Font Size question


Im using this font code on my site

<font size="2" face="Verdana, Arial, Helvetica, sans-serif">

I now have a lot of pages and I was thinking of switching to css. To make the work more easy.

I use this code

font-family: Verdana, Arial, Helvetica, sans-serif; 
font-size: 12px;

But if I compare the font size it doesn’t seems to be the same.
I think it’s because i use px in the second css code.
I’d like to know how much pixels is font size=2 ?