No. This is the calculator you were using to convert pixel widths to ems. Because the size of an em is based on the size of the base font, you have to set a base (or default) font size first, in order to use the calculator.
If you set your font size small, say 10px, an em will be smaller than if you set it to 16px. If you set your font size to 20px, an em will be larger. This is how using ems helps your design scale along with increases or decreases in font size. An em isn't a fixed size like an inch or a cm - or even a pixel. It's a flexible unit, based on font size.