I recently had a problem come up in a session that I wanted to see if there was more information on. One of my players is playing a Light Domain cleric; its 1st-level Warding Flare feature says, in part (PHB, p. 61):

When you are attacked by a creature within 30 feet of you that you can see, you can use your reaction to impose disadvantage on the attack roll, causing light to flare before the attacker before it hits or misses.

The cleric recently gained the ability to fly, and wanted to use his Warding Flare feature on an ally while flying. He was 30 feet up and 20 feet out, and when I calculated the distance between him and the target it should have been about 35 feet, so I thought he was out of range.

He argued that in D&D, it shouldn't matter so long as he is not more than 30 feet out, and 30 feet higher than the target. Which means his 30-foot-range ability can extend to 42 feet total.

I allowed it because it wasn't a big deal at the time, and I would just spend time looking for the rules after the session, but I haven't found any. Are there any good clarifications on 3D situations such as these?

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    \$\begingroup\$ Is the player by any chance a veteran of D&D 4e? \$\endgroup\$ – Ryan C. Thompson Oct 5 '19 at 14:32
  • \$\begingroup\$ He played 3.5 until I introduced him to 5e \$\endgroup\$ – Oxter Oct 5 '19 at 15:12
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    \$\begingroup\$ Welcome to RPG.SE! Take the tour if you haven't already, and check out the help center for more guidance. I've edited the question to cite the relevant feature; please make sure to note the edit matches your intent. (Though, as you note, the exact feature is not that relevant to the overall question.) \$\endgroup\$ – V2Blast Oct 6 '19 at 3:40

There are no special rules for 3D distance

Emphasis on the special here. The default rules already assume a 3 dimensional world. Hence why areas of effects are described as 3D geometrical shapes such as spheres, cubes or cylinders rather than 2D shapes.

The DMG pg. 249-252 has some optional rules for estimating how many creatures are caught up in an area when using theatre of the mind or anything else than doesn't involve precisely figuring out where everything is in relation to everything else and some rules for translating ranges into square grids ("If an area of effect is circular and covers at least half a square, it affects the square").

In general however, the rules assume normal euclidean distance, so your guess of "about 35" is roughly right (36.something but it doesn't matter at this point). The DMG recommends using things like tape measures when estimating distances on a tactical map and otherwise just letting the DM make a judgement call.

It's perhaps worth noting at this point that things like grids and tactical maps are considered optional rules in 5e, while the PHB and DMG offer a little guidance of how to play using them it's generally expected that groups come up with their own ways of managing such minutiae.

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    \$\begingroup\$ It may also be worth noting that the DMG includes an optional rule for "diagonals" (p. 252; emphasis mine): "When measuring range or moving diagonally on a grid, the first diagonal square counts as 5 feet, but the second diagonal square counts as 10 feet. This pattern of 5 feet and then 10 feet continues whenever you’re counting diagonally, even if you move horizontally or vertically between different bits of diagonal movement." \$\endgroup\$ – V2Blast Oct 6 '19 at 3:40

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