Nobody likes media queries in pixels. You wont press your well crafted content in an inflexible corset of absolute units. That’s why you will tend to use em. But why calculate the numbers by yourself?
Include the em meter JavaScript file while your development or use the bookmarklet. You will see a box at the bottom left corner of your viewport which shows you the current width and height in em.
The em ruler is more a fun tool but maybe useful for you. It will show a ruler with a stepping of 5 at the top of the viewport.