High ground rules

High ground is an important mechanic in Baldur's Gate 3 combat. Holding high ground over enemies grants a number of benefits including increased range and attack roll bonuses.

Attack roll modifiers

Targeting an enemy with an attack that will receive the +2 High Ground bonus to the attack roll.

Targeting an enemy with an attack that will receive the -2 Low Ground penalty to the attack roll.

If there is sufficient elevation difference between an attacker and target, an elevation modifier is applied to the attack roll. At 2.5 m / 8 ft above a target, an attacker receives a +2 "High Ground" bonus to attack rolls. At 2.5 m / 8 ft below a target, an attacker receives a -2 "Low Ground" penalty to attack rolls. When targeting an enemy, this modifier is displayed below its hit points bar with other modifiers.

The  Sharpshooter feat removes the low ground penalty for ranged weapon attack rolls.

These high ground rules apply to all attack rolls, even melee ones. However, since melee attacks seldom have a long enough range for elevation to matter, it rarely applies to them. There are a few exceptions, such as  Thorn Whip (a melee spell attack with a range of 9 m / 30 ft),  Fangs of the Fire Snake (a melee unarmed attack with a range of 6 m / 20 ft),  Diving Strike (which requires jumping from a higher elevation anyway), or  Charger (when charging up or down a slope with a large enough elevation change). On the other hand, attacks with Extra Reach weapons do not have extended vertical reach, so they do not have sufficient vertical range for high ground modifiers to apply.

Range extension

Normally for range considerations, only the horizontal distance between an attacker and target matters and vertical distance is ignored. Specifically, the targetable region of an ability is a vertical cylinder (where the attacker is at the centre of this cylinder) with a radius equal to the ability's range, and not a sphere.

Most ranged abilities gain increased range when targeting enemies at a lower elevation equal to the elevation difference. For example, a ranged weapon attack (range of 18 m / 60 ft) against a target 9 m / 30 ft below has a maximum horizontal range of 27 m / 90 ft. In this case, the true distance between the attacker and target is

$\text{Distance}=\sqrt{(27\text{m}{)}^2+(9\text{m}{)}^2}=28.4\text{m}$

which is well beyond the normal range of the attack. However, there is no corresponding range penalty when attacking a target at a higher elevation.

A ranged weapon's maximum effective range is its normal attack range. Beyond this, the attacker suffers a "Target outside normal range" penalty which applies Disadvantage to the attack roll. Unlike attack range abilities, this effective range is not extended by the high ground range extension. It is also measured using the true distance between the attacker and target, not just the horizontal component. Thus, maximum attack range and effective weapon range are equal at level elevations, but diverge when dealing with elevation differences. Practically speaking, this means this penalty is only encountered when extending the attack range with elevation, particularly high ground range extension. Spell attacks do not have a maximum effective range and do not suffer this penalty when attacking beyond normal range.