These formulas are not 100% accurate, but they are (as far as I know) the closest that anyone has been able to pin it down:
Chance of critical hit = attacker's dexterity / (500 + attacker's dexterity) + 0.2 per point of critical strike
CritMultiplier = 200% + 1% per point of attacker's dexterity - 1% per point of defender's dexterity
Maximum CritMultiplier = 300% dmg, Minimum critmultiplier = 100% dmg
Normal attack damage = (0.95+ attacker's strength / 100) x base weapon damage - defense
Critical hit damage = (0.95 + attacker's strength / 100) x base weapon damage x CritMultiplier
* base weapon damage is weapon damage shown with 0 strength, average damage = 5.5 x weapon tier
But the dex/crit modifiers have not been officially released so it's all speculation
