How aspect ratio works
Aspect ratio is the proportional relationship between an image's width and height, written as two numbers like 16:9. It stays the same no matter how large or small you scale the image, which is why it is the key to resizing without stretching or squashing.
The calculator simplifies any width and height to its lowest terms using the greatest common divisor, so 1920×1080 becomes 16:9 and 1280×1024 becomes 5:4. Enter a new width or a new height and it solves the missing side while keeping the exact proportions.
Common aspect ratios
16:9 is today's video and monitor standard. 4:3 is classic television and many tablets. 3:2 comes from 35mm photography and is common on cameras and some laptops. 21:9 is ultrawide, 1:1 is square (social posts), and 9:16 is vertical for phone video. Matching the right ratio avoids black bars and unwanted cropping.