A 30x30 or even 40x40 room would be far more satisfying. Fireball’s radius is 20ft, diameter is 40ft.
this post was submitted on 15 Apr 2026
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Fireball packing in a room is a classic wizard recreational math problem.
This isn't quite the same problem, though.
Wouldn't the wizard version be:
What is the arrangement of circles which can completely cover a given area, leaving no gaps, using the fewest circles?
In this case, the circles are allowed to overlap.
I'd be more be glad it's not Shadowrun, which has the chunky salsa rule with grenades in enclosed spaces.
Mathematically, a circle is defined by all points that have the same distance to the center point. In d&d on a grid, distances are measured in 5ft squares, where a diagonal distance is the same as an orthogonal distance. This results in mathematical circles being square on a 5e/5.5e d&d grid
so, one fireball, having a mathematical 20 ft radius, can completely fill a square room of 40x40 ft.
Yes I know the book has a circular template for spell effects. However, using that on a grid has the weird effect that a target can be in a place that is 20 ft removed from the center (diagonally) but at the same time not be in the area of effect of a fireball cast on that center point
I hate that the default grid mechanics in 5(.5)e are square circles, like, they really think people who want precise grid based combat aren't capable of using the alternating 5/10 ft. diagonals rule
I have used both types of distance calculation in different campaigns. I agree that the 'alternating' rule is better, but I've also seen that it confuses some players and in that case slows down combat.
I'm currently using the phb rule. My players have just gained access to fireball and similar AoE spells, and I use the actual circular spell effects
So far I have not seen the players actively abusing this, so for now I'm just ignoring the weird distance effects