http://havenandhearth.wikia.com/wiki/Category:WeaponsThe defense consumed by an attack is (sqrt(AW / BW) / 2) * AttackBarFullness, where AW is your attack's weight (based on the attack being used and its relevant skill value), and BW is the enemy's defensive weight (based on the maneuver they are using and the relevant skill value). Your attack weight is multiplied by from 0.5 to 2.0, depending on the combat advantage. See Combat: Attacks, Moves & Maneuvers for the weights of various moves.
So, as an example, let's say you have 70 UC and your enemy has 34 UC. You both have 100% full attack/defense bars, and you throw a Punch at them. Punch's attack weight is just based on your UC skill (as opposed to a sword/axe attack which would be based on MC). The amount of defense bar you will remove is equal to (sqrt(70 / 34) / 2) * 1 = 0.71, which is 71% of their defense bar.