Encounter Benchmark

An encounter may be deadly if the sum total of monster challenge ratings is greater than one quarter of the sum total of character levels, or half the sum total of character levels if the characters are above 4th level. A single monster may be deadly if its CR is greater than the average of character levels or greater than 1.5 x the average of character levels if they're above 4th level.

—Sly Flourish, Lazy Encounter Benchmark