The amount of radiation you are exposed to is reduced by this percentage. Radiation Resistance can be modified by the type of armor worn, and anti-radiation chems.”— Fallout In-game description
It reduces radiation damage by a percentage.
Rad-X temporarily increases Radiation Resistance.
Fallout, Fallout 2, Fallout Tactics, Fallout: New Vegas
- 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%.
Example: A starting Endurance of 5.
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.
- 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%.