D&D 5e Flanking Optional Rule vs Advantage Houserule

Flanking is optional, but its math is not small

Flanking is an optional rule in the 2014 Dungeon Master's Guide, not a default Player's Handbook combat rule. The common version gives advantage on melee attacks when two allies are on opposite sides of an enemy. Advantage itself is defined in PHB p.7. Because advantage is such a large modifier, adding flanking can reshape the value of class features, spells, Help actions, Pack Tactics, Faerie Fire, Reckless Attack, and familiar tactics.

The biggest table problem is not whether flanking is "realistic." The problem is whether it makes advantage too cheap. If walking around a target gives advantage every round, then many class features that create advantage stop feeling special. That is why many experienced DMs use +2 flanking, no flanking, or a stricter positioning rule instead.

How flanking advantage is calculated

If your base hit probability is p, advantage changes hit chance to 1 - (1 - p)^2. Disadvantage changes it to p^2. A flat +2 bonus is different: it usually adds 10 percentage points when the target number is not already capped by natural 1 or natural 20. That means advantage is much stronger than +2 in the middle of the d20 range.

Critical hit chance also changes. A normal attack crits on a natural 20, or 5%. With advantage, the chance of at least one natural 20 is 1 - (19/20)^2 = 9.75%. A flat +2 bonus does not change critical hit chance at all unless another feature changes the crit range.

Why advantage flanking can distort the table

Advantage flanking rewards melee movement, which is good. It also makes advantage so common that other tactical choices compete poorly. The Help action costs an action. Faerie Fire costs a spell slot and concentration. Reckless Attack exposes the barbarian. Guiding Bolt consumes a spell slot. If all of those are competing with "stand opposite the rogue," the cheaper tactic may dominate.

It can also punish large monsters. A single ogre, giant, or dragon that lacks legendary actions can be surrounded and attacked with advantage from multiple characters. The result is faster crit fishing and fewer meaningful boss turns. This does not mean advantage flanking is wrong, but the DM should know what the rule does before blaming encounter balance.

Three practical flanking variants

My default recommendation for most 5e tables is +2 flanking if the group loves grid tactics, and no flanking if the group plays theater of the mind. Advantage flanking is fun, but it should be a conscious high-action choice rather than a rule added because someone remembered it from another edition.

DM guidance for using flanking fairly

If monsters can flank, players should know that before combat begins. Symmetric rules matter. A pack of wolves with easy advantage feels very different when the DM remembers Pack Tactics and flanking in the same fight. Also define whether flanking works around corners, through walls, against amorphous creatures, against flying enemies, and on a hex grid. Ambiguity slows combat more than the rule itself.

Flanking works best when terrain creates choices. Narrow bridges, lava edges, difficult terrain, doorways, and opportunity attacks make "just walk around" less automatic. If every fight is a flat empty rectangle, flanking becomes a default bonus instead of a positional achievement.

Battle map examples that expose the difference

In a ten-foot corridor, flanking may be impossible because allies cannot stand on opposite sides of the target. In a wide throne room, flanking may be trivial because everyone can circulate around the boss. The same rule therefore has different value depending on map design. If a DM uses advantage flanking, narrow spaces become an indirect monster buff and open spaces become an indirect player buff.

Large and huge creatures need special attention. It is easy for multiple melee characters to surround a large creature, and many tables interpret opposite sides generously. If that grants advantage to everyone, a single large monster loses durability very quickly. Adding minions, hazardous terrain, reach, legendary actions, or movement reactions can keep the fight from becoming a stationary advantage pile.

Reach weapons are another edge case. A character with a glaive may threaten from 10 feet away. Decide whether that character can create a flank from reach or must be adjacent. Both rulings can work, but they produce different incentives. Allowing reach flanking makes polearms even stronger. Requiring adjacency makes flanking simpler but less simulationist.

Invisible, blinded, stunned, prone, restrained, and incapacitated creatures also need rulings. A flanker who cannot see or threaten the target probably should not create a meaningful flank. A target that is unaware may already be granting advantage through other rules. Put the guiding principle in one sentence: flanking should reward two active threats forcing a defender to split attention.

How flanking interacts with common class features

Rogues care because Sneak Attack requires either advantage or an enemy of the target within 5 feet under the class rules. Flanking advantage is not necessary for Sneak Attack if an ally is already adjacent, but it can improve hit chance and crit chance. That makes rogue turns more reliable, especially with one big attack per turn.

Barbarians care because Reckless Attack becomes less distinctive. If the barbarian can get advantage from positioning without giving enemies advantage back, reckless is used less often. That is not automatically bad, but it reduces one of the class's signature risk-reward decisions. A +2 flanking rule preserves more reason to use Reckless Attack when accuracy really matters.

Paladins care because advantage nearly doubles crit chance, and crits are the best time to spend Divine Smite. A paladin in an advantage-flanking table should be watched carefully during boss fights. The problem is not that smite crits are illegal; they are exciting and supported by crit dice rules. The problem is making the setup so cheap that every boss fight becomes a crit hunt.

Spellcasters care because several spells exist to create advantage or deny enemy defenses. Faerie Fire, Hold Person, restrained effects, invisibility, and familiar Help all lose relative value when every melee character already has advantage from walking. If your table enjoys those support spells, avoid making their main reward free every round.

Session-zero language for flanking

Flanking is exactly the kind of optional rule that should be named at session zero. A clean statement is enough: "This campaign uses no flanking," "This campaign uses +2 flanking on a grid," or "This campaign uses DMG-style advantage flanking, and monsters can use it too." That one sentence prevents players from building characters around a rule that appears or disappears without warning.

If you are changing an active campaign, test the rule instead of declaring it permanent. Run three combats with the variant and ask concrete questions afterward. Did melee turns get faster or slower? Did monsters drop too quickly? Did support spells feel worse? Did players move more? Those answers are better than theory arguments because they reflect your actual maps and party.

For encounter design, assume advantage flanking raises melee output significantly. Add terrain that makes surrounding enemies a choice, not a default. Doorways, difficult terrain, ledges, hazards, and mobile enemies all help. If you prefer simple monster math, use +2 or no flanking and let advantage come from explicit features and conditions.

Example ruling: rogue and paladin around an ogre

Suppose a rogue and paladin stand on opposite sides of an ogre. With advantage flanking, both melee attacks roll with advantage. The rogue becomes more likely to land Sneak Attack and more likely to crit. The paladin becomes more likely to land a smite and almost doubles the chance of seeing a natural 20. With +2 flanking, both characters still benefit, but their crit chance stays 5% and class features that grant advantage keep their value.

This is the whole design choice in miniature. Advantage flanking creates bigger moments and faster fights. +2 flanking creates positioning rewards without moving as many other mechanics. No flanking keeps advantage tied to explicit conditions, spells, and class features. Pick the experience your table wants, then build encounters with that math in mind.

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