Finds the matching side when an image or video is resized with its proportions kept. The original width and height are reduced to their simplest ratio, so you can see whether it is 16:9, 4:3 or something else.
This tool finds what the other side becomes when something is resized with its proportions kept: exporting video, scaling an image, laying out a screen.
Here and are the original width and height, the new width and the height sought. Keeping the proportions means scaling the height by exactly the factor applied to the width.
Dividing the width and the height by their greatest common divisor gives the familiar form, 16:9 or 4:3. The greatest common divisor of 1920 and 1080 is 120, and dividing through leaves 16 and 9.
Reducing a 1920 × 1080 video to a width of 1280 gives a height of 720. The ratio is 16:9 and the new frame holds 921600 pixels.
1280 × 1080 ÷ 1920 = 720. The width was reduced to two thirds, so the height follows to two thirds.
The height does not always come out whole. Reducing 1920 × 1080 to a width of 1000 gives 562.5, which has to be rounded to 562 or 563, bending the ratio very slightly.
Video encoders generally require both dimensions to be even, and some formats want multiples of 4 or 16. Take that into account when rounding.