The very first backpack in the whole Doom universe is found in a secret area in E1M2: Nuclear Plant.

The backpack is a powerup that doubles the player's ammo-carrying capacity; after picking up a backpack, the player is able to carry up to 400 bullets, 100 shotgun shells, 100 rockets, and 600 energy cell units at once. When picked up, the backpack also gives the equivalent of the small ammo item for each type of ammunition: 10 bullets, 4 shotgun shells, a rocket, and 20 energy cell units (doubled on the "I'm Too Young To Die" and "Nightmare!" skill levels). It is a misconception that the 10 bullet ammo is an exception to the small-pickup rule, as players are more familiar with the 5-bullet clips dropped by extremely common zombiemen, but the clip item placed as fixed pickups in levels does indeed contain 10 bullets.

If the player picks up a subsequent backpack, only the extra ammunition is collected. A backpack can be picked up even when the player cannot carry any more ammo of any type.

The backpack's effect lasts until the end of the episode in Doom and until the end of the game in Doom II. If the player is killed, ammo capacity returns to normal upon respawn.

Using the cheat code idkfa or idfa to give the player all weapons and maximum ammunition does not give the player a backpack (although some source ports change this behavior). The backpack is the only powerup which does not have a corresponding idbehold cheat code.

Backpacks are not included in the items percentage displayed at the end of each level.

Backpack data
Thing type8 (decimal), 8 (hex)
Appears inShareware Doom
Registered Doom
The Ultimate Doom
Doom II
Final Doom

Appearance statistics

The IWADs contain the following numbers of backpacks per skill level:

The Ultimate Doom282828
Doom II: Hell on Earth484848
TNT: Evilution313232
The Plutonia Experiment333333

See also

Items from Doom and Doom II
Health Health bonusStimpackMedikit
Armor Armor bonusArmorMegaarmor
Powerups BerserkComputer area mapInvulnerabilityLight amplification visorMegaspherePartial invisibilityRadiation shielding suitSupercharge
Keys Keycard • Skull key
Weapons FistChainsawPistolShotgunSuper shotgunChaingunRocket launcherPlasma gunBFG9000
Ammo BackpackClip4 shotgun shellsRocketEnergy cellBox of bulletsBox of shellsBox of rocketsEnergy cell pack