Weight is Shawn's secret formula that depends on: * PageRank (google toolbar) * Indexed pages (google api) * number of adverts shown your site You cannot calculate it , just add your site to the (famous) co-op system and you'll know what your weight is. Increasing either of the 3 aboved parameters will increase your weight.
It's like a 'currency'. You get weight and can spend weight. See the FAQ in my sig for an intro to the Co-op.