The ratio of the width to height of the picture (the horizontal size of the picture divided by its vertical size). Aspect ratios have a colon in the middle and start with 1 (for example, 1:1.5). To calculate the aspect ratio, divide the longest dimension of your picture by the shortest dimension. The result is the second component of the ratio. For example, 6 divided by 4 = 1.5 (aspect ratio is 1:1.5).