Blog / Fifth edition rules
Concentration checks in 5e, explained
Concentration is fifth edition's limit on how many ongoing spells one caster can keep running at a time, and the limit is one. If a spell's duration says it needs concentration, you hold it by concentrating; when something disrupts you, it ends. The most common disruption is damage, and that is where the roll comes in: when you take damage while concentrating, you make a Constitution saving throw against a DC of 10 or half the damage you took, whichever is higher. Fail it and the spell ends immediately.
That is the whole rule. Almost everything people get wrong about it comes from one of three places: what the roll actually is, how many of them you owe, and the fact that losing concentration is silent.
It is a saving throw, not a check
The phrase "concentration check" is universal and slightly wrong, and the difference is not pedantry - it decides what you add to the d20.
It is a Constitution saving throw. You add your Constitution modifier, and you add your proficiency bonus only if your class gives you proficiency in Constitution saves, which most casters do not have. A wizard with +2 Constitution rolls d20+2 whether they are level 1 or level 17. This is the reason concentration gets harder to hold as the campaign goes on rather than easier: the DC scales with the damage monsters deal, and your bonus does not scale with anything.
Which spells need it
Look at the spell's duration line. If it reads "Concentration, up to 1 minute" (or 10 minutes, or an hour), it needs concentration. If it just says "1 minute", or "Instantaneous", it does not.
The rough shape of it: spells that keep doing something to the battlefield tend to need concentration, and spells that do their work once and finish do not. Bless, Bane, Hold Person, Haste, Faerie Fire, Invisibility, Fly, Banishment and Polymorph are concentration spells. Fireball is not, because it goes off once and is over. Mage Armor is not, despite lasting eight hours, because the game decided it should not be.
The DC, precisely
The DC is 10, or half the damage taken, rounded down, whichever is higher.
So a 9 damage hit is DC 10, because half of 9 rounds down to 4 and 10 is higher. A 22 damage hit is DC 11. A 40 damage hit is DC 20, which a wizard with +2 Constitution fails on anything under an 18.
Two things follow that are worth holding onto. First, most hits in an average fight are DC 10, so a caster with a decent Constitution holds a spell through them most of the time. Second, the curve is brutal at the top: the DC has no ceiling, and a single big hit from a boss is a coin flip at best.
What breaks it without a roll
Some things end concentration outright, with no save offered:
- Casting another concentration spell. You do not choose which one to keep. The new one lands and the old one ends the moment you cast it, even if the new spell then fails or the target saves.
- Being incapacitated. Any effect that makes you incapacitated ends concentration. Stunned, paralyzed and unconscious all include incapacitated, so they all do it.
- Dropping to 0 hit points. This is the incapacitated rule again, arriving by the route it usually arrives by. Going down does not prompt a save; the spell is simply gone.
- Dying. For the avoidance of doubt.
- Choosing to stop. You can end concentration whenever you like, and it costs you nothing - no action, no reaction.
Notably absent from that list: being frightened, being poisoned, being knocked prone, being grappled, or casting a spell that does not require concentration. None of those touch it. You can cast a cantrip every round of a fight and hold Hold Person the entire time.
War Caster and Resilient (Constitution)
Two feats exist more or less to fix the maths above.
War Caster gives you advantage on Constitution saving throws made to maintain concentration. Advantage on a DC 10 save takes a +2 caster from roughly 65 percent to about 88 percent, which over a long fight is the difference between a spell that lasts and a spell that does not. It is the most-taken feat among full casters for exactly this reason.
Resilient (Constitution) gives you proficiency in Constitution saves and a +1 to the score. That is a flat bonus of your proficiency bonus, growing from +2 to +6 over a campaign, and unlike advantage it helps just as much against the enormous DCs as the small ones. Advantage is worth more at the middle of the curve; a flat bonus is worth more at the ends.
Where tables get it wrong
Several small hits are worse than one big one. Four hits of 8 damage is four separate DC 10 saves. One hit of 32 is a single DC 16 save. A +2 caster holds the single big hit 35 percent of the time and holds all four small ones about 18 percent of the time. Swarms of weak enemies are the real threat to concentration, and tables routinely forget to ask for a save on the third and fourth goblin.
Multiple sources in one turn are multiple saves. If a creature hits you twice, or you take damage from an attack and from a trap in the same round, you save once for each source, each against its own DC. It is not one save against the total, which is the version most tables play by accident and which is significantly more forgiving.
Losing it is silent. This is the big one. Nothing about the fiction changes when Bless ends. Nobody says anything. The party keeps adding the d4 to their attack rolls for three more rounds because the sheet does not say it stopped, and the numbers from that point on are wrong in a way that nobody will ever catch.
What this looks like when something tracks it for you
Concentration is a good example of a rule that is not hard, just relentless. It asks for a save at an awkward moment, several times a round, and the cost of skipping one is invisible. That is precisely the kind of rule a table drops, not out of ignorance but because a fight is already asking for enough attention.
On Rolegend the two halves of the game are separate systems, and only one of them is allowed to produce a number. Keystone Engine knows which spell you are concentrating on, computes the DC from the damage that was actually applied, rolls the Constitution saving throw with your modifier and with advantage if you have War Caster, and ends the spell on a failure. When you go down, the effect is cleared without a roll, because that is what D&D 5e says happens. ROWAN is told the outcome and writes it - the sudden slackness as the light around your allies goes out - and cannot re-roll it or argue with it.
The practical result is not that the game gets easier. It is that when Bless is still running on turn nine, it is really still running, and when it is not, you find out. What Keystone does and does not enforce is published rule by rule at /rules-coverage.