A simple pulley and drum may be used to lift a weight by applying a force with a rope which is wrapped about he outer diameter of one of the diameters.

F : W = r : R

W = (F x R)/r

F x R = W x r

R = (W x r)/F

F = (F x R)/R

r = (F x R)/W

Where:

W = Unknown weight (lbs, N)
F = Applied force (lbs, N)
R = Radius large diameter (in, mm)
r = Radius small diameter (in, mm)