[RPG] 20 foot square versus a 20 foot circle on a battle mat


A druid in my game used the Entangle spell, and it was pretty easy to know how a 20' square rendered on a battle mat with a 5' spacing grid. Now he also knows Spike Growth…

The Spike Growth effect manifests in a circle with a 20' radius, which had us debating what happened to the outer corners of the square (this might be a visual exercise for some). Imagine a 20' square on a grid with 5' spacing. Now plant a circle with a 20' diameter over that square. The 4 5×5' corners of the square are barely touched by the circle so I figure that they aren't affected by the spell. Furthering my thought process is that if it did affect these grid squares, then there would be no mechanical difference between a 20' square and a 20' diameter circle, and there would be no need for wizards to word things inconsistently.

So, using a battle mat how does a DM interpret the "corners" of a circle. Are there mechanics for this that I am missing? Failing that, are there any good interpretations anyone has heard of or used that I could try?

Best Answer

So, the best way to interpret this is to go back to the 3.5 version of D&D. In 4th Edition, there were no circular spell effects. Everything was a square to make things simple for the players and DM. Fifth Edition is going back to the earlier style of play where circles are more common, but on a grid, a circle isn't a circle at all (and they are not like the earlier poster submitted).

The "interpreted" circles are in the older Player's Handbooks and DM Guides, and they have been put up on the net in a number of places.

Here is one of the better guides that comes directly from an earlier stack exchange query (image below): What is a radius on a square grid?

And here is another PDF that one can cut for use on a battle map: http://www.enworld.org/forum/showthread.php?276089-Area-of-Effect-Templates

These exact templates are very likely to appear in the 5e Dungeon Master's Guide when it is out in November/December.

spell templates from earlier stack exchange answer