There are six levels of encumbrance in NetHack: Unencumbered, Burdened, Stressed, Strained, Overtaxed and Overloaded. The exact inventory weight which determines which of these levels you experience is based on Strength and Constitution.
The maximum possible carrying capacity without becoming burdened is 1000 weight units. A player can carry up to three times their capacity (maximum 3000 units) before becoming Overloaded, at which point any kind of action except dropping items or praying becomes impossible.
Your current carrying capacity is given by the formula (25 * (Str + Con)) + 50, which is then capped at a maximum of 1000. The maximum useful Str+Con is thus 38.
For the purposes of this calculation, a strength of 18/01–18/49 maps to 19, 18/50–18/99 maps to 20, and 18/** maps to 21. Gauntlets of power still count as strength 25, but because of the cap, this extra strength may not be of use (it will always be of use for elves, whose strength + constitution is otherwise capped at 18 + 16 = 34).
If you are currently levitating or riding a strong steed, your carrying capacity is always 1000.
|Burdened||capacity+1 to 1.5*capacity-1||3/4 normal||-1|
|Stressed||1.5*capacity to 2*capacity-1||1/2 normal||-3||Exercise Strength|
|Strained||2*capacity to 2.5*capacity-1||1/4 normal||-5||Exercise Strength, Abuse Dexterity|
|Overtaxed||2.5*capacity to 3*capacity-1||1/8 normal||-7*||Abuse Dexterity and Constitution|
* but see below.
Encumbrance will reduce your speed to the fraction shown above, after applying any adjustment for speed.
Your encumbrance category will interfere with your chances to-hit in melee.
If you are Burdened or greater, you:
- fall down stairs/ladders when going down.
- cannot jump except by casting the spell.
- could drown in water when punished if you cannot shed surplus weight.
If you are Stressed or greater, you also:
- cannot climb stairs/ladders.
- cannot move if you have <10hp and are not at your maximum hp.
- cannot kick.
- cannot mount your steed.
- cannot drag a heavy iron ball.
- Do not regenerate hp when moving unless you have regeneration.
- Do not regenerate Pw unless you have energy regeneration.
- consume an additional point of nutrition every odd turn.
If you are Strained or greater, you also:
- lose 1 hp every 30 turns if you are moving.
- lose 1 hp every 3 turns if you are attacking.
- cannot disarm traps.
- have a cursed loadstone considered your worst cursed item for the minor trouble cursed items (otherwise other things are worse).
If you are Overtaxed or greater, you also:
- lose 1 hp every 10 turns if you are moving.
- cannot attack a monster in melee.
- cannot throw or fire a weapon.
- cannot cast spells.
- cannot zap a wand.
- cannot use the read command.
- cannot engrave.
- cannot apply a tool.
- cannot eat.
- cannot use the #loot command.
- cannot teleport at will with teleportitis.
- cannot attempt to help a monster out of a pit.
- have the major trouble collapsing if you have also lost 4 or more points of Strength.
If you are Overloaded, you also:
- cannot move.
Your encumbrance level can be reduced by:
- Using a non-cursed bag of holding
- Gaining strength or constitution, including polymorphing into a stronger creature.
- Wearing a ring of levitation or levitation boots
- Riding a steed (Note that you cannot mount a steed if stressed or worse, but if you have at least basic riding skill, you can pick up items while riding.)
- Dropping items
- Stashing items (e.g. Wizards can stash their spellbooks at a level (e.g. Sokoban) with a heavy container and come back (e.g. branchport using The Eye of the Aethiopica) whenever they need to relearn a spell.)
Any encumbrance past unencumbered will slow your character down. It is therefore generally a Bad Idea to wander around carrying too much, and almost always a bad idea to enter combat overencumbered. If you are near your encumbrance limit and about to enter combat with a monster that can lower your strength (e.g. with poison), consider dropping a few extra items temporarily to avoid becoming overencumbered during the fight.
When being suddenly polymorphed into a small creature (especially via lycanthropy), it is common to become instantly overloaded, and unable to move.
Being encumbered is generally considered a bad idea, because it slows you down and speed is crucial to survival in NetHack; having a speed less than 12 is usually more dangerous than it seems, since the monsters can get in two hits in one player turn at times, increasing the chance of surprise deaths. Furthermore, climbing stairs becomes more difficult when Burdened (and impossible when Stressed or worse) as well as causing players to "fall down the stairs" when they attempt to descend (a fatal move when combined with a wielded cockatrice corpse).
Although being Stressed trains strength, there are far safer ways such as pushing boulders around.
Being stressed also slows down your rate of magical energy regeneration.