What is an aspect ratio?
An aspect ratio expresses the proportional relationship between width and height. Written as W:H (width to height), it defines the shape of a rectangle without specifying its actual size. A 16:9 rectangle is twice as wide as it is tall (roughly). A 1:1 ratio is a perfect square. The same ratio can describe a tiny thumbnail (320×180) and a massive cinema screen (3840×2160) — both are 16:9.
Aspect ratios appear everywhere in digital and physical media: video resolutions, photography sensor formats, screen dimensions, print paper sizes, thumbnail dimensions for social media platforms, and CSS percentage padding hacks for responsive containers.
Finding dimensions from a ratio
Use the "Find dimensions" tab when you know the ratio and one dimension. Enter the ratio (16 and 9), then enter the known width or height. The other dimension calculates instantly. Enter width 1920 with a 16:9 ratio and you get height 1080. This is the most common use case — you are designing a video thumbnail, banner, or card and need the correct height for a specific width.
Finding a ratio from dimensions
Use the "Find ratio" tab when you have specific pixel dimensions and need to know the ratio. Enter width 1170 and height 2532 (iPhone 14 resolution) and you see the ratio is approximately 9:19.5. The tool uses GCD (greatest common divisor) to reduce the ratio to its simplest form — 1920×1080 reduces to 16:9, 2048×1536 reduces to 4:3.
Common aspect ratios and their uses
16:9 is the universal widescreen ratio for video — YouTube, Vimeo, HD television, most desktop monitors, and presentation slides all use 16:9. 4:3 was the standard television and monitor ratio before widescreen and is still used in some presentation contexts. 1:1 (square) dominates social media — Instagram photos, profile pictures, and app icons. 9:16 (vertical) is the portrait mobile format used by Instagram Stories, TikTok, and YouTube Shorts. 21:9 is the ultra-widescreen ratio used by cinematic monitors and some gaming setups. 3:2 is the native ratio of most DSLR camera sensors. 4:5 is the portrait format used by Instagram feed posts (taller than square but not as tall as 9:16).
The common sizes table
For any ratio you enter, the tool shows a table of common pixel dimensions at that ratio — from 320px wide (mobile) to 7680px wide (8K UHD). Each row includes a label (1080p HD, 4K UHD, etc.) when the dimensions match a well-known resolution, plus the megapixel count. This is useful for understanding the full range of sizes you might need to export assets for, and for choosing the right source image size.
Responsive design applications
In CSS, maintaining an aspect ratio for responsive containers has historically required the "padding-bottom trick": a container div with padding-bottom: 56.25% (9/16 × 100%) maintains a 16:9 ratio regardless of width. Modern CSS provides aspect-ratio: 16/9 directly. The calculator gives you the exact percentage for the padding-bottom trick: (height/width) × 100. For 16:9: (9/16) × 100 = 56.25%.