Concentration, Holding a Spell Through a Fight

A Cleric casts Bless, and for the next minute three friends add a d4 to every attack and save. Then a goblin's scimitar lands, and the table asks: "Do you lose it?" That's Concentration. A lot of the best spells hang on it, and SRD 5.2.1 sets out the whole rule in one short glossary entry. Here it is, quoted, with the numbers worked out.

What it is

A spell's Duration line tells you. Bless reads "Duration: Concentration, up to 1 minute". The SRD defines the word:

Some spells and other effects require Concentration to remain active, as specified in their descriptions. If the effect's creator loses Concentration, the effect ends.

The time on the line is a ceiling, not a promise. "Up to 1 minute" is 10 rounds, since a round "represents about 6 seconds in the game world". You can also let go whenever you like: "The creator can end Concentration at any time (no action required)."

The three things that break it

The SRD lists them, and only them.

1. Another Concentration effect.

You lose Concentration on an effect the moment you start casting a spell that requires Concentration or activate another effect that requires Concentration.

One at a time. Start casting Bane while Bless runs and Bless ends before Bane lands. Spells without Concentration, like Cure Wounds or Sacred Flame, leave it alone.

2. Damage.

If you take damage, you must succeed on a Constitution saving throw to maintain Concentration. The DC equals 10 or half the damage taken (round down), whichever number is higher, up to a maximum DC of 30.

3. Incapacitated or dead.

Your Concentration ends if you have the Incapacitated condition or you die.

The Incapacitated condition says the same thing in its own words: "No Concentration. Your Concentration is broken." And the Unconscious condition carries Incapacitated with it ("You have the Incapacitated and Prone conditions"), so dropping to 0 Hit Points ends the spell too.

Nothing else on the list. Moving, attacking with a weapon, and casting a spell without Concentration all leave it standing.

The DC is nearly always 10

Half the damage only beats 10 when the hit is 22 or more. Most hits never get there.

So for most fights, the question is simply: how often does your Constitution save make 10?

A level 3 Cleric holding Bless

Our Cleric took the Protector order ("training with Heavy armor"), so wears Chain Mail (AC 16) and a Shield (+2): AC 18. Constitution 14 gives +2. Clerics' saving throw proficiencies are "Wisdom and Charisma", so no proficiency bonus on this one.

The second row is the trick. Bless says "Whenever a target makes an attack roll or a saving throw before the spell ends, the target adds 1d4". The Concentration save is a saving throw, so a Cleric who counts themself among the three targets adds the d4 to the save that keeps Bless alive. Across the four faces of the d4 the d20 needs 7, 6, 5 or 4, which works out to 62 rolls in 80: 77.5%. Nearly as good as a Sorcerer, whose class lists "Constitution and Charisma" saves.

How a round goes

A Goblin Warrior's Scimitar is "+4, reach 5 ft. Hit: 5 (1d6 + 2)". Against AC 18 it needs 14 or more on the d20: 35%.

• One goblin swings. Bless breaks only if it hits and the save fails: 35% × 35% = 12.25%. A Cleric who blessed themself: 35% × 22.5% = 7.9%.
• Two goblins both hit. Two saves, each its own roll. Keeping Bless through both: 65% × 65% = 42.25%, or 77.5% × 77.5% = 60% blessed.
• The Ogre. Its Greatclub hits for "13 (2d8 + 4)": still DC 10. The danger isn't the DC, it's the Hit Points. A level 3 Cleric with Constitution 14 has 8 + 2 at level 1 and 5 + 2 for each level after: 24. Two average Greatclub hits are 26, and at 0 the Cleric is Unconscious, Incapacitated, and Bless is gone with no save at all.

At the table

• Count your own spell. If the spell lets you choose yourself, as Bless does, include yourself when you can. It protects the spell.
• One save per hit. Two hits, two saves. A creature with several attacks is a bigger threat to Concentration than one big hit.
• The DC rarely moves. Don't fear the giant more than the swarm. Below 22 damage, every hit is DC 10.
• Stand behind someone. The surest save is the one you never roll. A caster holding Concentration wants the Fighter between them and the goblins.
• Say it out loud. When you cast a Concentration spell, tell the table. When you take damage, the DM knows to ask for the save, and nobody argues later.

The Book of Dice covers the d20 and the d4 in Part I, The Object, and how adding a die to a roll shifts the odds in Part III, The Rolling.

This work includes material from the System Reference Document 5.2.1 ("SRD 5.2.1") by Wizards of the Coast LLC, available at dndbeyond.com/srd. The SRD 5.2.1 is licensed under the Creative Commons Attribution 4.0 International License.