The rules text from Casting a Spell in the SRD 5.1 (the 2014 rules), with the rules around it and the 2024 (SRD 5.2) text that goes with it.
Some spells require you to maintain concentration in order to keep their magic active. If you lose concentration, such a spell ends.
If a spell must be maintained with concentration, that fact appears in its Duration entry, and the spell specifies how long you can concentrate on it. You can end concentration at any time (no action required).
Normal activity, such as moving and attacking, doesn't interfere with concentration. The following factors can break concentration:
Casting another spell that requires concentration. You lose concentration on a spell if you cast another spell that requires concentration. You can't concentrate on two spells at once.
Taking damage. Whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw to maintain your concentration. The DC equals 10 or half the damage you take, whichever number is higher. If you take damage from multiple sources, such as an arrow and a dragon's breath, you make a separate saving throw for each source of damage.
Being incapacitated or killed. You lose concentration on a spell if you are incapacitated or if you die.
The GM might also decide that certain environmental phenomena, such as a wave crashing over you while you're on a storm-tossed ship, require you to succeed on a DC 10 Constitution saving throw to maintain concentration on a spell.
Constitution save DC by damage
The DC is 10 or half the damage taken, whichever is higher:
Damage taken
Save DC
1–21
10
22–23
11
24–25
12
26–27
13
28–29
14
30–31
15
32–33
16
34–35
17
36–37
18
38–39
19
40–41
20
42–43
21
44–45
22
46–47
23
48–49
24
50–51
25
52–53
26
54–55
27
56–57
28
58–59
29
60–61
30
In the 2024 rules
The 2024 Incapacitated condition breaks Concentration as one of its effects (SRD 5.2): “Your Concentration is broken.”
Some spells last only as long as you concentrate on them: it says “Concentration” in the duration. You can concentrate on only one spell at a time, so casting a second concentration spell ends the first. You can drop concentration whenever you like, with no action needed.
Moving, attacking and casting spells that don’t need concentration don’t interfere. Three things break it: casting another concentration spell, being incapacitated or killed, and failing a Constitution saving throw after taking damage. The DC is 10 or half the damage you took, whichever is higher, so 22 damage means DC 11, and 30 means DC 15. Each source of damage is a separate save.
The DM can also call for a DC 10 Constitution save for something like a wave crashing over you on a ship’s deck.
Keeping concentration up
Stay out of reach. The simplest defense is not getting hit. Casters holding Bless, Hold Person or Haste should be the hardest target to get to.
Constitution matters. Sorcerers are proficient in Constitution saves, and the Resilient (Constitution) and War Caster feats from the Player’s Handbook make other casters much steadier. A Cloak of Protection adds 1.
Many small hits are worse than one big one. Five hits of 8 damage means five DC 10 saves. That’s why a swarm of goblins or a pack of wolves is so good at breaking a cleric’s Spirit Guardians.
Pick one spell that matters. Since you can only hold one, don’t trade a spell that’s winning the fight for a small bonus.
What changed in 2024
The 2024 rules keep the same core: one concentration spell at a time, and a Constitution save against DC 10 or half the damage taken, whichever is higher. The DC is now capped at 30, and the Incapacitated condition says outright that it breaks your concentration. Many spells changed whether they need concentration, so check each spell’s 2024 text.
Common questions
What is the DC to keep concentration?
10 or half the damage you took, whichever is higher. Anything up to 21 damage is DC 10; 30 damage is DC 15; 50 is DC 25.
Can I concentrate on two spells at once?
No. Casting a second concentration spell ends the first as soon as you cast it.
Does casting a spell without concentration break concentration?