This excel spreadsheet implements the monster creation and CR calculation guidelines from pages 274-281 of the Dungeon Master's Guide. It includes the steps listed from those pages that effect CR. Fill out the yellow spaces in the spreadsheet as appropriate. The Offensive, Defensive, and Average CR will be computed at the bottom of the first tab. Only the first tab is needs anything entered when calculating the CR of a monster. However, the Traits tab does need to be set up the first time.
To be fully useful, the Monster Features from pages 280-281 of the DMG need to be entered into the Traits tab on the spreadsheet. These were kept out for copyright purposes. Only the 39 entries that actually affect CR need to be entered.
One thing I noticed as I was calculating the CR of monsters in the Monster Manual that were spellcasters: when calculating damage per round of an area of effect spell, the number of creatures hit by the area of effect spell was half what was recommended in "Adjudicating Areas of Effect" on page 249 of the DMG.
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