Radiation resistance

Radiation resistance is a derived statistic in Fallout, Fallout 2, Fallout 3, Fallout Tactics, Fallout: New Vegas, Fallout 4, and Fallout 76.

It reduces radiation damage by a percentage.

Rad-X temporarily increases Radiation Resistance.

Fallout, Fallout 2, Fallout Tactics, Fallout: New Vegas

 * $$\text{Initial level}=\text{Endurance}\times2$$

Notes:
 * Average characters will have a 10% radiation resistance.
 * The Fast Metabolism trait sets initial radiation resistance to 0.
 * The Rad Resistance perk increases radiation resistance by 15% per rank. (10% per rank in FO1)
 * The Vault City Inoculations perk increases radiation resistance by 10%.
 * Rad-X temporarily increases radiation resistance by 50%. Half of it is lost after 24 hours, with resistance returning to its original value after another 24 hours.
 * In Fallout, rad resistance can go as high as 100% (Which is needed to survive The Glow). In Fallout 2, it is capped at 95%, in Fallout: New Vegas, however, it is capped at 85%.

Fallout 3

 * $$\text{Initial level }%=(\text{Endurance}-1)\times2$$

Example: A starting Endurance of 5.
 * $$(5-1)\times2=8\%$$

Permanently increasing Radiation Resistance

 * The Cyborg perk gives +10% Radiation Resistance.
 * The Rad Resistance perk gives +25% Radiation resistance.
 * The Survival Expert perk gives between +2% and +6% Radiation resistance.
 * The Pitt Fighter perk from The Pitt gives +3% Radiation Resistance.
 * The Booster Shot perk from The Pitt gives +10% Radiation Resistance.

Temporarily increasing Radiation Resistance

 * The Rad-X drug gives up to +75% Radiation resistance with the Medicine skill at 100.
 * An advanced radiation suit, the environment suit, Lesko's lab coat, Filtration helmet, a radiation suit and the modified utility jumpsuit will all increase Radiation Resistance while worn.
 * All power armor has some Radiation Resistance.

Notes:
 * In Fallout 3 rad resistance is capped at 85%.

Fallout 4 and Fallout 76
Damage resistance in both of the Creation Engine games is essentially the same and uses three separate armor types for the three available damage types:


 * DamageResist, used to determine resistance against ballistic or normal weapons - most conventional firearms, melee, and unarmed weapons, as well as creature attacks,
 * EnergyResist, used against weapons dealing energy damage,
 * RadiationResist, used against - what else? - radiation-based weapons.

To determine the damage reduction, the game first checks the following:
 * If target has 0 Armor = 0% DamageReduction
 * If Armor < 15% AttackDamage = 1% Damage Reduction
 * If Armor > 550x Attack Damage = 95% Damage Reduction

Otherwise, it performs the following calculation:


 * DamageReduction = 1-0.5*(AttackDamage / DamageResist)^0.3653

At extreme values, Energy damage can be nullified, but Ballistic or Normal damage will only be reduced by a max of 95%.