Counterspell and Dispel Magic in 5e

Counterspell stops a spell as it is cast; Dispel Magic ends a spell already running. In SRD 5.2.1, Counterspell makes the caster roll a Constitution saving throw against your spell save DC. On a failed save the spell fizzles and the action is wasted, but the caster keeps the spell slot. Dispel Magic ends any spell of level 3 or lower on its target, and needs an ability check for anything higher. Below: both spells quoted, then what they are worth against the SRD's own spellcasters.

Counterspell, the rule

Casting Time: Reaction, which you take when you see a creature within 60 feet of yourself casting a spell with Verbal, Somatic, or Material components

You attempt to interrupt a creature in the process of casting a spell. The creature makes a Constitution saving throw. On a failed save, the spell dissipates with no effect, and the action, Bonus Action, or Reaction used to cast it is wasted. If that spell was cast with a spell slot, the slot isn't expended.

Four things follow from that text.

• You roll nothing. The caster saves against your spell save DC. Raising your DC raises your odds.
• The level of the countered spell doesn't matter. No check, no upcasting. A level 3 Counterspell stops a level 9 spell as well as a cantrip.
• The enemy loses a turn, not a resource. The slot comes back. Your level 3 slot is spent either way.
• You must see the casting. An Invisible caster, or one using the Sorcerer's Subtle Spell (no components at all), gives you nothing to react to.

Counterspell is a Reaction, and the SRD allows one per round:

Once you take a Reaction, you can't take another one until the start of your next turn.

So a Wizard who casts Counterspell can't cast Shield until their next turn. Counterspell itself has a Somatic component, so an enemy can Counterspell your Counterspell, if it still has its Reaction.

Your Counterspell against the SRD casters

Three SRD stat blocks carry Counterspell, through a Reaction called Protective Magic: the Mage (CR 6, Con +0), the Archmage (CR 12, Con +1) and the Lich (CR 21, Con save +10). The Archmage also has Magic Resistance:

Magic Resistance. The archmage has Advantage on saving throws against spells and other magical effects.

Counterspell is a spell, so the Archmage rolls its save twice. The Lich has Legendary Resistance (4 a day, 5 in its lair): "If the lich fails a saving throw, it can choose to succeed instead."

Chance your Counterspell stops the spell, by your spell save DC:

A level 5 Wizard with Intelligence 18 has DC 15. Against a Mage that is a good trade. Against an Archmage it fails more often than it works. Against a Lich the numbers don't matter until its Legendary Resistance runs out: each of the first four counters just burns one use. That is still worth something, since every use spent on your Counterspell is one the Lich doesn't have against the party's Banishment.

Their Counterspell against you

The Mage casts at spell save DC 14, the Archmage at DC 17, the Lich at DC 20. You save with Constitution, and the Sorcerer is the only spellcaster in the SRD proficient in Constitution saves ("Constitution and Charisma"). The Wizard saves with Intelligence and Wisdom.

Chance your spell is countered:

The Mage's Protective Magic works 3 times a day. If a Con +2 Wizard casts three big spells into it, the Mage stops at least one 90.9% of the time, and 1.65 on average.

What to do about it:

• Bait the Reaction. Protective Magic covers Shield too. A Fighter's hit that the Mage blocks with Shield leaves it no Reaction for your Fireball.
• Cast from beyond 60 feet. Fireball reaches 150 feet. Counterspell reaches 60.
• Don't lead with your best slot. A countered spell keeps its slot, so the loss is your action. A Bonus Action spell is the cheapest thing to have countered.

Dispel Magic, the rule

Choose one creature, object, or magical effect within range. Any ongoing spell of level 3 or lower on the target ends. For each ongoing spell of level 4 or higher on the target, make an ability check using your spellcasting ability (DC 10 plus that spell's level). On a successful check, the spell ends.

Using a Higher-Level Spell Slot. You automatically end a spell on the target if the spell's level is equal to or less than the level of the spell slot you use.

Dispel Magic sits on eight class lists: Bard, Cleric, Druid, Paladin, Ranger, Sorcerer, Warlock and Wizard. It is an action with a range of 120 feet, and it only touches ongoing spells. An instantaneous Fireball has nothing left to dispel.

The check is a plain ability check: your spellcasting modifier, no Proficiency Bonus. Chance to end a higher-level spell with a level 3 slot:

Bardic Inspiration adds to ability checks, so it helps here: a +4 caster against a level 5 spell goes from 50% to 67.5% with a d6, 72.5% with a d8. When the ally under Banishment or Polymorph matters, spend a level 4 or 5 slot and skip the roll.

Dispel Magic also works on enemies' protections. The SRD Mage's AC 15 includes Mage Armor, whose base AC is "13 plus its Dexterity modifier". Dispel it and the Mage drops to AC 12, so a Fighter with +5 to hit goes from 55% to 70%. The Mage casts Mage Armor at will, though, so the trade is your action for its action.

The Lich has Dispel Magic at will. Its check is Intelligence +5, so it ends your level 4 Greater Invisibility 60% of the time, every turn it tries.

Where this leaves you

• Counterspell is now a saving throw. Raise your DC; against Magic Resistance or Legendary Resistance, expect to fail.
• A countered spell keeps its slot. Both sides lose an action; only the counterspeller loses a slot.
• One Reaction a round: Shield or Counterspell, not both.
• Dispel Magic ends level 3 spells for free. Above that, roll or upcast.

Counterspell lives or dies on one d20, the Constitution save. The Book of Dice covers the d20 in Part I, The Object.

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.