YiffieMon:Mass
Humanoid characters first calculate their BMI on by rolling 3D6 on Table 2.10a: BMI A characters Mass in Kilograms, can be calculate with the formula:
HiM * HiM * BMI
Where HiM is the character's Height in Meters and BMI is the character's Body Mass Index
Non humanoid character roll on Table 2.10b: Mass Factors and multiplying by character size.