IS there such thing ? Like lets say i can bench press 120 and i weight 100 and my friend can bench press 180 but weighs 150 ...is there any way to calculate who is stronger for their weight ?

Strength <> Weight Ratio

Proportions, I believe. Someone correct me if that's not the way to calculate it. Anyways, you both can bench press 120% of your weight. You want to get it into a percentage of how much of their body weight they can lift, so you basically set up two fractions, one with the weight one can lift over their body weight and the other as X over 100. Then you cross multiply, divide, and get X by itself. I'm time restricted, so here's a link: http://www.shodor.org/UNChem/math/r_p/index.html