Aspect Ratio Calculator

What is an aspect ratio?

An aspect ratio is the proportion between an image's width and its height, written as width to height like 16:9. It describes the shape of the frame, not its size, so a tiny thumbnail and a 4K video can share the exact same ratio.

Think of it as the recipe for a rectangle. The first number is how wide the frame runs for every unit of the second number in height. A 16:9 frame is 16 units across for every 9 down, which is why it looks like a long, low rectangle. Change the two numbers and you change the shape: 1:1 is a perfect square, 9:16 is a tall portrait, and 21:9 is the stretched-out cinema look. The pixels can scale up or down all day, but as long as that proportion holds, the shape stays identical. That's the whole reason a YouTube video fills a phone screen the same way it fills a TV. Both screens are 16:9, so the picture fits edge to edge with no black bars. Get the ratio wrong and you'll see those bars, or worse, a squashed or stretched image where everyone looks too thin or too wide.

How do you calculate an aspect ratio?

To calculate an aspect ratio, divide the width by the height to get a decimal, then simplify the two numbers by their greatest common divisor. The result is the ratio in its lowest terms, written W:H. The aspect ratio formula is simply width divided by height.

Here's a worked example you can follow by hand. Take a 1920 by 1080 image. First, divide 1920 by 1080 and you get 1.78, which is the decimal ratio. Now simplify the pixel counts: the greatest common divisor of 1920 and 1080 is 120. Divide both by 120 and you get 16 and 9, so the ratio is 16:9. That's the same answer the decimal hinted at, since 16 divided by 9 is also 1.78. The greatest common divisor is the largest number that divides evenly into both sides, and finding it is what turns an ugly pair like 1920:1080 into a clean 16:9.

You don't have to do this by hand, of course. The calculator's Find mode runs the greatest common divisor for you the moment you type both sides, so you'll see the simplified ratio and the decimal at once. If you're sizing for a screen and need the raw pixel count instead, our inches to pixels tool covers that side of the job.

Common aspect ratios chart

These seven ratios cover almost everything you'll meet in photo, video and screen work. The decimal column is width divided by height, so you can compare shapes at a glance: the bigger the decimal, the wider the frame. A 16:9 monitor sits at 1.78, while a square Instagram post sits at 1:1.

Reading the chart in plain words: 16:9 is the modern widescreen default that fills 1920x1080 and 4K screens alike. 4:3 is the squarer shape of old TVs and the iPad. 3:2 is what most DSLR sensors and 35mm film shoot, which is why a 3000x2000 photo crops oddly into a 16:9 video frame. 21:9 stretches wide for ultrawide monitors and cinema, 1:1 is the Instagram square, and 9:16 is the vertical shape behind phone video, Stories and Reels. At the far end, 2.39:1 is true cinemascope, the extra-wide ratio behind most theatrical films. You'll also see 1.91:1 thrown around for social link previews, which sits between 16:9 and 2:1.

How do you resize an image and keep the same aspect ratio?

To resize without distortion, lock the ratio and change only one side. New height equals new width divided by the ratio. So a 16:9 image scaled to 1280 wide needs a height of 720, because 1280 divided by 1.78 lands on 720. The other side always follows the formula.

This is where most resizing goes wrong. People grab a corner handle, drag it freely, and end up with a 16:9 photo squeezed into a 4:3 box. Everyone in the shot gets stretched, and the fix is annoying. The safe move is to pick your target ratio first, set one dimension to whatever you need, then let the math set the other. If you want a 16:9 banner at 1600 wide, the height has to be 900. If you flip to a 9:16 vertical at 1080 wide, the height jumps to 1920. The ratio is the constant, and one side drives the other. The calculator's Resize mode does this for you: pick a ratio chip, type a width, and it fills in the matching height instantly, so you never have to reach for a calculator or risk a stretched result. It works in reverse too, so type a height and it solves for width.

Need a specific 16:9 size worked out? The sibling 16:9 aspect ratio calculator is dialed in for that single ratio with a ready-made size table. And if you're going the other way, from a physical print size to pixels, the pixels to inches page handles that conversion.

How do you find the aspect ratio of an image?

To find an image's aspect ratio, you only need its pixel dimensions. Read the width and height from your file's properties, then feed them to the tool and it returns the simplified ratio. It takes three steps and a few seconds.

1. Find the pixel dimensions of your image. On Windows, right-click the file, choose Properties, then Details. On a Mac, select it and press Command plus I. You're looking for two numbers like 1920 x 1080.
2. Open the Find mode above and type the width into the first box and the height into the second. Decimals work, but pixel counts are usually whole numbers.
3. Read the result. The tool shows the simplified ratio like 16:9, the decimal like 1.78:1, and the common name so you know exactly what shape you're working with.

That's the fast aspect ratio finder workflow, and it beats squinting at a photo trying to guess. If the simplified ratio comes back as something odd, like 683:384, your image was probably cropped off-ratio at some point, and you'll want to recrop it to a clean ratio before exporting. For the background on why pixel density and ratio are two different things, the DPI guide lays it out, and the full theory lives in our what is aspect ratio guide.