Aspect Ratio Calculator

What Is an Aspect Ratio?

An aspect ratio compares a rectangle's two sides by division rather than by size. Divide 1920 by 1080 and you get 1.778; divide both by the largest number that goes into each — 120 — and you get the same relationship written as 16 and 9. Because it is a comparison, scaling both sides by any factor leaves it untouched, which is how a 1280 × 720 thumbnail and a 3840 × 2160 television share one shape.

Order is part of the notation, and the convention is width first. 16:9 is landscape; 9:16 is the identical shape stood on end, which is what a phone records when it is held upright. The calculator reads its first two boxes in that order, so 1920 × 1080 returns 16:9 and 1080 × 1920 returns 9:16.

Television's shape is fixed by standards rather than habit. The ITU publishes the parameter recommendations broadcast equipment is built against — Rec. BT.709 for high definition, in force in its sixth revision since June 2015, and Rec. BT.2020 for ultra-high definition, in its second since October 2015 — which is why an HD frame and a UHD frame come back from this calculator as the same shape at different pixel counts.

Reducing 1920 : 1080 to 16 : 9 is a greatest-common-divisor problem, and you can run one on any pair of numbers with the GCF Calculator.

Aspect Ratio vs Resolution

Resolution counts pixels; aspect ratio describes shape, and the two travel independently. Feed the calculator 1280 × 720, 2560 × 1440, 3840 × 2160 or 7680 × 4320 and every one returns the same headline, 16:9. Change the shape instead and the pixel count stops predicting anything: 1920 × 1200 holds 2,304,000 pixels against 1920 × 1080's 2,073,600 — 11% more picture — and it comes back 8:5. Whether an image fits is decided by the ratio; how sharp it looks is decided by the resolution.

Two Ways to Write One Shape

The colon form and the decimal form say the same thing. 16:9 divided out is 1.778, and film has always quoted the decimal directly — 1.85:1 and 2.39:1 are decimals with a 1 bolted on. The panel leads with the colon form and prints a decimal only when the shape is not one it can name: 1920 × 1080 returns “16:9 — HD/4K video, most screens” with no decimal anywhere, while 2560 × 1080 returns “Ratio 64:27 (≈ 2.370:1)”.

You can also enter a decimal shape directly. Type 2.39 and 1 in the two original boxes and the headline reads 2.39:1, with “Ratio 2.39:1 (≈ 2.390:1)” under it; type 239 and 100 and you get 239:100 instead, the same proportion in whole numbers. Either way a target width of 1920 returns a height of 803.35.

How Do You Calculate an Aspect Ratio?

Two operations answer every question a ratio raises: reduce, then scale.

The Aspect Ratio Formula

W is the original width and H the original height. Both have to be measured the same way; the arithmetic never asks what the unit is.

GCD = greatest common divisor of W and H

Ratio      = (W ÷ GCD) : (H ÷ GCD)
New height = new width × H ÷ W
New width  = new height × W ÷ H

The calculator runs the first two of those. The third line is here because you will need it: the tool solves for a height from a width, so when your constraint is a height instead, do that one multiplication yourself and then feed the width back in to check it.

Step by Step

• Write down the original width and height in one unit. Pixels, millimeters and inches all work, provided you do not mix them.
• Find the greatest common divisor of the pair. The tool uses Euclid's algorithm: replace (a, b) with (b, a mod b) and repeat until the second number is zero. For 1920 and 1080 the chain runs (1920, 1080) → (1080, 840) → (840, 240) → (240, 120) → (120, 0), so the divisor is 120.
• Divide both original figures by that divisor. 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9, which is 16:9.
• To resize, multiply your new width by the original height and divide by the original width. Hold the full-precision value while you work.
• Round once, at the end. Rounding an intermediate figure and then dividing by it is how a resize picks up a pixel of error it never needed.

Worked Example: 1920 × 1080 Scaled to 1280 Wide

Enter 1920, 1080 and 1280, then press Calculate. The panel fills with four lines:

• 16:9
• 16:9 — HD/4K video, most screens
• 1920 × 1080 scaled to width 1280 → height 720 (1280 × 720)
• Scaling rule: new height = new width × 1080 ÷ 1920

The headline is the reduction, 1920 ÷ 120 against 1080 ÷ 120. The third line is 1280 × 1080 ÷ 1920, which lands on exactly 720 because 1280 is a clean two-thirds of 1920. Ask for 1000 instead and the same arithmetic returns 562.5 — correct, and unusable as a pixel dimension. A target of 1536 returns 864, and 320 returns 180.

Aspect Ratio Examples, Worked Out

Seven runs through the calculator, with the figures it hands back.

Two rows repay a second look. A4 paper measures 210 × 297 mm and reduces to 70:99, a decimal of 0.707. One divided by the square root of two is 0.7071, and 70 ÷ 99 is 0.7071 to the same four places: the ISO 216 series is defined so that halving a sheet across its long side reproduces the shape, and only that irrational proportion does it. Whole millimeters cannot land on it exactly, so 70:99 is as close as A4 gets.

The 1366 × 768 row is the other one. A genuine 16:9 panel 768 pixels tall would be 1365.33 pixels wide, and since a third of a pixel cannot be built, those panels shipped at 1366 and the shape reduces to 683:384 — 1.779 against 16:9's 1.778. Scale it to 1024 wide and the height comes back 575.72 rather than a whole pixel. Round that to 576 and you are on 1024 × 576, which does reduce to exactly 16:9.

Reduce-then-scale works on any pair of quantities, not only screen sizes — for mixes, shopping quantities and map scales, use the Ratio Calculator.

Aspect Ratio Chart: Shapes, Decimals and Resolutions

Each shape below is what the calculator prints for the resolutions beside it. The decimal column is width ÷ height, rounded to three places.

The two ultrawide rows carry a warning. The 21:9 on the box is a marketing round number: 2560 × 1080 reduces to 64:27 at 2.370, 3440 × 1440 reduces to 43:18 at 2.389, and a true 21:9 would be 7:3 at 2.333. The three shapes sit close enough together to share a category and far enough apart that a pixel-exact layout has to commit to one of them.

16:10 is the same problem seen from the other side. It is a real shape — 1920 × 1200 and 2560 × 1600 are both it — but 16 and 10 share a factor of 2, so lowest terms are 8:5, and 8:5 is the headline the panel prints. That is why the row above is filed under 8:5.

The 16:9 Resolution Ladder

YouTube publishes the resolutions it wants for the default 16:9 shape. Six of the eight reduce to exactly 16:9. Two do not.

480 pixels of height needs 853.33 pixels of width to be exactly 16:9, and 240 needs 426.67. Neither is a whole number, so the ladder rounds outward to 854 and 426, and the calculator reports what those figures actually say: “Ratio 427:240 (≈ 1.779:1)” and “Ratio 71:40 (≈ 1.775:1)”, against a true 16:9 of 1.778. The gap is under a pixel and invisible in playback. It shows up only when you check the arithmetic and wonder why two rungs of a 16:9 ladder refuse to reduce to 16:9.

How to Read Your Result

A successful run always returns four lines, in the same order.

• The headline: the ratio in lowest terms and nothing else — 16:9.
• The shape line: a name when the calculator recognizes the shape, such as “16:9 — HD/4K video, most screens”, and otherwise the ratio again with its decimal, such as “Ratio 8:5 (≈ 1.600:1)”.
• The scaled line: your original figures, your target width and the height that preserves the shape — “1920 × 1080 scaled to width 1280 → height 720 (1280 × 720)”.
• The scaling rule with your own numbers in it. It arrives as a labeled row, so the panel shows “Scaling rule: new height” on the left and “new width × 1080 ÷ 1920” on the right.

A “Copy result” button under the breakdown copies the headline and the three lines below it as plain text. The fourth line loses its on-screen spacing on the way to the clipboard: label and value are lifted from the same row, so it pastes as “Scaling rule: new heightnew width × 1080 ÷ 1920”, run together. The first three lines paste exactly as they read.

The Seven Shapes the Panel Names

Seven ratios arrive with a description attached, and the wording is fixed:

• 16:9 — HD/4K video, most screens
• 4:3 — classic TV, iPad
• 3:2 — 35mm photo, many laptops
• 5:4 — 8×10 prints
• 1:1 — square (Instagram grid)
• 9:16 — vertical video (Reels/TikTok)
• 2:3 — portrait photo prints

Everything else falls through to the “Ratio X:Y (≈ D:1)” form, and that includes shapes as ordinary as 8:5, 4:5 and 3:4. A portrait phone photo at 3024 × 4032 returns “Ratio 3:4 (≈ 0.750:1)” even though its landscape twin, 4:3, is named. The absence of a name says nothing about how common the shape is.

What the Decimal Tells You

The decimal is width ÷ height, which sorts orientation at a glance: above 1 is landscape, exactly 1 is square, below 1 is portrait. An ultrawide reads 2.370. A portrait feed post at 1080 × 1350 returns “Ratio 4:5 (≈ 0.800:1)”, and A4 held upright returns “Ratio 70:99 (≈ 0.707:1)”. The further a figure sits from 1 in either direction, the more a square crop will take out of it.

When the Panel Refuses

All three fields are required and all three open empty, so the amber “Check your inputs” notice is easy to meet. Two things trigger it, and a third behavior gets mistaken for it.

• A blank box is caught before the formula runs, and the message names the field the way the form labels it: “Enter a value for Original Width.” Leave two blank and they are listed together — “Enter a value for: Original Width, Original Height.” The third field keeps its parenthetical, so it reads “Enter a value for New Width (to scale to).”
• A zero or a negative number does reach the formula, which has a guard of its own. Zero or less in either original box returns “Enter the original width and height”; zero or less in the target width returns “Enter the new width to scale to”.
• The third behavior raises no notice at all: nothing recalculates as you type. The tool runs on Calculate and clears on Reset, and because the boxes start empty there is no result waiting when the page loads — the 1920, 1080 and 1280 you can see are gray placeholders, not values.

Crop, Letterbox or Stretch: Fitting One Ratio Into Another

When a source shape and a destination shape disagree, three things can happen, and the CSS object-fit property names all three. Cover scales the picture until it fills the box and clips whatever hangs over the edge. Contain scales until the whole picture fits and pads the space left over — letterboxing above and below, pillarboxing at the sides. Fill stretches the picture to the box and changes its shape.

Two of those are choices. Crop when the edges are expendable and the subject carries the frame; pad when every part of the picture holds information. Stretching is the third, and it is the only one of the three that changes the proportions of the subject itself.

Working Out the Crop

Hand the calculator the destination shape as its original dimensions, then ask for the largest version that fits inside what you already have. A 6000 × 4000 photo is 3:2; an 8 × 10 print stood upright is 4:5. Enter 4, 5 and a target width of 3200, and the panel returns “4 × 5 scaled to width 3200 → height 4000 (3200 × 4000)” — the full height of the frame, so 3200 × 4000 is the biggest 4:5 rectangle the photo contains.

That crop throws away 6000 − 3200 = 2800 pixels of width, 46.7% of it, 1400 from each side if you center it, and keeps 53.3% of the original area. A gentler pair costs far less: a 4032 × 3024 phone photo into a 16:9 frame. Enter 16, 9 and 4032 and the height comes back 2268, so the crop is 4032 × 2268 — 756 pixels off the height, exactly a quarter of it, with three quarters of the picture surviving.

Working Out the Padding

Padding is the same arithmetic with the other dimension pinned, and it takes one step the calculator will not do for you, because the tool solves for a height from a width. A 1080 × 1920 vertical clip inside a 1920 × 1080 player has to shrink until its height is 1080, so multiply the frame height by the clip's width over its height: 1080 × 1080 ÷ 1920 = 607.5.

Feed that back in and the tool confirms it. Enter 1080, 1920 and 607.5 and the panel returns “1080 × 1920 scaled to width 607.5 → height 1080 (607.5 × 1080)”. The clip occupies 607.5 of the frame's 1920 pixels and leaves 1312.5 pixels of padding, 656.25 on each side — 68.4% of the player's width is not video.

YouTube describes the same behavior from the platform side. Its help page says the player adapts to each video's shape, and that for a 9:16 clip on a computer browser it may add more padding of its own, white by default and dark gray under the dark theme. It also advises against baking bars into the file, because pre-padded video stops the player resizing itself to the viewer's device.

Limits: When This Calculator Does Not Apply

Six things the tool does not do, or does differently from what you might expect.

• The reduction needs whole numbers. A greatest common divisor is defined for integers, so 8.5 × 11 letter paper stays 8.5:11 in the headline. The decimal is still right — the panel reads “Ratio 8.5:11 (≈ 0.773:1)” — and entering 17 and 22 instead returns “Ratio 17:22 (≈ 0.773:1)” with an identical scaled height of 7.76 at a target width of 6.
• Lowest terms are not always a useful shorthand. A 3456 × 2234 laptop panel reduces to 1728:1117, which tells you nothing; its decimal, 1.547, tells you plenty. When the reduced terms come out in the hundreds or thousands, read the decimal and ignore the colon.
• The height is rounded to two decimal places, and an extreme shape can round away entirely. Enter 100000, 1 and a target width of 50 and the panel prints “100000 × 1 scaled to width 50 → height 0 (50 × 0)”, because the true answer, 0.0005, does not survive that precision.
• Pixels are assumed square. The calculator compares two numbers and cannot know whether the grid they describe is displayed with square pixels. Formats that store non-square pixels have a stored shape and a displayed shape that differ, and only the second one is what a viewer sees.
• It scales one pair of dimensions and stops there. It does not convert to a print size at a given DPI, estimate a file size, or judge whether an upscale will hold together — a 320 × 180 thumbnail taken to 1920 wide is arithmetically 1080 tall and visually a mess.
• It does not know what any platform accepts today. Upload specifications change; the arithmetic does not. Take the required shape from the platform's own documentation, then bring it here to turn it into pixel dimensions.

Screens and televisions are advertised by their diagonal, which follows from the width and height you have just worked out — square both, add, take the root, or use the Pythagorean Theorem Calculator.