Defense | Dark Souls 3 Wiki

Defense in Dark Souls 3 is the numbers listed under the "Defense" section on the right side of the second page of the equipment menu (you can swap between pages with Y on controller and F on keyboard) and is not to be confused with Absorption, the numbers listed to its direct right.  Contrary to popular belief it is not a flat reduction and is calculated with these formulas.

Defense Statistics

- If DEF >8x ATK, deal damage equal to 0.10 * ATK
- If DEF >ATK, deal damage equal to (19.2/49 * (ATK/DEF-0.125)^ 2 +0.1) * ATK
- If DEF >0.4x ATK, deal damage equal to (-0.4/3 * (ATK/DEF-2.5)^ 2 +0.7) * ATK
- If DEF >0.125x ATK, deal damage equal to (-0.8/121 * (ATK/DEF-8)^ 2 +0.9) * ATK
- If DEF <0.125x ATK, deal damage equal to 0.90 * ATK

The maximum damage a player can deal through defense and before absorption is 90% of their attack, conversely the minimum a player can deal is 10% of their attack.

Because defense is in brackets changing damage by even by a small amount can cause a large change in damage. This can cause attacks to do more damage than  expected. For example the Ultra Greatsword weapon class does 175% damage on a fully charged heavy attack, thats 75% more than a light attack, so the player would expect to deal 75% more damage to the enemy than they did with the light attack, however, often times the player will deal more than 75% because they will move into a different defense bracket. The phenomena also occurs with small amounts of damage.