FFmpeg command to scaling video proportionately to input

In FFmpeg without knowing the size of the input frame, its resolution can be changed proportionately using the ih and iw parameters of the scale filter

for example to create a half sized video, we can use the next command

ffmpeg -i input.mpg -vf scale=iw/2:ih/2 output.mp4

Command for 90% sized video

ffmpeg -i input.mpg -vf scale=iw0.9:ih0.9 output.mp4