Firstly, the requirements to wield the weapon is different to scaling. Requirements are clearly defined (usually in STR/DEX) as digits, eg: S - 25, D - 15.
Secondly, scaling works from S through to E, S being the best, E being the worst. The scale goes S, A, B, C, D, E.
Finally, (this is going from the DkS2 values, though I don't see why the DkS3 values should differ too greatly), the bonuses granted from scaling go like this:
- S 140%–200%
- A 100%–139%
- B 75%– 99%
- C 50%– 74%
- D 25%– 49%
- E 1%– 24%
So, if you have a weapon that has an S scaling, you will be granted between a 140 - 200% bonus for that stat.
Again, if you have a weapon that has a C/C scaling in STR/DEX, it doesn't matter which stat you level up, it will still only grant you a 50% - 74% upgrade, regardless of which stat you level up.
EG:
+15 Club, 24 Strength, 2 hands.
◾ BasePhysical: 182
◾ StrengthScaling: 132% (A)
◾ StrengthRating: 76% (effectively 36 Strength because two-handing)
BonusStrength = BasePhysical × StrengthScaling × StrengthRating BonusStrength = 183 × 132% × 76% ≈ 183
PhysicalRating = BasePhysical + BonusStrength + BonusDexterity + BonusChaosPhysical PhysicalRating = 182 + 183 + 0 + 0 = 365
TotalAttackRating = PhysicalRating + MagicRating + FireRating + LightningRating TotalAttackRating = 365 + 0 + 0 + 0 = 365
Source
Best Answer
Two-handing a weapon in DS3 works a bit differently than in previous games.
The most important change is the addition of Weapon Arts, which, depending on the weapon, can do a great deal of damage, counter, or give versatility to your moveset. In any of these cases, your offensive potential is augmented.
Two-handing a weapon also can allow you to properly equip a weapon you do not meet the strength requirement for, as two-handing will add 50% of your strength towards meeting the requirement.
Lastly, for damage output, there is an increase, but it varies from weapon to weapon.
See this Reddit thread: