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For the perk, see Rad Resistance. 
Radiation Resistance is a derived statistic in Fallout, Fallout 2, Fallout 3, Fallout: New Vegas, Fallout 4, Fallout 76 and Fallout Tactics.
It reduces radiation damage and can be temporarily increased by using RadX.
Contents
Fallout, Fallout 2, Fallout: New Vegas, Fallout Tactics
Temporarily increasing Radiation Resistance
 Any temporary increase to Endurance.
 The space suit increases Radiation Resistance by 40% while worn, as will various other items of clothing and armor.
 RadX temporarily increases Radiation Resistance:
 In Fallout, Fallout 2, and Fallout Tactics, RadX 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: New Vegas, RadX increases Radiation Resistance by for 4 minutes per dose taken (the durations add, not the magnitude), up to 75% at 100 Medicine (enough to reach the cap of 85% with only 5 Endurance).
Permanently increasing Radiation Resistance
 Any permanent increase to Endurance.
 The Rad Resistance perk increases Radiation Resistance by 15% per rank. (25% in New Vegas, where it only has 1 rank)
 In Fallout 2, the Vault City Inoculations perk increases Radiation Resistance by 10%.
Notes
 Average characters will have a 10% Radiation Resistance.
 The Fast Metabolism trait sets initial Radiation Resistance to 0.
 In Fallout, Radiation 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
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 RadX drug gives up to +75% Radiation Resistance with the Medicine skill at 100.
 The advanced radiation suit, the environment suit, Lesko's lab coat, Filtration helmet, radiation suit and the modified utility jumpsuit will all increase Radiation Resistance while worn.
 All power armor (other than tribal power armor) has some Radiation Resistance, some more than others.
Notes
 In Fallout 3, Radiation Resistance is capped at 85%.
Fallout 4
Radiation Resistance works similar to the other games, but is more crucial because radiation damage itself is much more dangerous than previous titles, lowering a player character's maximum health by 1 percent for every 10 Rads absorbed. Resistance is not gained from Endurance any longer, but now appears on much more equipment, such as vault jumpsuits and gas masks, as well as from perks.
Resistance effects are still present on radiation suits, now called a hazmat suit, as well as all forms of power armor, which can be modified to offer even more protection; wearing either of these renders the wearer largely resistant to background radiation, but never completely immune.
One dose of RadX is no longer an incredibly potent, easytouse form of protection, as it only offers 100 Rad Resistance. However, multiple doses can be taken with each one stacking for the duration.
Rad Resistance does not work in straight percentage terms as in earlier games. Instead it is a variable function that works identically to Fallout 4's Damage Resistance and Energy Resistance functions.
Power armor can provide over 1,000 Rad Resistance; this helps significantly but still does not make one completely immune to radiation. The basic principle is that if the rads/second being received is exactly equal to a character's Rad Resistance, they will take half damage. If one goes swimming (10 rads/s in water) with a Rad Resistance of 10, they will only take 5 rads/s. Higher or lower resistance relative to the radiation level modifies the percent of damage one takes, but the increasing and decreasing effects are not linear: even 5 Rad Resistance will significantly mitigate lowlevel radiation, but rads will still seep through the toughest of power armor.
The Hazmat suit has an additional 98% reduction in rads, beyond its Rad Resistance, making it far more effective even than heavily customised radresistant power armor, yet even the Hazmat suit cannot fully block all rads in the worst environments such as the Glowing Sea.
An example of radiation damage that a character can experience while in a river near Sanctuary (v. 1.10.120.0.1):
Radiation Resistance  0  1  2  3  4  5  6  7  8  9  10  12  15  20  30  50  100  200  300  400  500  750  1000 

Damage taken  10  9  9  7  6  6  6  5  5  5  5  4  4  3  3  2  2  1  1  1  1  1  <1 
Another example of radiation damage that a character can experience while in a middle of Crater of Atom (v. 1.10.120.0.1):
Radiation Resistance  0  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  34  49  75  124  227  499  1515 

Damage taken  25  25  22  19  17  16  15  14  13  12  12  12  12  11  11  10  10  10  10  9  9  9  9  9  9  8  8  8  8  8  8  7  6  5  4  3  2  1 
Because of this, exposure to radiation will be relatively severe if one is unprotected, but most sources of radiation will become less hazardous as one acquires Radiation Resistance.
Permanently increasing Radiation Resistance
 Rad Resistant perk grants +10 Radiation Resistance per level, up to a maximum of +30 (or +40 with the extra level added in Far Harbor). These values are unchanged in Survival mode.
 Astoundingly Awesome Tales #10: Grants +5 Radiation Resistance.
Temporarily increasing Radiation Resistance
 Baked bloatfly: +20 Radiation Resistance for 10 minutes.
 Glowing blood pack: +75 Radiation Resistance for 10 minutes.
 RadX: +100 Radiation Resistance for ? minutes.
 Vegetable soup: +25 Radiation Resistance for 1 hour.
Apparel
 Vault jumpsuit: +10 Radiation Resistance
 Various gas masks and similar items (sack hoods, etc): +2 to +15 Radiation Resistance
 Hazmat suit: +1,000 Radiation Resistance.
 A full set of any Power armor: +1,050 Radiation Resistance. Can be further increased with the Lead plating mod.
 Robes of Atom's Devoted: A unique variant of the Children of Atom gear. Radiation Resistance maxes out at over 2500 making the wearer completely immune to environmental radiation, but with the unmarked effect of when total Rads exceeds 900, health returns to 100% (albeit it cannot be seen behind the Rad bar) and any food or drink based rads will tick but not affect the wearer as long as rads stay above 900.
 Morgan's space suit: +1,000 Radiation Resistance. The suit can also be modified with ballistic weave to provide Damage Resistance alongside its Radiation Resistance.
Normal armor pieces can be found with, or modded to have, the Leaded mod, which adds to Rad Resistance whenever that armor piece is equipped (worn).
Fallout 76
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Temporarily increasing Radiation Resistance
 Rad Resistant perk card gives the player character +10 Radiation Resistance at Rank 1, with up to +40 Radiation Resistance at Rank 4.
