Damage Threshold

Damage Threshold is a combat statistic in Fallout, Fallout 2, Fallout Tactics: Brotherhood of Steel, and Fallout: New Vegas. It was removed in Fallout 3, leaving Damage Resistance as the only armor statistic. However, it was restored in Fallout: New Vegas, before being removed yet again in Fallout 4.

Fallout, Fallout 2, Fallout Tactics
In the original Fallout games, DT (Damage Threshold) is one of three stats by which a character can reduce or avoid damage. The other stats are AC (Armor Class) and DR (Damage Resistance). DT occupies the "middle step" for combat and simulates the effect of how tough armor can immediately stop a bullet cold, no matter how many successful shots are fired (like a real-life tank).

More specifically, after AC is checked for a successful hit and before DR is checked to further reduce the damage, any incoming damage is immediately reduced by DT:
 * $$Adjusted = \text{max}(0,\ Base - DT)$$

Unlike in Fallout: New Vegas, damage can be completely negated by DT. In such a case, the player gets a message that an attack dealt "no damage," which can be frustratingly common when using weaker weapons against power armor or Wanamingos.

Fallout: New Vegas
Fallout: New Vegas mostly moves away from the exclusive use of Damage Resistance (DR) in Fallout 3, towards Damage Threshold (DT). Like in previous games, DT can function as an outright subtraction: if a shot is fired with a damage value of 40 against a DT of 10, then 10 damage will be ignored.

However, the sequence of calculations differs compared to previous games and makes the math a little bit more involved than in Fallouts 1 and 2. Thus, no discussion of damage is complete without referring to the combined statistics of DR and, after it, DT. This is because, contrary to previous games, DR is applied first. The value after this adjustment depends on any DR a player has, which is capped to a DR of 85 (85%).
 * $$Damage_{DR}=Damage \times \frac{100-\text{min}(DR, 85)}{100}$$

DT applies after that, with a limit: the damage taken cannot be lower than 20% of the value after DR (in other words, it cannot reduce an attack 0.2x of the adjusted damage before DT was applied).


 * $$Damage_{Final}=\text{max}(Damage_{DR} - DT, Damage_{DR} \times .2)$$

So, to evaluate our earlier example, if a shot of damage 40 is fired against a character with 95 DR as well as the previous 10 DT, the damage is first reduced to 6 (95 DR is capped down to 85% reduction), and then reduced by 10. The final result (-4) is less than 20% (1.2) of the pre-adjusted original (6), so the final damage is instead 1.2.

Damage which is reduced to below the 20% minimum value is indicated with a shield icon next to the health bar (either the PC, or the non-player character's). When all of an enemy's DT (if it's not 0 to begin with) is overcome with armor piercing attacks, then a broken shield icon will appear in its place instead. The shield icon is shown during V.A.T.S. and on the HUD's health bars once damage has been inflicted upon a target and will remain for 3 seconds afterwards.

The default value for DT is set at 0; this is then modified mainly by the DT value of armor and clothing worn. Other attributes can affect the DT value of a character, both permanently or temporarily. perks and consumables can increase the player's DT, while the Kamikaze trait reduces it. When attacking, the use of ammunition can modify the target's DT either way, with Hollow Point generally increasing their DT, and AP decreasing it.

Bugs

 * Despite DT being the determining factor for armor in Fallout: New Vegas, when repairing armor, merchants will list DR instead. All armors when viewed from the repair screen have a DR of 0. This is fixed in patch 1.1.1.
 * In the initial release of Fallout: New Vegas, all creatures such as super mutants, robots, or deathclaws had twice as much DT as intended (in other words, twice the amount of DT as was entered in their G.E.C.K. page). This was corrected in the 1.2 patch. A similar bug affects the DR of creatures in Fallout 3, which has not been corrected.
 * In Fallout: New Vegas, the only articles of clothing that provide DR are the rebreather, which is acquired during the quest Volare!, the Vault 11 jumpsuit and the Scientist outfit.
 * While wearing one of the said items above or after having taken Med-X, Slasher, or battle brew, your Pip-Boy 3000 will crossfade between showing your DR and DT. Taking the DR items off or having your DR chems expire while your Pip-Boy has DT partially faded out or completely replaced with DR will keep your DT partially faded out or render it completely invisible. Removing the DR item or putting away your Pip-Boy only when the armor display is fully on DT is highly recommended.