Aspect Ratio Correct Formula

Hey Guys, I'm having a hard time creating / understanding the correct formula on witch the aspect ratio works based on the give Pixels width and Pixels height

Searching on google 2 site with the same formula will give 2 answers :
Example : the old and well known resolutions : 1024 x 768 => 4 : 3 (standard monitor) and 1280 by 720 => 16 : 9 (hd video)

Upon recreating the formula : https://www.easycalculation.com/area/aspectratio.php will give incorrect answers like 1.33:1 (256:192) and 1.78:1 (320:180). The correct way would be here https://calculateaspectratio.com

as for the number of pixels i know that width *height is the correct formula (i hope.)