Spell Slots, Save DCs and Components, How Casting Works
A new player picks a Wizard, opens the spell list, and asks the question every table hears: "How many times can I cast this?" SRD 5.2.1 answers it in two pages. Here are the rules, quoted, with the dice worked out for a level 3 Wizard with Intelligence 16.
Spell level and slots
Every spell has a level from 0 to 9, which is indicated in a spell's description. A spell's level is an indicator of how powerful it is. Cantrips—simple spells that can be cast almost by rote—are level 0.
Spellcasting is taxing, so a spellcaster can cast only a limited number of level 1+ spells before resting. Spell slots are the main way a spellcaster's magical potential is represented.
The SRD's own example is our Wizard:
For example, a level 3 Wizard has four level 1 spell slots and two level 2 slots.
Six level 1+ spells a day, then. Cantrips cost nothing and never run out.
A small spell fits a big slot
Imagine a spell slot is a groove of a certain size—small for a level 1 slot and larger for a higher-level spell. A level 1 spell fits into a slot of any size, but a level 2 spell fits only into a slot that's at least level 2.
When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell takes on the higher level for that casting.
Many spells grow when they do. Magic Missile "creates one more dart for each spell slot level above 1". Burning Hands' damage "increases by 1d6 for each spell slot level above 1".
Finishing a Long Rest restores any expended spell slots.
A Wizard gets a little back sooner. Arcane Recovery, once per Long Rest, after a Short Rest:
The spell slots can have a combined level equal to no more than half your Wizard level (round up), and none of the slots can be level 6 or higher.
At level 3 that is two levels' worth: one level 2 slot or two level 1 slots. The two level 2 slots stay the scarcest thing on the sheet.
Casting without a slot
Cantrips. A cantrip is cast without a spell slot.
The Ritual version of a spell takes 10 minutes longer to cast than normal, but it doesn't expend a spell slot. To cast a spell as a Ritual, a spellcaster must have it prepared.
Rituals are free slots for anyone with ten spare minutes. Detect Magic before opening the chest costs nothing if nobody is chasing you.
One slot per turn
On a turn, you can expend only one spell slot to cast a spell. This rule means you can't, for example, cast a spell with a spell slot using the Magic action and another one using a Bonus Action on the same turn.
A cantrip plus a slotted spell in one turn is fine. Two slotted spells is not.
Components
If the spellcaster can't provide one or more of a spell's components, the spellcaster can't cast the spell.
Thus, a creature who is gagged or in an area of magical silence can't cast a spell with a Verbal component.
A Somatic component is a forceful gesticulation or an intricate set of gestures. A spellcaster must use at least one of their hands to perform these movements.
These materials aren't consumed by the spell unless the spell's description states otherwise. The spellcaster must have a hand free to access them, but it can be the same hand used to perform Somatic components, if any.
A Component Pouch or Spellcasting Focus stands in for any material that is neither consumed nor priced. A Wizard with a hand on each end of a rope casts nothing with S or M.
You must have training with any armor you are wearing to cast spells while wearing it.
The two numbers
Spell save DC = 8 + your spellcasting ability modifier + your Proficiency Bonus
Spell attack modifier = your spellcasting ability modifier + your Proficiency Bonus
Intelligence 16 is +3; a level 3 character has Proficiency Bonus +2. Save DC 13, spell attack +5. Write both at the top of the spell page.
The Wizard against a goblin
The SRD's Goblin Warrior: AC 15, 10 Hit Points, Dexterity save +2.
| Spell | Cost | How it lands | Goblin drops |
|---|---|---|---|
| Fire Bolt (1d10) | Cantrip | Attack +5, hits on 10+: 55% | 8.2% |
| Magic Missile (3d4+3) | Level 1 slot | Never misses | 68.75% |
| Magic Missile (4d4+4) | Level 2 slot | Never misses | 98% |
| Burning Hands (3d6) | Level 1 slot | Dex save, fails on 1–10: 50% | 31.25% each |
| Burning Hands (4d6) | Level 2 slot | Dex save, 50% | 47.8% each |
Fire Bolt averages 3.3 damage a turn against that AC, crits included. It wears a goblin down over three turns; it rarely ends one.
Magic Missile ends one goblin about two times in three for a level 1 slot. In a level 2 slot it is close to certain, and the four darts can split instead: two darts (2d4+2) drop a goblin only 6.25% of the time, so splitting rarely pays.
Burning Hands is the crowd spell. A failed save takes 3d6, which reaches 10 on 62.5% of rolls; a success takes half, which never reaches 10. Three goblins in the Cone: 0.94 dead on average from one level 1 slot. Upcast to level 2, 1.4.
At the table
Track slots with tick boxes, not memory: four small, two large. Say the slot level out loud when you cast. And before the fight, check your hands: a dagger and a torch leave no free hand for Somatic.
The Book of Dice covers the d4, d6 and d10 in Part I, The Object, and reading damage dice 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.
