how thick will a 1/2 inch wide rebar become if flatened to 2/16 inch thick?

I was planing on making a sword, considering I allready have a fire pit, charcoals, and a rebar. I need to know this because I am choosing to make a buster sword style guard, feel free to post any tips on making swords or other weapons ( especially bladed ones)

Assuming you start with a circular cross-section with a diameter of 1/2 inch, and then flatten this into a rectangle, and assuming this cross-section retains the same area... you get a width of pi/2, or approximately 3/2 = 1+1/2 inches.

That's simple math.  The actual result you'll get is approximately this, i.e. something kinda close. That's what the word approximation means. ;-)

See picture.