Common aspect ratios explained
Short answer
An aspect ratio expresses width relative to height as two numbers, such as 16:9. It describes shape, not size, so any image at 16:9 has the same proportions whether it is 1280x720 or 1920x1080.
What an aspect ratio means
An aspect ratio is width divided by height, reduced to its simplest whole numbers. A 1920x1080 image reduces to 16:9 because both numbers divide by 120. Two images with the same ratio have the same shape even if their pixel dimensions differ.
The ones you will meet most
16:9— widescreen video, YouTube, most monitors and TVs9:16— vertical video for phones, Reels, Shorts, and Stories1:1— square, common for profile pictures and feed posts4:3— older displays, many cameras, and presentation slides4:5— portrait photo posts that take more vertical space in a feed
When you resize for a specific platform, keep the target ratio so the image is not stretched. If the source ratio differs, you will need to crop or add padding.