1. Measure the entire length of the concrete slab. This is your slab length, or "SL. As an example, assume your SL is 50 feet. 2. Calculate rebar length for the slab  "
http://www.ehow.com/how_8110962_calculaterebarle...

You have to find a chart that will indicate the weight per unit of measure for the appropriate rebar size. Than take that and multiply it by the the length of the rebar. For example
http://wiki.answers.com/Q/What_is_the_formula_to_c...

Sometimes you just don't have a calculator on you but you do have the Internet in front of you. There are search engines that will allow you to type in an equation and it will solve
http://answers.ask.com/Reference/Other/where_can_i...

Mohammed: Go the the link below, look inside the document to find a table of rebar weights by size. Source(s) http://www.globalsecurity.org/military/l….
http://answers.yahoo.com/question/index?qid=200905...
