[RPG] a radius on a square grid

battle-mapdnd-3.5espells

So last night, my D&D group ran into an interesting question. The spell Entangle says:

Area: plants in a 40-ft radius spread.

We (like a lot of people, I think) play D&D on a square grid, where each square represents a 5'x5' area. In that context, how do you find the area when you're given a radius?

The books seem unclear, probably since the definition of a radius is fairly simple. My suggestion was to treat it the way you treat range: start at the center, and any square you can get to with 8 five-foot 'steps' is affected. Someone else thought it should be a square that was 80'x80'; that is, a 'circle' that's 80 feet across, since a lot of other 'circles' in D&D are actually treated as squares. That's a simply huge area, though — bigger than the board we use. We wound up using a 40×40 square, since my counting would take more time and the 80×80 square was huge. 40×40 was still big enough to encompass the entire fight.

My question: how does a radius measurement (and by extension, circles in general) work on a square grid?

Best Answer

You can extrapolate from these spell area diagrams and the rules for determining the exact radius in squares.

Spell Areas

Regardless of the shape of the area, you select the point where the spell originates, but otherwise you don't control which creatures or objects the spell affects. The point of origin of a spell is always a grid intersection. When determining whether a given creature is within the area of a spell, count out the distance from the point of origin in squares just as you do when moving a character or when determining the range for a ranged attack. The only difference is that instead of counting from the center of one square to the center of the next, you count from intersection to intersection.

You can count diagonally across a square, but remember that every second diagonal counts as 2 squares of distance. If the far edge of a square is within the spell's area, anything within that square is within the spell's area. If the spell's area only touches the near edge of a square, however, anything within that square is unaffected by the spell.

(The diagram and text are from Pathfinder, but these rules have not changed since 3.5.)

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