Common ratios
16:9 is standard for video and most monitors (1920 × 1080), 4:3 is older screens and many photos, 1:1 is square, and 3:2 is typical for camera sensors. Divide width by height and simplify to find it: 1920:1080 reduces to 16:9.
Resizing
To keep the shape, change both sides by the same factor. Shrinking 1920 × 1080 to 1280 wide means the height must be 720. The Image Resizer has a keep-aspect option that does this for you, and the Ratio Calculator simplifies a ratio or solves a proportion.
Common misunderstanding
Changing only the width distorts the picture. If you need a different shape, crop instead of stretching. See how to resize an image.