Spell Areas on a Grid in the d20 System

In AD&D 1st and 2nd Edition, miniatures rules were optional or even vestigial. Gygax wrote, “I don't usually employ miniatures in my RPG play. We ceased that when we moved from CHAINMAIL Fantasy to D&D” (link).

With D&D 3rd Edition and the d20 System, the new designers worked to make a closer connection with miniature play on a grid (and collection of said miniatures). However, in its first appearance (2000), there were no hard-and-fast rules for such things as measuring distances or placing spell areas on the grid. There is a section in the DMG on “Using Miniatures and Grids” (p. 67-69), but it counsels, “Realize ahead of time that you will have to make ad hoc rulings when applying areas onto the grid.” None of the guidelines there appear in the 3.0 PHB or the Open-Gaming SRD core rules document. (In fact, the text suggestions and illustrations are at many points contradictory.)

These rules became more solidified in the D&D 3.5 Revision, the SRD, and the later Pathfinder game. A first formalization was, “The point of origin of a spell is always a grid intersection.” This was suggested in the 3.0 DMG, but appeared nowhere in the 3.0 PHB or SRD, and was contradicted in the PHB illustration for the meteor swarm spell, for example. A second formalization was, “When measuring distance, the first diagonal counts as 1 square, the second counts as 2 squares, the third counts as 1, the fourth as 2, and so on.” Note that this rule appeared nowhere in the 3.0 text, and the text that does exist subtly contradicts it (see: “majority of a grid square” rule, DMG p. 69; and do the integral calculus).

In the somewhat ambiguous era of 3.0, this author wrote an essay on the different possible interpretations at the time, and put forward three different options to make the system well-defined.  With greater perspective, it is now easier to interpret the 3rd Edition designers’ intent along the lines of the 3.5 rules (which do in fact match the 3.0 DMG spell illustrations, if not the text). While this authors’ graphics for spell templates proved to be quite popular (continuously the top download on the Superdan.net site through 2017!), it seems due time to make them more concise and easier to locate.

Area Accounting

Below is an accounting, for both 3.0 and 3.5, of how many times each of the areas is referenced in the given spell listings. In theory, this highlights which areas would be the most used, necessary, and recommended for having a physical template.

Coloration indicates inclusion in the new document: Green items are included at scale for printing; Yellow included but cannot be at tabletop scale; Red are excluded from the document. Generally, 3.5 reduced variation in its areas: The one-off types in 3.0 (burning hands and meteor swarm) are removed in 3.5. Whereas 3.0 used circles of any radius 5 to 40 feet freely, 3.5 mostly only uses 10, 20, and 40 feet. The variable-range, triangular-style cones in 3.0 were replaced by fixed-range quarter-circle cones in 3.5.

Another important category is not included above: the many conjuration-type spells (such as summon monster) which have a restriction like, “no two of which can be more than 30 ft. apart”. Now, this is not exactly the same as a 30-foot circle, but it’s very close (we can place 3 creatures each 30 feet apart, and require a 35-foot radius circle to enclose them all). There are 44 such spells in 3.0, and 58 such spells in 3.5. So this would suggest that one should have a 30-ft circle on hand for such castings.

Large-Scale Implications

One may note that the implication of the 1-2-1-2 diagonal metric is that for any quarter-circle, the far edge gets shorter by one box for every 2 steps taken – in different directions on each side of the diagonal line itself. This means that in the limit, normalizing for scale, our circles really look like octagons on the grid. Below is an example for a 100-ft radius circle (center and edge only shown) as per this rule. However, this is a larger radius circle than any actually specified in the game, so this behavior will usually not be apparent to players.

Other Options

Disclosure: The d20 System rules provided an enticing mathematical subject of investigation. That said, I haven’t actually played a d20 System game, or use these rules on a grid, in many years. Generally I play classic D&D where miniature usage is relatively rare, and when it does happen, the movement rules are very rough and casual (I may let people count each diagonal as “one”, or else hand out a ruler, depending on mood).

Likewise, for spell-effect areas, I wouldn’t actually use grid templates on paper, as they seem too burdensome, fussy, and difficult to place with miniatures in the way. What I have used in the past are either an ink-pen compass, or a set of brass rings of appropriate sizes, which I found to be much easier to use.

Brass ring

Ink compass


Click the link below to access the document on revised spell areas on a grid for the d20 System.


  1. This is good work.
    Some random thoughts.
    I have to say we never gave the topic much worry. When we started playing it was a mix of basic and 2nd ed (those were the books we had) and heavy use of minis/grid. Negotiating with the DM about who/which squares were in the spell area was part of the process. So when 3e came around, we just plowed on through with our established approach. When we did read the 3.x rules it did always seem contradictory. Spells at the intersection even though the caster is in the "middle" of the square. Some quarter circles contain more squares than others, what if I want to fire my spell at a 30* angle instead of 45* to catch that one extra guy or two.
    I wonder if this is why some aspects of 4e went to "X number of squares" instead of trying to account for the variations between linear distance and grid lines.

    1. Thanks!

      Totally agree that these square-by-square formalization are a troubling pit to fall into. In particular, the center-on-a-vertex idea is really surprising and counterintuitive when a new player runs into it, for exactly the reason you mention (and likewise on the target side).

  2. Funny, from a 1E/2E perspective, I always assumed a spell with radius or diameter must have a target point as a grid intersection. Otherwise, if you count from the edge of a target square, your diameter will always be 5' greater than it should be. Also, using metal rings is an awesome idea, and you could make them with some wire instead of trying to find precise ones. My method for finding the area of a Fireball, for example, is to have the player choose where the target is and the DM draws the area. In this way it's a bit unpredictable for the player, and trying to blast a few enemies in melee with friends is risky. Of course it requires that the players trust that the DM is acting as a neutral arbiter and isn't going to screw them. As for determining whether someone is in or out of the area, I'll grab a ruler and see if the radius hits the center of the victim's square. But it might be nice to have a "no damage on save, half on fail" for anyone in a partially-affected square.

    1. I agree that the original missile spells need some variation. I still use the optional rule from Chainmail: pick a target, roll 2d6: each pip under 7 is 1" short, each pip over 7 is 1" long.

    2. I like that.
      I am tossing with the idea to let the magic user roll a D30. 11-18 means 1 square in the direction of N, NE, E etc. and 21-28 2 squares in these directions. All other result means 'spot-on'.
      So, no more high accuracy fire ball droppings etc.

    3. Ha, I will steal your idea, but let them also roll a d8 for direction (and distance is abs(7-number of pips) )