Windows Display Settings dialog only lists the actual resolution. Ideally, they should also mention the aspect ratios as well. Never mind. Here are Surface Pro Aspect Ratios for your reference. Most of us nowadays need 16:9 resolution.
Download Excel file containing ratios for all available resolutions.
The conversion formula is also included in the file.
Some weird looking aspect ratios are for backward compatibility.
For more details read this Wikipedia article.
