Part of an ongoing series of 5e Rules notes.

It’s a classic, so much so that it’s inspired a dozen memes. It’s every magic-user’s favorite 3rd Level spell: Fireball!

So, what does that bad boy look like?

A bright streak flashes from your pointing finger to a point you choose within range and then blossoms with a low roar into an explosion of flame.

Each creature in a 20-foot-radius sphere centered on that point must make a Dexterity saving throw. A target takes 8d6 fire damage on a failed save, or half as much damage on a successful one.

The fire spreads around corners.

It ignites flammable objects in the area that aren’t being worn or carried.

From PHB 241.

What’s not to like? Hitting everyone in a large radius with 8d6 of fire damage?  It is Teh Awesome.

Of course, Fire is one of the most commonly resisted damage types (largely “thanks” to Fireball), and the save is on DEXterity, which is something a lot of bad guys have in abundance.

But, still, it’s pretty damned amazing. No wonder all the spellcasters cannot wait to get it, and then to use it.

So let’s talk about the rules.

The Rules of Fireball

Fireball has, traditionally in D&D, been a debate about physics, calculating the volume of the fireball, then the volume of the room, then figuring out the blowback if the latter is smaller than the former, etc.

5e has simplified this. Though the spell talks about an “explosion of flame,” the consensus is that, RAW, it acts more like a volume that is suddenly filled with roiling flame, as long as there is an open channel within range.

There’s no ka-boom that roars down the hallway like in Backdraft. 

Walls and doors, etc., block the effect.

All this does mean that Fireball can affect folk out of line of sight. The following picture (source unknown) illustrates:

The magic user on the steps casts Fireball in the middle of the corridor ahead. (Properly speaking, spells should anchor on an intersection, not in the middle of a square or an edge. But I digress.)

The lady around the corner gets hit, even though she’s out of Line of Sight from both the caster and the center of the spell, because the fireball spreads around the corner — within the 20 foot radius.

The figure in the room, though, is not hit because the doors are all closed. If the upper door by the lady was open, though, that figure would get hit, even if it’s a lot longer to walk from the center of the spell to that figure than 20 feet.

(I’ve seen some suggestion that the line-of-explosion has to go through full squares; that seems to be a DM call, though. Under that suggestion, the figure in the room would not get hit if the upper door was open, because the effect has to go through half-squares. This gets solved, though, by using a squares template, especially on a VTT, rather than drawing a circle.)

(And, no, we’re not going to worry, for purpose of area of effect, whether the doors catch on fire and burn through.)

References: 1 2 3 4 5 6