What is aspect ratio?

Reading the notation

The first number is always the width and the second the height, so 16:9 means sixteen units across for every nine units down, and 9:16 is the same shape stood on its end. Ratios are written in their smallest whole numbers where possible: 1920 by 1080 divides by 120 to give 16:9, and 1080 by 1350 divides by 270 to give 4:5. Some shapes do not reduce neatly, which is why the wide post shape is usually written as 1.91:1 instead of 191:100. On the web the CSS aspect-ratio property uses the same idea, taking a width over a height and keeping that proportion as the box grows or shrinks.

Working one out

Divide the width by the height and you have the ratio as a single number. A 1500 by 500 header gives 3, a 1080 by 1920 story gives 0.5625. To find a missing dimension, multiply or divide by that number: a 1.91:1 image that must be 1200 pixels wide needs to be 1200 divided by 1.91, or about 628 pixels tall, which is exactly the figure LinkedIn's ads guide recommends. Comparing two shapes is the same sum. If the numbers differ by more than a few percent, one will have to be cropped, padded or distorted to fit the other, and the bigger the gap, the more is lost.

Paper has a ratio too

ISO A paper sizes all share one ratio, 1 to the square root of 2, about 1:1.414. Fold an A3 sheet in half, cut along the fold, and you get two A4 sheets of exactly the same shape, which is the whole point of the system: A0 at 841 by 1189 millimetres halves down through A1, A2 and A3 to A4 at 210 by 297 and A5 at 148 by 210. A layout therefore enlarges from A4 to A2 without cropping. North American Letter at 8.5 by 11 inches is about 1:1.294, a squatter shape, which is why an A4 design printed on Letter paper either loses a strip or gains a margin.

When the ratio matters more than the pixels

Pixels can be added or removed by scaling without changing what the picture shows. A ratio change cannot happen without removing something or adding empty space. That is why platforms specify shapes first: LinkedIn's single image ads, for example, accept 1.91:1, 1:1 and a mobile only 1:1.91, plus 2:3 and 4:5, and anything else is fitted into one of those frames. It is also why a photograph that looks well composed on a camera, usually 3:2 or 4:3, can lose a head or a product when placed into a square or a thin banner. Choose the final shape before choosing the photograph.

Common mistakes

Stretching an image to fill a box of a different shape, which distorts faces, logos and circles in a way viewers notice instantly. Assuming 16:9 is standard everywhere, when most feeds favour square and portrait. Confusing a ratio with a size, and scaling a small image up because its shape is right. Cropping a group photo to a narrow banner and cutting people off at the edges. And treating 1080 by 1350 and 1080 by 1080 as interchangeable, when the extra 270 pixels of height is exactly where a portrait post gets its advantage.

Where ratios show up in the flyer builder

Every canvas in the flyer builder has a fixed ratio, listed in the table, and switching a page between them swaps in the design's own layout for the new shape rather than stretching the old one. Uploaded images fill their slot with a cover fit: the image is scaled until the slot is full and the overflow is cropped from the centre, never letterboxed, and there is no control to move the crop. AI generated images avoid that loss by being made at whichever of eight ratios is closest to the slot's own width and height, and no text is ever rendered into them. When the generator picks designs for a size, it matches them by aspect ratio within 12 percent.

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