I’ve been playing 5e for a while now, and mercifully this hasn’t come up yet, but it’s definitely on it’s way.

When a caster casts burning hands, how is that properly mapped onto a standard 1″ battle mat? The easy response is a 3×3 square, and that’s the method I’ve told myself I’m going to use. But is there a better template?

The 3×3 grid works great if the caster casts on a corner, it matches the description of a cone perfectly (each square is 5′, 10, 15′ away for the purposes of grid math). However, when a caster casts the spell in a cardinal direction (N/S/W/E), that math breaks down. now a second or even third square is adjacent with a 3×3 square and the spell description has fall apart.

What are the appropriate grid shapes for a 15′ cone?

Guidance for mapping areas of effect defined by continuous lines onto discrete grid squares or hexes is provided on page 251 of the DMG in the section on using miniatures in combat.

It’s short and sweet: follow the rules for laying out the shape of the area “as normal” to find what targets are under/within the shape; for circular areas, at least half of the square/hex must be covered to be affected.