here is an easy formula.
Hornady Index of Terminal Standards (HITS)
HITS = Bullet Weight2 / Bullet Diameter2 / 7000 x Impact Velocity / 100
Alternatively, recognizing that some of the terms constitute the sectional density:
HITS = Bullet Weight x Sectional Density x Impact Velocity / 100
Having calculated this value, one then compares the score to four categories:
500 or Below -- Small game weighing less than 50 lbs
501 to 900 -- Medium game with a body weight of 50 to 300 lbs (e.g., deer, black bear and caribou)
901 to 1500 -- Large and heavy, but non-dangerous game weighing from 300 to 2000 lbs (e.g., elk, moose, African plains game, bison, etc)
1501 or Above -- Dangerous game of any weight (including, for example, big cats)