An aspect ratio describes width relative to height. Reducing it removes a common factor without changing the proportion. The order matters: 16:9 is width to height, while 9:16 describes the opposite orientation.
Divide integer dimensions by their greatest common divisor
For 1920 × 1080, the greatest common divisor is 120. Dividing both terms by 120 gives 16 and 9, so the exact reduced ratio is 16:9. Both dimensions must be divided by the same number.
Convert finite decimals before reducing
For 1.5 × 1, move both decimal points one place to obtain 15:10, then divide by 5. The result is 3:2. Using a common decimal scale keeps the original proportion exact.
A ratio is not an output size
The ratio 16:9 describes a family of proportional dimensions such as 1280 × 720 and 1920 × 1080. It does not choose the pixel count, file format, or publishing requirement. Set one target side to obtain a specific pair.