On page 249 of the DMG, under Adjudicating Areas of Effect, it has this table with conversions for estimating how many targets are hit with an area of effect spell when using a battle grid. I’ve found that in both of my current campaigns, bringing out the battle grid usually slows things down significantly, and doesn’t always add to the interest of the combat. Sometimes it’s just a fight and there aren’t a lot of interesting tactical options available, but people still spend more time counting squares and so on. In brief, it’s less fun.
When using “theater of the mind” to run crunchy systems like D&D, though, the big problem is areas of effect. Who gets hit? Can you avoid hitting allies? Etc. So what I did, as a resource for myself at least, is I went through the various areas of effect I could find in the PHB and ran them through the formulas on page 249. Here are the results, with an added option of a random roll to determine targets:
| 15’ arc (burning hands) | 2 targets | 1d4 |
| 15’ radius (darkness) | 3 targets | 1d6 |
| 30’ radius (auras) | 6 targets | 1d12 |
| 30’ cone | 3 targets | 1d6 |
| 60’ cone | 6 targets | 1d12 |
| 20’ radius (fireball) | 4 targets | 1d8 |
| 40’ radius (conjure volley) | 8 targets | 2d8 |
| 60’ radius (daylight) | 12 targets | 2d12 |
| 100’ line (lightning) | 3 targets | 1d6 |
| 10×10’ cubes (fire storm) | 20 targets | 2d20 |
Did I miss an area of effect here? Did I forget something?
Hope these are helpful, mind-thespians!
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>unfinished 
You round up, so 100′ line would target 4 creatures, not 3 for example. As for the die rolls, I really like the idea. It’d be hard for me to implement in my own games though.. hmmm… Will have to see if there’s some easy way of implementing it. Like a “this many creatures targeted gives this kind of die roll” (like 3 targets = d6, 5 targets = 2d4, etc.) Gives some variance, which could be good.
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Thanks for the comment! My thinking with the rounding down is that it would encourage the die-roll, as in with my own players, just because it’s a bit more dynamic (or has the potential to be). If you’re guaranteed hitting 4 targets, there’s really little incentive to roll 1d6 for the total, at least in my own thinking. But one could easily not use the die rolls and just add one to all of the totals in the table – good point.
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Sorry I know this was several years ago, but you need to put the spells point of origin in a square, not on the junction of the grid. You measure from the square of origin.
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