How the Shield Spell Works in 5e
Shield is a level 1 Sorcerer and Wizard spell you cast as a Reaction when an attack roll hits you. You get +5 to AC until the start of your next turn, and the bonus counts against the attack that triggered it, so a hit that beat your AC by 4 or less becomes a miss. It also blocks Magic Missile completely. It costs a spell slot and your Reaction for the round, and a Critical Hit still hits. Below: the spell quoted, the timing rules around it, and what it saves a level 1 Wizard against goblins and an Ogre.
The rule
Casting Time: Reaction, which you take when you are hit by an attack roll or targeted by the Magic Missile spell
An imperceptible barrier of magical force protects you. Until the start of your next turn, you have a +5 bonus to AC, including against the triggering attack, and you take no damage from Magic Missile.
Range: Self. Components: V, S. Duration: 1 round. No Concentration.
Three things follow from the wording:
- You cast it after the hit. The trigger is being hit, so you never waste it on an attack that was going to miss. You only waste it on a hit it can't turn.
- It lasts the whole round. Every attack against you until the start of your next turn faces AC + 5, not just the first one.
- A natural 20 still hits. A 20 on the d20 hits whatever your AC is, so Shield can't stop a Critical Hit. The +5 still covers the rest of the round.
A Reaction, once a round
The Reaction rule:
When you take a Reaction, you can't take another one until the start of your next turn.
So Shield, Counterspell and an Opportunity Attack share one slot each round. If you Shield the goblin's sword, the enemy caster's spell gets through.
There is one more limit, from the spellcasting rules:
On a turn, you can expend only one spell slot to cast a spell.
The limit is per turn, and it counts every creature's turn. If you cast Magic Missile with a slot on your turn and an Opportunity Attack hits you while you walk away, you can't cast Shield with a slot until that turn is over. On the goblins' turns you can. The Magic Initiate feat gets around this: a Wizard-list pick can be Shield, cast "once without a spell slot" per Long Rest.
How many hits it turns
Shield only turns hits that beat your AC by 0 to 4. Here is how often a hit falls in that range, for a Goblin Warrior (Scimitar +4) and an Ogre (Greatclub +6):
| Your AC | Goblin hits, then with Shield | Ogre hits, then with Shield | Share of hits turned (goblin / Ogre) |
|---|---|---|---|
| 12 | 65% → 40% | 75% → 50% | 38% / 33% |
| 15 | 50% → 25% | 60% → 35% | 50% / 42% |
| 18 | 35% → 10% | 45% → 20% | 71% / 56% |
The higher your AC, the more of the hits that land are close calls, so Shield turns a bigger share of them. A Wizard in Mage Armor gets more from each casting than one in a robe.
What it saves a level 1 Wizard
A level 1 Wizard with Dexterity 14 and Constitution 14 has 8 Hit Points and AC 12, or AC 15 under Mage Armor. Goblin Warriors swing for 1d6 + 2. The Wizard casts Shield on the first hit of the round. If that hit can't be turned (a 20, or a roll that clears AC + 5), it lands, and the +5 still covers the attacks after it.
Chance the Wizard drops, without Shield → with it:
| Attackers this round | AC 12 | AC 15 (Mage Armor) |
|---|---|---|
| 1 goblin | 13.6% → 9.4% | 11.1% → 6.9% |
| 2 goblins | 48.8% → 26.3% | 34.4% → 16.3% |
| 3 goblins | 73.6% → 43.5% | 55.8% → 26.6% |
The more attacks come in, the more Shield is worth, because one casting covers all of them. Against three goblins, Mage Armor and Shield together cut the chance of going down from 73.6% to 26.6%.
One Ogre swing, at 2d8 + 4, drops an 8 HP Wizard 71.7% of the time at AC 12 and 57.4% at AC 15. With Shield the odds are 47.9% and 33.6%.
Shield against Magic Missile
Magic Missile never misses: three darts of 1d4 + 1 Force damage, 10.5 on average, more with a higher slot. Shield is the one SRD spell that says you "take no damage from Magic Missile". The trigger is being targeted, so you see it coming and cast before the darts land.
The SRD's Mage and Archmage both carry it:
Protective Magic (3/Day). The mage casts Counterspell or Shield in response to the spell's trigger, using the same spellcasting ability as Spellcasting.
Three uses a day means three rounds of +5 AC, on top of the Mage's slots. Read Counterspell vs Dispel Magic for baiting that Reaction out before your big spell.
Shield next to other AC boosts
| Source | Bonus | Cost | How long |
|---|---|---|---|
| Shield spell | +5 | Reaction, level 1 slot | Until the start of your next turn |
| Shield (armor) | +2 | Takes a hand; needs Shield training | While you wield it |
| Shield of Faith | +2 | Bonus Action, level 1 slot, Concentration | Up to 10 minutes |
| Mage Armor | Base AC 13 + Dex | Action, level 1 slot, no armor | 8 hours |
Shield of Faith works on any creature within 60 feet and lasts a whole fight. The Shield spell only protects you, for one round, but at +5 it is bigger than anything else on a level 1 slot. The full AC math is in Armor Class, What Each Point Buys. And with only two level 1 slots at level 1, Spell Slots, Save DCs and Components covers what else wants them.
When to cast it
- When you're hit and several attackers are still to come. The table above: the third goblin is where Shield pays.
- When a hit would drop you. On a hit worth more than your remaining HP, cast it.
- When Magic Missile targets you. It's a sure 10.5 damage blocked.
- Not when Counterspell is the bigger need. If an enemy caster is about to act, keep your Reaction.
Odds are exact d20 and dice counts. Rules and stat blocks from the SRD 5.2.1: Shield, Shield of Faith, Mage Armor, Magic Missile, Magic Initiate, Reactions, Casting Spells and the Monsters chapter.
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.
