I just want to know the how many "px" occupy the single " ". so that i can calculate and give the padding instead of
It's not possible to know this accurately, because it will depend on the font's metrics and the way it's rendered. A non-breaking space is usually rendered with the same width as a regular space in the same font, it just suggests to the browser not to wrap at that point or collapse the space.
You should never rely on fonts rendering a particular way in order to line up design elements on the page. Specify distances in units that are appropriate, and don't use non-breaking spaces in situations for which they aren't suitable.
You could start with a value of, say, around
0.4em. But if you absolutely have to exactly match the width of a non-breaking space, you are using a non-breaking space incorrectly.