An aspect ratio is the width of an image divided by its height. It says nothing about the pixel count: 1200 x 675 and 3840 x 2160 are both 16:9. To reduce a size to a ratio, divide both numbers by their greatest common divisor. For 1920 x 1080 the divisor is 120, which gives 16:9. 1200 x 630 reduces to 40:21, which is usually written as its decimal, 1.91:1.
A platform crops an image whose ratio it does not support. Instagram does this to photos outside its range. Crop to the target ratio yourself and you decide what is cut.
Resizing without distortion
To change the width and keep the shape, scale the height by the same factor: new height = new width × old height ÷ old width. A 4000 x 3000 photo resized to 1080 px wide becomes 1080 x 810. The calculator above does this for any target width.
Cropping to a new ratio
Resizing never changes the ratio, so a 4:3 photo cannot become a 3:1 X header by resizing alone. You have to crop. The largest 3:1 area inside a 4000 x 3000 photo is 4000 x 1333 px. The calculator shows this crop size when your image does not match the preset.
Practical rules
- Upload at or above the stated size. Instagram scales photos narrower than 320 px up to 320 px wide, and an enlarged image looks soft.
- Keep text in the middle. Different screens show different parts of a YouTube banner, so put text and logos inside the 1235 x 338 safe area.
