An aspect ratio describes an image's shape independently of its pixel count. Understanding it helps you predict whether a file will fit a layout or require cropping.
Divide width by height
For an image 1600 pixels wide and 900 pixels tall, dividing both numbers by 100 gives 16:9. An 800 by 450 image has the same ratio even though it contains fewer pixels. The shape is shared; the amount of image information is not.
Compare source and destination
If a square image goes into a wide frame, you must either crop it, add surrounding space, or distort it. Distortion stretches recognizable objects and is usually easy to notice. Choose the treatment deliberately.
Keep a reversible copy
Save the original file before making a crop. Check the destination's current requirements when preparing an actual upload, since different surfaces can use different treatments. A ratio calculation helps with composition, but it does not replace checking how the image appears in the interface where viewers will encounter it.