All the way back in Original Dungeons & Dragons, the fireball (then two words) used to take up the full volume of a space. So, if you cast a fireball in a room to small to contain it, the blast would expand to other rooms.
For the curious, those two inches translate to 20 feet. This size persisted through time.
In 5th Edition, the fireball is a sphere of fire with a 20-foot radius:
That 20′ radius means fireball takes up 33,510 ft3 of space! However, that’s only in open space. In 5th Edition, a fireball expands around corners, but not beyond its 20′ radius. Sage Advice confirms:
But…what happens if we apply the “old school” backdraft rules in a dungeon?
We know that a fireball will fill a standard dungeon room. But how many?
Let’s assume a standard dungeon has 10-foot ceilings. This is probably high…
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