What an aspect ratio is
The aspect ratio is the proportion between the width and the height of a picture, screen or video, written as two numbers such as 16:9. A 1920 × 1080 screen has the ratio 16:9 because dividing both sizes by their greatest common divisor (120) leaves 16 and 9. Two sizes with the same ratio have the same shape, so one can be scaled to the other without stretching.
How to use it
What is the ratio? Enter a width and a height to see the simplified ratio, the decimal value and the nearest common ratio, with a small preview of the shape. Resize to a ratio takes a ratio (type it or choose one of the buttons) and either a new width or a new height, and works out the other side, rounded to a whole number. Use it to size a video thumbnail, crop a photo for a profile picture, or fit a design to a screen.
Questions
Why is my result a pixel off? Sizes must be whole pixels, so the other side is rounded. Which ratio should I pick? 16:9 for most videos and screens, 1:1 for square posts, 9:16 for vertical video and stories, 4:3 or 3:2 for photos. To actually resize a picture, use the Image Resizer.