Concentration in D&D 5e: Every Rule, Edge Case, and How to Never Forget It Mid-Fight
The complete concentration ruleset: what breaks it, the real cost of the Con save, double-casting myths, and the 10 spells that waste concentration too often.
What concentration actually says
A spell marked (Concentration) means you can maintain only one such spell at a time. That's the whole rule — everything else is interpretation. The text lives in the spellcasting chapter; our rules mirror keeps it searchable instead of buried on page 203 of a 320-page PDF. When you cast a second concentration spell, the first one ends instantly. No save, no negotiation. That simple rule drives more table decisions than most people realize.
The damage save: DC 10 or half damage, whichever is higher
Concentration breaks when you take damage and the check fails. The formula matters: DC = max(10, damage/2). A wizard with +2 Con saves vs 31 damage is rolling DC 15 needing 8+: about a 45% failure chance from a single fireball. Two goblin arrows of 5 damage each? DC 10 twice, because the rule applies per damage instance, and each hit forces its own check. This is the single most-underused detail in the game: spreading damage across many small hits is mathematically better against casters than one big hit. Our tracker's Concentration checkbox on each card logs every save automatically, so nobody argues about what the DC was.
Myths worth killing
(1) “Casting Haste is always free because you're concentrating anyway” — wrong, it ends other concentration spells instantly. (2) “Invisibility doesn't matter for concentration” — you still take damage when you're discovered. (3) “Concentration ends when you cast anything” — only concentration spells. Casting Shield (no concentration) never breaks Hold Person. These three myths create more table-arguments than the rule itself.
The spell list, honestly sorted
Best-per-slot: Hold Person (paralysis doubles attacks, no save to keep), Cloud of Daggers (guaranteed damage per turn, no save), Spike Growth (area denial with zero action cost), Hex (5e's cleanest damage tax). Worst-value: spells that want you to stand still in melee while everyone else is dying, or that compete with better uses of your bonus action. We rank them against the spells database rather than opinions — the search filter (school, level, concentration-only) makes the argument mostly unnecessary.
The real cost nobody computes
A Sorcerer concentrating on Spirit Guardians spends level-9 slots knowing one arrow can end it. That's not a flaw, it's the balance lever — the designers pay for sustained area damage with fragility. The correct play is the one the rules imply: maintain concentration only while its value exceeds the value of the next spell you might need. Early in the fight, Control; mid-fight, Defense; if the fight's already won, let it go. Keeping this ledger mentally is hard, which is exactly why running a campaign means your tools should do the bookkeeping — see the characters sheets that carry the save modifier next to the HP bar.
Building a habit at the table
Declare concentration out loud when you start it, declare it when you drop it, and put a visible marker on the initiative tracker. Three sentences per session ends 95% of disputes. If you want the tool that makes that marker automatic, our initiative-tracker puts it on the same screen as combat.