A creature’s space is the area in feet that it effectively controls in combat, not an expression of its physical dimensions. A typical Medium creature isn’t 6 feet wide, for example, but it does control a space that wide. If a Medium hobgoblin stands in a 5-foot-wide doorway, other creatures can’t get through unless the hobgoblin lets them.

A creature’s space also reflects the area it needs to fight effectively. For that reason, there’s a limit to the number of creatures that can surround another creature in combat. Assuming Medium combatants, eight creatures can fit in a 5-foot radius around another one.

Because larger creatures take up more space, fewer of them can surround a creature. If five Large creatures crowd around a Medium or smaller one, there’s little room for anyone else. In contrast, as many as twenty Medium creatures can surround a Gargantuan one.

Volumetric Shapes

From time to time, there will be a description or effect that is described using a volumetric shape. These shapes will, in a virtual environment, “snap to the grid”. In Theater of the mind, they will do whatever is described.

BowlA 3-Dimensional half spherical shape, bisected equally, without a base, descending vertically.
BoxA 3-Dimensional square or rectangle, open on one side
Closed DomeA 3-Dimensional half spherical shape, bisected equally, with a base.
ColumnA 3-Dimensional circular form rising vertically, closed on the top and bottom.
ConeA 3-Dimensional circular with one circular base and a curved surface that connects the base and the vertex, open on the circular end.
CubeA 3-Dimensional square, equal on all sides, enclosed.
DomeA 3-Dimensional half spherical shape, bisected equally, without a base, rising vertically.
HemisphereA 3-Dimensional partial sphere, always having a curve and open in at least one area. A Hemisphere operates horizontally.
OvoidA 3-Dimensional egg-shaped form, enclosed on all sides.
PillarA 3-Dimensional circular form, rising vertically, open on the top and bottom.
PrismA 3 Dimensional rectangle, unequal sides, enclosed.
PyramidA 3-Dimensional polyhedron for which the base is a polygon and all lateral faces are triangles coming to a point, fully enclosed.
RingA circluar space with a vertical height never greater than 9 feet, a form of Pillar.
SphereA 3-Dimensional round form, enclosed on all sides.
ToroidA 3-Dimensional circular form with an open center, like a doughnut.

A form can always be adjusted in general dimensions, but the volume will remain the same. So, one could shape a sphere into an ovoid or a cube into a rectangle, but the volume of that that sphere or cube in its new form must remain the same. One cannot, however, change the core structure – you cannot change a sphere into a cube, or a cube into a pyramid.

Squeezing into a Smaller Space

A creature can squeeze through a space that is large enough for a creature one size smaller than it. Thus, a Large creature can squeeze through a passage that’s only 7 feet wide. While squeezing through a space, a creature must spend 1 extra foot for every foot it moves there, and it has Disadvantage on attack rolls and Dexterity saving throws. Attack rolls against the creature have Advantage while it’s in the smaller space.

Creature Size

Size Categories
Micro1 in by 1 in up to 3 in
Minute3 in by 3 in up to 6 in
Bitty6 in by 6 in up to 9 in
Tiny9 in by 9 in up to 12 in
Wee12 in by 12 in up to 18 in
Little18 in by 18 in1 ½ ft. to 2 ft.
Small24 in by 24 in2 ft. to 3 ft.
Short3 ft. by 3 ft.3 ft to 6 ft
Medium6 ft. by 6 ft.6 ft to 9 ft
Large9 ft. by 9 ft.9 ft to 12 ft
Big12 ft. by 12 ft.12 ft to 15 ft
Huge15 ft. by 15 ft.15 ft to 18 ft
Gigantic18 ft. by 18 ftup to 21 ft 
Massive21 ft. by 21 ft. up to 24 ft
Immense24 ft. by 24 ft. up to 27 ft
Monstrous27 ft by 27 ft. up to 30 ft
Humongous30 ft. by 30 ft up to 36 ft
Gargantuan36 ft. by 36 ft up to 42 ft
Colossal42 ft. by 42 ft up to 48 ft
Titanic48 ft. by 48 ft up to 54 ft

Each creature takes up a different amount of space. Size Category Table shows how much space a creature of a particular size controls in combat. This also applies outside of combat and is used to describe the amount of space that a being takes up. You may note that it uses a 3-foot rule, instead of a 2½ foot rule.

Objects sometimes use the same size categories.

Moving Around Other Creatures

You can move through a nonhostile creature’s space. In contrast, you can move through a hostile creature’s space only if the creature is at least two sizes larger or smaller than you. Remember that another creature’s space is difficult terrain for you.

Whether a creature is a friend or an enemy, you can’t willingly end your move in its space.

If you leave a hostile creature’s reach during your move, you provoke an opportunity attack.

Climbing Another Creature

Once during your move, you can climb a creature that is at least two sizes larger than you and that is within 5 feet of you. Doing so costs an amount of movement equal to half your speed. To stay on a creature requires a Strength (Athletics) or Dexterity (Acrobatics) check against a DC of the creatures AC plus Dexterity modifier. This will persist as a roll each turn until either the creature is immobile, or you get off or fall off. The larger of the two can take an action each turn to attempt to dislodge you, rolling its Strength against either your Strength or your Dexterity – your choice.

While on the creature, it has disadvantage on attacks against you, and you have advantage on attacks against it. Note that being dislodged from a height means falling damage. Getting off of your own accord takes a full action, just like dismounting.

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