How Spellcasting Works in D&D 5e: Spell Slots and Concentration

26 March 2026

Spellcasting is one of the most powerful and most confusing parts of D&D 5e. New players who choose a spellcasting class often spend their first few sessions unsure what they can cast, when they can cast it, and why their slots don’t regenerate after every fight. Here’s everything you need to understand to actually use magic effectively.

Cantrips: the unlimited option

Cantrips are level-0 spells. You can cast them as many times as you want — they don’t use any resources. They’re your default option when you’ve run out of spell slots or want to conserve them.

Most cantrips deal modest damage or provide a useful utility effect. A Wizard’s Fire Bolt deals 1d10 fire damage and can be cast every turn forever. A Cleric’s Sacred Flame deals 1d8 radiant damage and can bypass some enemy resistances.

As you level up, cantrips scale automatically — typically dealing extra dice of damage at levels 5, 11, and 17.

When to use cantrips: When you don’t want to spend a spell slot. When a fight is going well enough that you don’t need to escalate. When you’re going to cast every turn and want to save your better spells.

Spell slots: your limited resource

Spell slots are your “fuel” for casting spells of level 1 and above. Each time you cast a spell of level 1 or higher, you expend one slot of the appropriate level or higher.

You regain all expended spell slots after completing a long rest (at least 8 hours of sleep/light activity).

The slot system:

• At level 1, most spellcasters have 2 first-level slots.
• By level 5, they have slots of level 1, 2, and 3.
• By level 9, they have slots up to level 5.
• At level 17+, they have slots up to level 9.

Short rests (1 hour of rest without combat) don’t generally restore spell slots — except for Warlocks, who are a special case.

Upcasting: spending higher slots for bigger effects

You can always use a higher-level spell slot to cast a lower-level spell. Many spells scale upward when you do this.

For example, Cure Wounds heals 1d8 + your spellcasting modifier when cast with a 1st-level slot. Cast it with a 2nd-level slot and it heals an additional 1d8. A 3rd-level slot, another 1d8.

Some spells don’t scale at all when upcast — they’re equally effective regardless of slot level. Read your spells to find out which ones are worth upcasting.

Rule of thumb: Upcasting healing spells and damage spells with clear scaling text is usually worth it. Upcasting spells that don’t specifically mention scaling is usually a waste of a higher slot.

Concentration: the most important rule new spellcasters miss

Some of the most powerful spells in D&D require concentration. You can only concentrate on one spell at a time. If you cast a second concentration spell, the first one ends immediately.

The more important rule: When you take damage while concentrating, you must make a Constitution saving throw to maintain concentration. The DC is 10, or half the damage you just took — whichever is higher.

Fail the save, the spell ends.

This means:

• You should have a decent Constitution score if you rely on concentration spells.
• Standing in melee range as a concentration spellcaster is risky.
• The War Caster and Resilient (Constitution) feats both help maintain concentration.

Concentration spells include (among others): Bless, Hex, Hunter’s Mark, Hold Person, Polymorph, Concentration Spells, Hypnotic Pattern, Conjure Animals.

Non-concentration spells like Fireball, Magic Missile, and Healing Word require no sustaining — cast them and they’re done.

Prepared spells vs. spells known

Different classes handle spell selection differently.

Spells known (Bard, Ranger, Sorcerer, Warlock): You have a fixed list of spells that you know. You can only cast spells from that list, and you can’t change which spells you know between sessions (you swap one out when you level up).

Prepared spells (Cleric, Druid, Paladin, Wizard): You have access to a larger spell list (your full class spell list or spellbook), and each day you choose which spells to prepare. Only prepared spells can be cast that day — but you can change your preparations every long rest.

A Cleric with 14 Wisdom and 3 levels in Cleric can prepare 3 + 2 = 5 spells per day, chosen from the full Cleric spell list. A Wizard can prepare a number equal to their Intelligence modifier + their Wizard level.

Practical difference: Prepared-spell classes are more flexible but require more decisions. Known-spell classes are simpler but less adaptable to unusual situations.

Spellcasting ability

Your spells that require a saving throw use a spell save DC:

Spell save DC = 8 + proficiency bonus + spellcasting ability modifier

When you cast a spell that attacks (like Fire Bolt or Eldritch Blast), you make a spell attack roll:

Spell attack modifier = proficiency bonus + spellcasting ability modifier

The spellcasting ability varies by class:

• Intelligence: Wizard
• Wisdom: Cleric, Druid, Ranger
• Charisma: Bard, Paladin, Sorcerer, Warlock

Put your highest stat into your spellcasting ability. Every point of modifier increases both your attack rolls and your save DCs.

Components: verbal, somatic, material

Most spells require components to cast:

• Verbal (V): You must speak aloud. Silenced characters can’t cast V spells.
• Somatic (S): You must make gestures with at least one hand. Restrained or bound characters struggle with S components.
• Material (M): You need a physical item. Most material components are trivial (a pinch of sand, a bit of wool) and are assumed to be in your component pouch. Some expensive components (noted with a gold piece cost in the spell) must be present and may be consumed.

In practice, most players don’t track material components unless they’re expensive or consumed. Your DM will usually tell you if they matter.

Ritual casting: free spells with time

Some spells have the ritual tag. Classes that can ritual cast (Clerics, Druids, Wizards, some Bards) can cast these spells without expending a spell slot — but it takes 10 extra minutes.

Ritual spells include Detect Magic, Identify, Comprehend Languages, and Find Familiar. If you have time and don’t need the spell immediately, ritual casting is free. Use it liberally.

The short version

• Cantrips are free and unlimited.
• Spell slots are limited and come back on a long rest.
• Concentration is fragile — protect it.
• Prepared-spell classes choose daily; known-spell classes have a fixed list.
• Your spellcasting ability modifier determines your DC and attack bonus — maximise it.

Spellcasting rewards preparation. Know your three most-used spells well and you’ll handle most situations. Learn the rest over time.

The full spellcasting rules are in Chapter 10 of the Player’s Handbook, followed by all spell descriptions in Chapter 11. (Affiliate link — we earn a small commission at no cost to you.)