An encumbrance rating is a penalty applied by almost all body armour and bardings. This is a measure of how much your defensive equipment gets in the way of your attacks, spellcasting, and attempts to evade enemy attacks. Multiple sources of encumbrance rating stack with each other. Shields also have a penalty, but it is handled differently and is never actually named in game.
In general, the heavier your equipment, the greater the encumbrance rating will be. You can reduce the impact of your encumbrance rating by increasing your strength or Armour with the exception of the stealth penalty, which is defined from the encumbrance. For the purposes of evasion it is best to have a strength equal to at least the encumbrance value minus two, though further strength will still provide you with diminishing returns. Reducing your to hit penalty is the hardest, and even with a strength of 72 and armour skill of 27 (the maximum values) both crystal and gold dragon armour suffer a penalty of 2, so you will need to improve your weapon skill and fighting to account for this.
See the Table of Armours for numerical details on all items with encumbrance ratings.
See this spreadsheet (and/or make a copy) for an armour penalty calculator: Calculator
Adjusted body armour penalty
This penalty is not applied directly, rather it is used in evasion, to hit, and cast chance. For to hit the scale used is 20, while for evasion and cast the scale used is 100. This syntax is the same for the other formula.
Armour to hit penalty
This amount reduces the accuracy of your melee attacks and decreases the chance of performing an unarmed auxiliary attack without the corresponding mutation.
After calculating this formula; first the result of the division by 20 is randomly rounded up or down, then a value from 1 to the penalty is randomly selected as the final penalty applied for each attack. Typically this means that the highest possible penalty value is much less likely than any other roll.
Adjusted evasion penalty
Where B is the base dodge bonus (formula shown), S is the relationship between strength and encumbrance, d is dodge skill, D is dexterity stepped down if it is above 24, and F is normalized size factor.
This penalty along with the evasion penalty impacts your ability to evade attacks. Due to the change of function when encumbrance nears strength, this penalty makes it a good idea to have a strength equal to or larger than two minus the encumbrance of the armour.
The size factor is normalized around medium size, and can be derived from the following table:
|Small: Kobold, Halfling||1|
|Medium: Most Races||0|
|Large: Troll, Ogre, Centaur, Naga||-1|
This penalty is subtracted from your stealth score.
If this added to your shield penalty is positive then it is added to your failure chance. See spell success for more information.
Encumbrance Penalty Analysis
By calculating the second derivative of the dodge penalty plus the evasion penalty (formula shown above), we can measure the gains from additional strength for any given character. Using this information we can determine what value you would want to raise your strength to for the greatest benefit. The graph on the right shows a spike in gains from strength at two lower than the encumbrance rating of the armour, however a few points after that spike tend to be fairly beneficial as well. This relationship leads to the rule of thumb for strength that you should at least raise it to the same number as your encumbrance, however more strength is still likely to have a noticeable impact.
Armour no longer slows down unarmed melee attacks, and some of the mathematics have changed between 0.13 and 0.17.
Encumbrance ratings were introduced in 0.13, replacing the EV penalty system.