How does the DMG compute challenge rating?

The Dungeon Master's Guide derives a defensive CR from hit points adjusted by armour class, and an offensive CR from damage per round adjusted by attack bonus or save DC. The final CR is the average of the two, rounded to the nearest CR on the table.

How is the AC adjustment applied?

Each HP band has an expected AC. If your monster's AC is two points above or below that expected value, the defensive CR shifts up or down by one step per two points of difference. Very tanky or very squishy monsters are rated accordingly.

What counts as damage per round?

Use the average total damage the monster deals across its first three rounds of combat, then take the mean per round. Include multiattack, but assume its most damaging realistic action sequence rather than a perfect or worst case.

Should I trust the number blindly?

Treat it as a strong starting point. The DMG maths captures raw numbers but not abilities like flight, invisibility, legendary actions, or area control, which can make a monster play above its CR. Adjust based on how it actually performs at the table.

What proficiency bonus does my CR use?

Proficiency bonus rises with CR: it is plus 2 up to CR 4, plus 3 from CR 5 to 8, plus 4 from CR 9 to 12, and so on. The tool reports the matching proficiency bonus and the XP award for the final CR alongside the result.

D&D Custom Monster CR Builder

Homebrewing a balanced 5e monster means getting its challenge rating right so encounters land at the difficulty you intend. This builder runs the Dungeon Master’s Guide method: a defensive CR from hit points and armour class, an offensive CR from damage and accuracy, averaged into a final number.

How it works

The DMG splits CR into two halves that are then averaged:

defensive CR = CR(hit points), shifted ±1 per 2 points AC away from expected
offensive CR = CR(damage/round), shifted ±1 per 2 points to-hit/DC away from expected
final CR     = round( (defensive CR + offensive CR) / 2 )

Each side is looked up on the DMG’s “Monster Statistics by Challenge Rating” table, which pairs HP ranges and damage ranges with expected AC and attack values. The averaging stops a monster that is a glass cannon or an unkillable pillow from being rated purely on one extreme.

Example and tips

A monster with 130 HP, AC 15, 30 damage per round, and a +5 to-hit lands near CR 5. If you raise its AC well above the expected value for its HP, its defensive CR climbs and pulls the final CR up. Remember the table cannot see special abilities: flight, multiple legendary actions, or strong crowd control let a monster punch above its computed CR, so playtest and nudge the result to match real table performance.