What an aspect ratio is
An aspect ratio is the shape of an image: its width compared with its height, written as width:height. Two images with the same ratio have the same shape at any size, so 1080×1350 and 720×900 are both 4:5. Social networks size their feed slots by ratio, which is why the same photo can look full in one feed and cropped in another.
Find the ratio of a size
Type a width and a height and the calculator reduces them to the smallest whole numbers with the same shape. It also gives the decimal ratio, which is handy for comparing shapes that do not reduce to tidy numbers, and tells you whether the image is landscape, portrait or square.
Find the missing side
When you resize an image, you usually know one side and the ratio. Tick Lock the ratio, then type the new width: the height follows. If the exact height is not a whole number of pixels, the calculator rounds it and shows the exact value, so you know how far off the result is.
Match it to a social media size
Below each result is a list of social media image sizes with the same ratio, taken from our image sizes page. When nothing matches exactly, it lists the nearest sizes instead, so you can see which placement will crop your image the least. To crop and resize the image itself, use the image resizer on the same page, then schedule the post with PostBoost.