Whether you are working in HD or SD, knowing the right aspect ratios to convert your video for web distribution is key. By choosing the right ratios, you can insure that you will get best picture possible.
Sometimes, you are restricted to the size of a web page window, but you still can choose a better ratio than simply picking one out of the air.
Below is a chart of the best ratios to use.
(Chart compiled by Robert Reinhardt)
You can also download the Best Aspect Ratio Chart (Excel).