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What's the Challenge Rating of a 10th-level NPC stripped of all equipment? What's the CR of a ghoul outfitted with the same amount of gear as a 12th level NPC? Just enter a creature's CR and the total value of its gear into these form fields, and this JavaScript calculator will guestimate the adjusted CR.

How does this work? The D&D Challenge Rating system provides useful guidelines as to the impact of an encounter on a standard party of 4-5 PCs. A key premise of this system is that a single classed NPC of X levels (and no monstrous level adjustment) is also a CR X encounter.

But only part of this challenge rating is due solely to class levels, and another portion is due to benefits provided by equipment. In an interesting discussion on usenet, Doug Lampert suggested that a good rule of thumb is to assume that 1/3 of an NPC's CR value is due to equipment.

Doug then offered a pretty handy guestimator to assess the impact of equipment on any creature's CR. Because D&D has standardized wealth values for each NPC level, it is possible to take the total value of a creature's equipment, find a matching level on the NPC wealth tables, and then use this "gear level" to estimate how this equipment affects challenge rating.

First, get the creature's base CR. For an NPC, this is simply the NPC's level; for a monster this is simply the monster's out-of-the-book CR. Second, take the total value of the creature's usable equipment and find the closest matching entry on the NPC Wealth by Level table; this give you the Gear Level. With the base CR and Gear Level in hand, a simple weighted average provides an adjusted challenge rating:

Challenge Rating (with equipment) = ((2 x Base CR) + Gear Level)/3

Thus, a 9th-level NPC has a base CR of 9. If this character were outfitted with the standard 9th-level gear (12,000 gp total value), the CR works out to be ((2 x 9) + 9)/3 = CR 9. The same NPC with only 1st-level gear would be ((2 x 9) + 1)/3 = CR 6. The same NPC with gear appropriate to an 18th-level character (130,000 gp total value) is ((2 x 9) + 18)/3 = CR 12.

Keep in mind this is only a rough estimate. In my experience, Doug's system is pretty reliable up to the most extreme cases. In particular, this formula tends to overestimate the benefit of really good equipment on really weak creatures, so view those situations with a critical eye.