I can't define fomula for weight

[JusteLoneWolf]

Hello, i have a problem, ican't define the formula used to define the weight of an item.

because:
if weight is 1.19 equal 20lb

but
if weight is 1.15 equal to 16lb

but if weight is 2 equal to 105lb

but if weight is 1.0.91 equal to 10lb

So what is the formula ?