — which is a different constraint from an aspect ratio.
  • Safe areas and title-safe framing are out of scope.

Frequently asked questions

What aspect ratio is 1920x1080?

16:9. The greatest common divisor of 1920 and 1080 is 120, giving 16:9 exactly, with a decimal form of 1.7778.

How do I convert a 16:9 video to 9:16 without losing the subject?

You cannot avoid the loss arithmetically — covering a 3840x2160 frame into 9:16 leaves 1215 px of the original 3840, so 1312.5 px is cut from each side. What you can choose is where those 1215 px come from. Use contain instead if you would rather add bars than crop.

Is 4K always 3840x2160?

YouTube’s recommended 16:9 table lists 2160p as 3840x2160. Other 4K frame sizes exist in production formats, and Vimeo’s playback guidance lists 4K as up to 2160px high and 4096px wide. Enter your actual dimensions rather than assuming.

Why is 480p listed as 854x480 and not 853x480?

Because 480 x 16/9 is 853.33, which is not a whole number, and even dimensions are preferred by many codecs. 854x480 has a decimal ratio of 1.7792 against 16:9’s 1.7778 — close enough to display as 16:9, and this is why the calculator reports a nearby known ratio alongside the exact reduced one.

Does changing aspect ratio change my file size?

Not by itself. File size comes from bitrate and duration. Cropping changes how many pixels share the bitrate, not the bitrate. See video bitrate explained.

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