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Related: What exact dimensions does a physical cone AoE template need to have?

The rules on areas of effect for spells state (PHB, p. 204):

A spell’s effect expands in straight lines from the point of origin. 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.

However, the description of the fireball spell states, in part:

The fire spreads around corners. [...]

In the case of someone hiding around a corner, I would personally rule that they have total cover.

Does this mean that the flame from a fireball will ignore total cover?
Or does it only apply to one specific condition under which a creature can have total cover from another (for instance, if the creature was inside of a welded steel box)?

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RAW Fireball appears to ignore full cover, if there is a path fully within range of the effect that can reach the target, as this spell specifically travels around corners.

If there are no corners this spell can successively travel around within it's effect radius to reach the target, then the target cannot be reached by the effect, and has total cover, causing it to ignore the effect entirely.

Note, there is no limit specified by the spell, stating how many corners can be traveled around, or how arbitrarily complex the path it must follow can be. This leads me to conclude, if you can draw a line that remains entirely within the effect radius that reaches the target, they are hit by the spell.

Also, another thing to note, this is not a point originated explosion as in normal physics, but rather a magical effect that fills any area it can reach with magical flames.

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    \$\begingroup\$ This, if playing with minis/online the easiest way to handle fireballs is use a spell template overlay the player/GM can move around and see all areas affected. \$\endgroup\$
    – Joshua Aslan Smith
    Commented Jan 12, 2017 at 19:30
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    \$\begingroup\$ @JoshuaAslanSmith Yep, it's a very good way to do it. Keep in mind there are very specific cases where the overlay alone will be incorrect. For instance, standing in the interior corner of a building, covered by the AoE, but the spell originates from outside, with no doors or windows within the AoE. \$\endgroup\$
    – Randomorph
    Commented Jan 12, 2017 at 19:32
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    \$\begingroup\$ Might also be worth referencing Crawford's response: "Your cover is foiled if an effect spreads around it and reaches you." It relates more to non-total cover and the associated bonuses to Dex saves - but it would also cover the case where there's an obstruction between the creature and the point of origin of the spell, but part of the AOE is able to have an unobstructed path to the creature. \$\endgroup\$
    – V2Blast
    Commented Jul 25, 2018 at 21:07
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Yes if it can get to you.

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 was what's important in old editions, but not 5e. In a typical 10 foot wide, 10 foot high dungeon corridor, 33510.3 cubic feet exploded roughly 160 feet in each direction from its point of origin.

So since the range is only 150 feet the caster would be caught in the blast in older editions.

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 5e 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.

If you are within a 20 foot radius of the point of origin, regardless of how many corners there are, and water could get to you, then the fireball will get you. If you were sealed in cement, then no it won't.

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    \$\begingroup\$ I think you are flirting with a potentially correct answer, but you've included so much unrelated mathematics and physics into an answer mostly focused on whether or not elves can throw fireballs in the woods and hit eachother behind those trees that the elves and fireballs are being lost in the arithmetic. I think you can improve this answer substantially by excising the math and just focusing on the relevant sections from the rulebook and appropriate comparisons (i.e. if water could get to you, then the fireball will). \$\endgroup\$
    – Pyrotechnical
    Commented Jul 25, 2018 at 21:12
  • \$\begingroup\$ Yeah, the 1E/2E fireball was very bad in dungeons as it usually filled all the corridors including the part with you in it. \$\endgroup\$
    – Michael Shopsin
    Commented Oct 18, 2021 at 18:59
  • \$\begingroup\$ From a pragmatic point of view, i think this would overcomplicate things... Players could argue that "the wall has cracks and gaps that the fireball would penetrate", or "the fire spreads through the mouth, noses and God knows which other Froghemoth holes". A total cover should be considered as "sealed" for that purpose, in my opinion. \$\endgroup\$
    – Alan Régis
    Commented Jan 9, 2023 at 14:45
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Fireball ignores total cover as long as there is a corner within reach of the spell

Fireball:

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.

And this is the area of effect rulling:

A spell’s Effect expands in straight lines from the point of Origin. 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.

The fireball text allow it to circunvent some limitations of an area of effect spell, as long as there is a corner to expand. So a target behind a wall with a corner within the radius of fireball will still be affected. If there are no corners (the character is inside a locked building, for example) the fireball area of effect will not include past the cover.

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