Aspect Ratio Dimensions: 16:9, 4:3, 21:9

This page starts from the ratio: pick 16:9, 4:3, 21:9 or type your own, give it one pixel dimension, and it returns the other. If you already have a file and want to resize it without working out its ratio first, the resize an image and keep its aspect ratio page derives the ratio from the dimensions you have.

An aspect ratio is the proportional relationship between something’s width and its height, written as two numbers like 16:9. It describes the shape, not the size: a 16:9 frame is the same shape whether it’s a phone thumbnail or a cinema screen.

Common aspect ratios:

• 4:3, classic TVs and older monitors (1024×768, 1600×1200)
• 16:9, the widescreen standard (1920×1080, 3840×2160)
• 16:10, many laptop screens (1920×1200, 2560×1600)
• 21:9, ultrawide monitors (3440×1440, though see below)
• 1:1, square, used for Instagram posts

“21:9” is a marketing name, not a ratio, and it is the one thing on this page worth knowing before you trust a number. A 3440×1440 ultrawide is really 43:18, which works out at 2.389:1 against 21:9’s 2.333:1. Set the ratio to 21:9 and type 3440 and you get 1474 pixels tall, not the 1440 your monitor actually has. Use the second calculator below to read the true ratio off a resolution you already have.

Finding a missing dimension: Given a ratio W:H and one side, the other follows:

• Height = Width × (H / W)
• Width = Height × (W / H)

So a 16:9 image 1920 pixels wide is 1920 × (9/16) = 1080 pixels tall.

The point people miss is that aspect ratio and resolution are different things. 1920×1080 and 3840×2160 are both 16:9, even though one packs four times the pixels. Resolution is how much detail you get; aspect ratio is only the shape.

It’s also why fitting a 16:9 video onto a 4:3 screen forces a choice: add black bars (letterboxing), crop the sides off, or stretch everyone into funhouse proportions. You can’t change the shape of a frame without either losing part of the image or padding it with bars.