Backpack

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 type	8 (decimal), 8 (hex)
Appears in	Shareware Doom Registered Doom The Ultimate Doom Doom II Final Doom
Radius	20
Height	16
Sprite	BPAK
Class	Item Pickup

Appearance statistics

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

Game	1-2	3	4-5
Doom	22	22	22
The Ultimate Doom	28	28	28
Doom II: Hell on Earth	48	48	48
TNT: Evilution	31	32	32
The Plutonia Experiment	33	33	33