What is aspect ratio?

Aspect ratio is the proportional relationship between an image's or video's width and height, written as two numbers separated by a colon, like 16:9 or 1:1. It describes shape, not size: a 1920x1080 video and a 640x360 thumbnail can both be 16:9, because what matters is the ratio between the two dimensions, not their absolute pixel count. A square photo is 1:1, meaning width equals height. A widescreen video is 16:9, meaning the width is 16 units for every 9 units of height. A phone screenshot is often 9:16, taller than it is wide. Aspect ratio matters in practice because platforms, printers, and video players expect a specific shape, and a file with the wrong one either gets cropped to fit or padded with empty bars (letterboxing) to preserve the whole frame.

The two numbers in an aspect ratio are a simplified ratio of width to height, the same way 1:2 and 2:4 describe the same proportion. To find a file's aspect ratio, divide its pixel width by its height and match the result to a known ratio: 1920 divided by 1080 is 1.78, which is 16:9. 1080 divided by 1080 is 1.0, which is 1:1. 1080 divided by 1350 is 0.8, which is 4:5. Reversing the division flips the ratio to 9:16 instead of 16:9, which is exactly what happens when the same content rotates from landscape to portrait. You don't need to do this math by hand for every file: pixeltools' resize tool shows live pixel dimensions and locks its presets to the correct ratio automatically.

16:9 and 9:16 are the same ratio rotated 90 degrees. 16:9 is landscape, wider than tall, and is the standard for TVs, YouTube videos, and desktop monitors. 9:16 is portrait, taller than wide, and is the standard for phone-first vertical video: Instagram Reels, TikTok, Snapchat, and Stories across nearly every platform. A 1920x1080 video is 16:9; rotate it and it becomes 1080x1920, which is 9:16. The two aren't interchangeable just because they share the same digits: a video shot in 16:9 and dropped into a 9:16 slot doesn't rotate itself, it gets cropped down to a narrow vertical strip of the original frame, or letterboxed with bars on the sides. That mismatch, shooting landscape when you need vertical or the reverse, is one of the most common aspect-ratio mistakes.

pixeltools' resize tool hardcodes exact pixel dimensions for over 20 social image slots (src/lib/resize-presets.ts), and the real numbers show a genuine pattern: landscape post and link-preview formats converge tightly around 1.9:1 even though they come from unrelated platforms. LinkedIn's post preset is 1200x627, which is 1.91:1. Facebook's post preset is 1200x630, which is 1.90:1, identical to the generic Open Graph preset that every link-preview card on the web uses (also 1200x630). Reddit's post image is 1200x628, also 1.91:1. Twitter and Bluesky land nearby but not identically, at 1200x675, an exact 16:9 (1.78:1). None of these platforms coordinated with each other. They independently arrived at close to the same landscape shape because it's a comfortable width-to-height proportion for a thumbnail sitting above a headline and caption.

Every story and reel format in pixeltools' preset list is identically 1080x1920, whether it's labeled Instagram Story, Facebook Story, TikTok Video, Pinterest Idea Pin, Snapchat Story, or WhatsApp Status. That's not a loose convergence like the 1.9:1 post cluster above, it's the exact same dimensions across six unrelated platforms, because 1080x1920 is precisely 9:16: the vertical mirror of the 16:9 standard that phone cameras and TV screens already share. Vertical video became the default the moment phones became both the primary camera and the primary screen, so every platform building a full-screen mobile video feed converged on the same shape rather than each inventing its own. The practical upshot: a graphic or video built at 1080x1920 works as-is across nearly every story and reel format on the market, with no per-platform resizing needed.

Two things happen: cropping or letterboxing. A resize or upload step that forces your file into a slot's exact pixel dimensions will crop off whatever doesn't fit, usually from the edges, keeping the center. Drop a 1200x630 landscape post graphic into a 1080x1920 vertical story slot and you lose most of the image, since only a narrow vertical strip of the original 1.9:1 frame fits inside a 9:16 frame. The alternative, letterboxing, keeps the whole image but adds solid bars to fill the extra space, which reads as intentional on some platforms and lazy on others. Neither is usually what you want by default. The real fix is picking the crop deliberately: pixeltools' crop tool for images and crop video tool for footage both let you set an exact target ratio and reposition the frame before exporting, instead of letting a platform's automatic crop decide for you.

1:1, a square like 1080x1080, covers Instagram and Threads square posts and most profile pictures. 4:5, like 1080x1350, is Instagram and Threads' portrait post format, the tallest ratio their feed displays without cropping. 16:9, like 1920x1080 or 1200x675 at social-post size, is the standard for YouTube, widescreen video, HD wallpapers, and Twitter and Bluesky post images. Roughly 1.9:1, like 1200x630 or 1200x627, covers Facebook and LinkedIn post images, Reddit post images, and the generic Open Graph preview image that link cards use across the web. 9:16, like 1080x1920, is the universal story and reel format shared identically by Instagram, Facebook, TikTok, Snapchat, Pinterest, and WhatsApp. Outside social media, 4:3 is the older standard-definition video and print-photo ratio, and 21:9 shows up on ultra-wide monitors and cinematic video.