Bounded accuracy

Bounded accuracy is a design principle in Dungeons & Dragons 5th edition which limits the numeric bonuses to d20-based rolls which accrue with character level. While such bonuses were significant in earlier editions of the rules, the designers of D&D 5th edition aimed to achieve various gameplay improvements by limiting their extent.

Design

Mechanics

In Dungeons & Dragons 5th edition, player characters and monsters add a number known as a proficiency bonus to certain rolls, such as attacks with weapons they are proficient with and ability checks using skills they are proficient with. That number starts at +2 at first level, and increases to a maximum of +6 at 17th (or for monsters, up to +9 at challenge rating).

In earlier editions of the D&D rules, the equivalent bonuses were much higher, such as +20 at 20th level, and bonuses from feats and items were much higher and easily come by. The decision to limit proficiency bonus in 5th edition was intentional on the part of the designers, and fulfilled several design purposes.

Particular bounded accuracy features in D&D 5th edition include limiting the level-based bonus to attacks, saving throws and skills to +6; limiting magic weapons and other items to +3; limiting player character ability scores to 20 in most instances; removing feats which grant a bonus to attacks; and giving advantage in beneficial circumstances rather than numeric circumstance bonuses.

Monsters likewise have limited statistics. For example, in D&D 5e, the challenge rating 30 tarrasque has only an armor class of 25, compared to AC 35 in D&D 3.5 (where it is only CR 20), AC 43 in D&D 4e, and AC 54 in Pathfinder 2nd edition.

Design goals

In 2012, during the development of D&D 5th edition, designer Rodney Thompson cited several intended benefits of bounded accuracy.

In D&D 4th edition, level-based bonuses were often matched by equivalent increase in target number; for example, as a character's attack bonus increased, so did the armor class of level-appropriate monsters. Characters did not succeed more frequently at higher levels, but merely kept pace with level-appropriate challenges. Bounded accuracy was intended to resolve this by removing the assumption that target numbers would scale with level, allowing players to see tangible benefit from numeric increases.

Bounded accuracy would decrease the difference between proficient and non-proficient characters. A d20 roll which is easy for a specialized character (e.g. a rogue using Stealth) would still be within the range of achievable results by a non-specialized character, even at high level.

Low-level monsters would continue to be usable at higher level, as their attack bonus and AC would allow them to remain meaningful threats to player characters. In designing D&D 4th edition, James Wyatt noticed that large groups of low-level monsters were ineffective due their low AC and attack bonuses, which posed little threat to higher-level PCs. 4e's solution to this had been the minion, a monster type with level-appropriate statistics but which would be killed by a single hit. Bounded accuracy allowed D&D 5e to do away with the minion type, allowing large groups of standard enemies (such as a horde of orcs) to pose a meaningful threat to a high-level party. Conversely, large groups of low-level NPCs could now plausibly threaten a higher-level opponent, such as a group of peasants fighting a dragon.

Target numbers for rolls such as skill checks could now remain fairly constant. This adds realism and verisimilitude, such that the DC to achieve a given outcome is consistant and easily understood by both DMs and players. If breaking down a door requires a DC 15 check, this will remain true even at high level; it is no longer necessary to challenge high-level PCs with adamantine doors.

History

Unbounded accuracy

In Dungeons & Dragons 3rd edition (2000), characters gained numeric bonuses to attack rolls, saving throws, and skill points which increased significantly with character level. For example, a fighter added a bonus equal their full character level to attack rolls, while a wizard added half their level to attack rolls; similarly, a character can have 3 plus their level from ranks in a skill, while a character without ranks this skill has zero.

In addition, characters often gained new magic items which added numeric bonuses, and rules allowed players to buy or craft magic items. D&D third edition's character customization rules also allowed numerous options such as feats and prestige classes which could further increase numeric bonuses.

The result is that, at high level, a challenge appropriate for a trained character may be completely impossible for an untrained one. For example, Bastion of Broken Souls (2002) has a DC 35 Knowledge (the planes) check to reveal certain lore, which would be completely impossible if the party does not contain a member specializing in that skill.

Dungeons & Dragons 4th edition (2008) had a similar feature. The numbers were more constrained than in third edition, with player characters adding a value equal to half their character level to many rolls and statistics, including armor class, initiative, skills, defenses, and attack rolls. However, the difference at high level can still be significant, with level bonus giving a difference of +15 at level 30.

Additionally, rolls tended to scale with character level to provide a balanced encounter. For example, in Dungeon Master's Guide 2 (4e) (2009), p.63, rules are given for a boulder which requires an Athletics check to push. At first level, the difficulty class is 15. At 30th level, the difficulty class is 33, meaning that a character of godlike power still has an equally difficult time pushing boulders; they just tend to encounter more challenging boulders. The result is that D&D 4th edition characters often do not become more capable with level; they just keep pace with the numbers.

Bounded accuracy

The observation that Dungeons & Dragons was better balanced at lower numbers was previously made in E6, a fan variant of D&D 3.5 which caps all character classes at level 6 in order to better explore the "gritty fantasy" tier of play. That variant effectively caps attack bonuses at +6 and magic weapons to +2. Benefits include that the DM can continue to use low-level monsters, characters never become invulnerable, and DMs retain a better sense of the numbers.

The term "bounded accuracy" first appeared in an article on the Wizards of the Coast website, posted in the Legends and Lore column by Rodney Thompson on June 4, 2012.

Bounded accuracy would subsequently appear as a core feature of Dungeons & Dragons 5th edition. Conversely, D&D's competitor Pathfinder 2nd edition (2019) did not adopt this change.