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Swarm of Rats 5e: Swarm Rules, Damage Math and Clearing the Sewer
A swarm of rats is a CR 1/4 Medium swarm of Tiny beasts with AC 10, 24 hit points and resistance to bludgeoning, piercing and slashing damage. It bites for 7 (2d6) piercing, but only half that once it's down to half its hit points. It can share a creature's space, squeeze through any rat-sized hole, and can't be grappled, knocked prone, restrained, stunned, frightened or charmed. The 2024 swarm has only 14 hit points and bites for 5 (2d4), but it gains a 30-foot climb speed and a 5-foot reach, so it no longer has to crawl into your space to attack.
Keen Smell.The swarm has advantage on Wisdom (Perception) checks that rely on smell.
Swarm.The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Tiny rat. The swarm can't regain hit points or gain temporary hit points.
Actions
Bites.Melee Weapon Attack: +2 to hit, reach 0 ft., one target in the swarm's space. Hit: 7 (2d6) piercing damage, or 3 (1d6) piercing damage if the swarm has half of its hit points or fewer.
Stat block from the SRD 5.1 (2014 rules) by Wizards of the Coast, licensed under CC-BY-4.0. Check an encounter with this monster in the encounter calculator.
Swarms have their own rules, and they're all in the stat block's Swarm trait:
•It can occupy another creature's space, and vice versa. Moving through it or standing in it is allowed.
•It can move through any opening large enough for a Tiny rat: drains, gaps under doors, cracks in a wall.
•It can't regain hit points or gain temporary hit points. Damage sticks.
The 2014 Bites have a reach of 0 feet and target one creature in the swarm's own space. To attack, the swarm has to move into a character's square. Its damage drops from 7 (2d6) to 3 (1d6) once it has 12 hit points or fewer, which models the swarm thinning out.
Its resistance to bludgeoning, piercing and slashing doubles its effective hit points against weapons. A longsword hit for 7 becomes 3 after resistance (rounding down). Against a party of fighters, 24 hit points behave like 48.
Swarm of Rats in the 2024 Rules
SRD 5.2.1 makes the swarm smaller, faster on walls, and simpler.
Swarm.The swarm can occupy another creature’s space and vice versa, and the swarm can move through any opening large enough for a Tiny rat. The swarm can’t regain Hit Points or gain Temporary Hit Points.
Actions
Bites.Melee Attack Roll: +2, reach 5 ft. Hit: 5 (2d4) Piercing damage, or 2 (1d4) Piercing damage if the swarm is Bloodied.
Stat block from the SRD 5.2.1 (2024 rules) by Wizards of the Coast, licensed under CC-BY-4.0. Check an encounter with this monster in the encounter calculator.
2014 Swarm of Rats
2024 Swarm of Rats
HP
24 (7d8 − 7)
14 (4d8 − 4)
Speed
30 ft.
30 ft., climb 30 ft.
Bites
+2, reach 0 ft. (creature in its space), 7 (2d6)
+2, reach 5 ft., 5 (2d4)
Damage when hurt
3 (1d6) at half HP or fewer
2 (1d4) while Bloodied
Keen Smell
Advantage on smell-based Perception
Removed
Saves
No proficiencies
Dexterity save +2
Resistances and immunities
Resists B/P/S; immune to 8 conditions
Unchanged
What it means: the 2024 swarm dies faster (14 hit points, about 28 effective against weapons) but is easier to run. With 5-foot reach, you don't have to resolve swarms crawling into characters' squares, and the climb speed lets rats come down walls and across ceilings. "Bloodied" is the 2024 term for having half your hit points or fewer, so the half-damage rule is the same idea in both editions.
How a Swarm of Rats Fights
On its own, a swarm is weak. Its danger comes from numbers, terrain and the fact that weapons barely hurt it.
Damage per round. Against AC 14, a +2 bite hits on a 12 or better (45%). Against a wizard with AC 12, it hits on 10 or better (55%).
Swarm vs target
Hit chance
Expected damage
2014 swarm, healthy, vs AC 14
45%
about 3.2
2014 swarm, at 12 HP or less, vs AC 14
45%
about 1.6
2024 swarm, healthy, vs AC 14
45%
about 2.3
2014 swarm, healthy, vs AC 12
55%
about 3.9
That's low per swarm. Three 2024 swarms around a level 1 wizard average about 8 damage a round; three 2014 swarms average about 12 if you let them pile into the same space. Either way, a wizard with 8 to 12 hit points goes down in a round or two.
Pick the soft target. Rats are animals, but they're not stupid. They go for the person who is bleeding, alone or holding food. A 2014 swarm moves into that character's space and stays there.
Come from everywhere. A swarm can come out of a drain, a wall crack or a pile of sacks. Two swarms entering from opposite sides of a room force the party to split their attention. In 2024, the climb speed lets a swarm come down a wall or along the rafters and land in the middle of the party instead of at the front line.
Keep moving. The swarm can't heal. Once it's below half and doing 1d6 or 1d4, the real rat behavior is to scatter. Remove it and let the next swarm arrive from a fresh hole.
Running a Swarm of Rats Encounter
A swarm of rats is worth 50 XP. Using the 2024 budget for four level 1 characters (200 Low, 300 Moderate, 400 High):
Under the 2014 Dungeon Master's Guide guidelines (used by our encounter calculator), three swarms are 150 XP, doubled to 300 for a group of three to six creatures. That's exactly the Hard threshold for four level 1 characters. Four swarms (400 adjusted XP) hit Deadly.
The 2014 math overstates swarms for parties with any area damage, and understates them for a party of four weapon users. Adjust for your table.
Where swarms fit:
•Level 1 to 2, as the main fight. Two or three swarms in a sewer or cellar are a full first-session encounter.
•Level 3 to 5, as a complication. Add a swarm to a fight with a bandit leader to tie up the fighter.
•Any level, as a hazard. A swarm that pours out when a door opens is a great way to make a stealthy party noisy.
How Players Beat a Swarm of Rats
•Use area spells. Swarms resist weapons but not fire, thunder, cold or radiant. Burning Hands (3d6, average 10.5) or Thunderwave (2d8, average 9) can hit two or three swarms that have moved into the party. Against the 2024 swarm's 14 hit points, a failed save on either nearly kills it.
•Fireball ends them.Fireball deals 8d6 (average 28), more than a 2014 swarm's 24 hit points on a failed save.
•Hold your ground with Spirit Guardians or Moonbeam. At higher levels, Spirit Guardians or Moonbeam damage every swarm that enters or starts its turn in the area. Since swarms can't heal, the damage adds up.
•Torches do 1 fire damage. Swinging a burning torch as a weapon deals 1 fire damage in both editions. It's not resisted, but it's only 1. Fire as an area effect (oil, a brazier, a spell) is what clears rats.
•Skip most control spells. Swarms are immune to Charmed, Frightened, Grappled, Paralyzed, Petrified, Prone, Restrained and Stunned, so hold, grapple, trip and fear effects are wasted. Hold Person can't even target a Beast. Sleep is edition-dependent: the 2014 spell doesn't affect creatures immune to being charmed, so it fails, while the 2024 spell only exempts creatures that don't sleep or are immune to Exhaustion, so it works.
•Find the source. In a sewer or warehouse, swarms keep coming until the party deals with whatever is drawing them. That could be food, a carcass, a rat-controlling NPC or a magic item.
The Pipes of the Sewers. Both SRDs include a magic item, the Pipes of the Sewers, that calls swarms of rats and can make a swarm friendly. In 2024, a swarm near someone playing them makes a DC 15 Wisdom save or becomes Friendly; in 2014, it's a contested check. A villain with these pipes is a great reason a city has a rat problem, and a great treasure for a party bard.
Swarm of Rats Encounter Ideas
The grain tithe. A temple granary has been stripped bare. The priests blame thieves. In fact a rat-catcher with the Pipes of the Sewers has been sending swarms through the drains as revenge for being cheated on his fee. Every night at midnight, three swarms arrive. Find him and the swarms stop coming.
Plague ship. A merchant cog drifts into harbor with no crew visible. Below decks, swarms of rats nest among the cargo, and the hold is dark and cramped. The cargo is valuable spice, the crew is hiding in the captain's cabin, and the rats have eaten through the rope holding the mainsail.
The collapsed cellar. The party fights a bandit gang in an inn. During the fight, a fireball or a dropped lantern sets the cellar alight, and four swarms boil up through the floorboards, fleeing the fire. The rats don't care which side anyone is on.
For a lone rat or a familiar, see the rat stat block. Giant rats run as a pack using Pack Tactics, which plays quite differently from a swarm.
Frequently Asked Questions
What CR is a swarm of rats in 5e?
A swarm of rats is CR 1/4 (50 XP) in both editions. It has AC 10 and 24 hit points in the 2014 rules, 14 in the 2024 rules.
How much damage does a swarm of rats do?
The 2014 swarm bites for 7 (2d6) piercing, or 3 (1d6) when it has half its hit points or fewer. The 2024 swarm bites for 5 (2d4), or 2 (1d4) while Bloodied. Both attack at +2.
Do swarms take half damage from weapons?
A swarm of rats resists bludgeoning, piercing and slashing damage in both editions, so weapon damage is halved. Spells and other damage types like fire, thunder and radiant hit it for full damage.
Can a swarm of rats be grappled or knocked prone?
No. A swarm of rats is immune to the Charmed, Frightened, Grappled, Paralyzed, Petrified, Prone, Restrained and Stunned conditions in both editions.
Can a swarm of rats enter my space?
Yes. A swarm can occupy another creature's space and vice versa. The 2014 swarm has to be in your space to bite you, because its Bites have a reach of 0 feet. The 2024 swarm has a 5-foot reach.
Stat blocks: System Reference Document 5.1 (2014) and 5.2.1 (2024), Wizards of the Coast, CC-BY-4.0. Tactics, encounter math and ideas are Loreify's own.
This page includes material from the System Reference Document 5.1 and 5.2.1 by Wizards of the Coast LLC, available under the Creative Commons Attribution 4.0 International License. Loreify is not affiliated with Wizards of the Coast.