For monster carrying capacity, see just that. 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.
If your total inventory weight is more than your carrying capacity, you become burdened or worse, and slow down. You can carry up to three times your capacity before becoming overloaded, at which point most actions except dropping items or praying become impossible.
Your current carrying capacity is given by the formula (25 * (Str + Con)) + 50.  Next, if you are polymorphed, this capacity is rescaled (except for nymphs): normally by your current monster form's corpse weight / 1450, but not down if it is strong, or by size / 2 if weight is 0. Note that strong monsters automatically get strength 18/**.. If you are currently levitating, on the Plane of Air, riding a strong steed, or polymorphed into a nymph, your carrying capacity is set to 1000. The capacity is then capped at a maximum of 1000. The maximum useful Str+Con is thus 38 (unless polymorphed into a non-strong monster lighter than a human, such as a lich). Finally, your carrying capacity is reduced by 100 for each wounded leg unless you are flying, levitating, or on the Plane of Air.
For the purposes of this calculation, a strength of 18/01–18/31 maps to 19, 18/32–18/81 maps to 20, and 18/82–** 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.)
|Category||Weight||Speed||To-hit||Exercise||Message on increasing level||Message on decreasing level||Warning message|
|Unencumbered||≤ capacity||normal||none||N/A||Your movements are now unencumbered.||N/A|
|Burdened||capacity+1 to 1.5*capacity-1||3/4 normal||-1||Your movements are slowed slightly because of your load.||Your movements are only slowed slightly because of your load.||You have a little trouble lifting <item>.|
|Stressed||1.5*capacity to 2*capacity-1||1/2 normal||-3||Exercise Strength||You rebalance your load. Movement is difficult.||You rebalance your load. Movement is still difficult.||You have a little trouble lifting <item>.|
|Strained||2*capacity to 2.5*capacity-1||1/4 normal||-5||Exercise strength, abuse dexterity||You stagger under your load. Movement is very hard.||You stagger under your load. Movement is still very hard.||You have much trouble lifting <item>.|
|Overtaxed||2.5*capacity to 3*capacity-1||1/8 normal||-7*||Abuse dexterity and constitution||You can barely move a handspan with this load!||(no message)||You have extreme difficulty lifting <item>.|
|Overloaded||≥ 3*capacity||normal*||-9*||You collapse under your load.||N/A||There is <item> here, but you cannot lift any more.|
* but see below.
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 less than 10 HP 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.
- See also Weight.
Some of the most common weight culprits are:
|body armor||30 to 450|
|weapon||1 to 180|
|The Orb of Fate||150|
|shield||30 to 100|
|helm||3 to 50|
|1,000 gold pieces||10|
Think hard what you really need to carry. Pack rats die faster.
Your encumbrance level can be reduced by:
- Using a non-cursed bag of holding
- Gaining strength or constitution, including polymorphing into a stronger creature
- 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
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.
Being encumbered is generally considered a bad idea, because speed is crucial to survival in NetHack. Having a speed of less than 12 is usually more dangerous than it seems, since the monsters can sometimes get in two hits in one player turn, 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 wielding a cockatrice corpse).
If you have a good reason to move around encumbered, prepare to drop items if you are attacked. You can pick them up after the fight. This goes especially for corpses you are dragging to an altar to sacrifice; they are likely to become too old if you take up more turns fighting while carrying them. If you are relocating a stash, put it into a sack or bag of holding, so you can drop the container if you are attacked. If you need something from the container during the fight, just #loot it; this takes the same time as applying a held one. Don't accidentally drop crysknives or scrolls of scare monster.
You should not try to train strength by being stressed. There are far safer ways, such as pushing boulders around. Being stressed also slows your HP regeneration and blocks Power regeneration.