An aspect ratio is the proportional relationship between width and height, written as W:H (for example 16:9). It stays the same when you scale an image up or down.
What is the 16:9 aspect ratio?
16:9 is the standard widescreen ratio used by most TVs, monitors, and YouTube videos. 1920×1080 and 3840×2160 are both 16:9.
How do I calculate an aspect ratio?
Divide the width and height by their greatest common divisor. For 1920×1080 the GCD is 120, which gives 16:9.