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The fireball spell is clearly described as a sphere with a 20' radius.

In most case, that is applied at ground level since that's where most creatures are. (We can actually see an example of the area in: 20 foot square versus a 20 foot circle on a battle mat.)

If I think in terms of the law of physics, I would assume that for a fireball cast with its center on the ground, all the energy which is expected to go down has to go somewhere else. Similarly, if casting that same fireball in a small room or a corridor, wouldn't that affect people further than 20' radius around?

How does this actually work according to the rules of the game though?

Are we expected to just lose all the extra area of effect, or does the fireball expand outward to fill its full normal volume? So in a corridor, would you only affect 40' diameter and the extra energy that would create the rest of the sphere just never materializes (the effect is blocked by the walls of the corridor), or do I calculate the volume and fill that much space in the nearby corridors?

Specifically, I'm interested in understanding how to interpret the rules in this case.

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Unlike some previous editions in which the fireball will adjust based on the space available and spread down corridors if contained, 5th ed simply states that the fireball will take up the amount of space listed as 20' radius sphere and spread around corners to fill that space, never expanding to exceed that distance from the point of origin. This specifically breaks the line of sight rule on page 204.

So if cast at the entrance of a 10x10 room with a 5' corridor leading into it, it shall fill that room and spread 20' down the corridor.

Fireball PHB PG 242
The fire spreads around corners.

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Oddly enough the laws of physics forgot magic. The energy is created magically and fills the unobstructed radius,per the PHB. Unobstructed includes any open connected space.

No expansion is required as the fireball is created in place (as an instantaneous expansion) , centered on the target(for a pseudo physics explanation).

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The fire would be blocked by the walls.

From the Area of Effect section on page 204 of the PHB:

If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell’s area. To block one of these imaginary lines, an obstruction must provide total cover, as explained in chapter 9.

Since the walls of a corridor or small room would provide total cover for anything beyond them, the spell's actual area of effect is limited by the size of the corridor.

To clarify, I'm not saying it doesn't spread around corners: the text of the spell clearly states that it does and specific beats general. What I am saying is that the walls would condense/contain some of the fire. If, as Alexis Wilke states, "the fireball is cast with its center on the ground" than you don't magically have a 20 foot radius semi-circle of fire beneath the ground because the area beneath the ground is not included in the spell's AoE due to the text I previously quoted.

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    \$\begingroup\$ As pointed out by Tashio, the fireball has text that clearly says it goes around corners. But it is good to know the snippet that you gave because other spells will be affected by that. \$\endgroup\$ – Alexis Wilke Oct 13 '15 at 7:44
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Dunno if anyone posted this already but a Fireball is a 20 foot radius sphere according to PHb pg241.

A sphere with a 20 foot radius has: a volume of 33510.3 cubic feet an area of 5026.55 square feet And a circumference of 125.664 feet

The volume is what's important. In a typical 10 foot wide, 10 foot high dungeon corridor, 33510.3 cubic feet explodes roughly 160 feet in each direction from its point of origin.

So since the range is only 150 feet (again PHb of 241) the caster will be caught in the blast.

Now just to complicate things a little, the definition of a sphere is "a set of points in three dimensional space that are located at and equal distance r (the radius) from a given point (the center point).

Does that mean the Fireball is hollow and only burns around the surface of the sphere? If so, does it maintain its 20 foot radius into the corridor and burning the ground or into other rooms around it rather than using the tunnel like the barrel of a cannon?

Since RAW say it "blossoms with a low roar into an explosion of flame" and "spreads around corners" I interpret that to mean it doesn't just appear as a hollow sphere of fire, but grows into one. I still give that a maximum of 20 foot radius and don't have it expand mathematically to full 33510.3 volume capacity because the spell specifically states 20 foot radius and not 33510.3 volume.

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    \$\begingroup\$ Hi there! How has this worked out in your games? DnD 5e is purposefully non-simulationist, but that doesn't mean you can't run it that way. Please keep in mind, though, this is a rules question and not one of real-world physics. Also, check out the tour while you're here! It's a good way to get oriented with the site. \$\endgroup\$ – Jason_c_o Jun 29 '18 at 17:05
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In 4th edition D&D and onwards, WotC started simplifying the game mechanics. While it does make the game easier to learn or figure out, it does lead to some occurrences like the one you mention here where the Fireball magically ends at a radius of 20.'

As a possible house rule, you could have it be that Fireball affects 16 five-foot squares and if you cast it in a narrow hallway it expands out both directions until those 16 squares are met. I think this is the usual ruling from earlier editions, as I remember hearing plenty of stories about narrow hallways and fireball shenanigans.

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    \$\begingroup\$ This doesn't appear to answer the question currently? (The question isn't seeking house rules, it's seeking a clarification on how it actually does work.) \$\endgroup\$ – SevenSidedDie Oct 29 '15 at 2:17
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The Blast of a Fireball has a radius of 20' so that takes up 4/3*pi*20^3 of space or 33,510 feet of space. Just use LWH to calculate your area the blast should be spread evenly when dealing with areas less that the 40'x40'x40' sphere of fire.

Example you cast fireball at a 4 way intersection the halls are all 6' wide by 10' high. L*W = 60 so just 33,510/4=8,377.5 (4 corridors) then 8,377.5/60 = length of spread down each hall is 139.625' pending it running into another room which causes it to spread out reducing how far it travels.

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    \$\begingroup\$ Welcome to the site! Please note that this question is explicitly about what the rules say should happen, not what physics suggests should happen (to the extent that physics applies to magic). \$\endgroup\$ – Oblivious Sage Apr 24 '17 at 21:34

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