pulley drivetrain ratio

this may be a dumb question, but here goes..
i wish to spin a "Lazy susan" with an electric motor and pulleys at approx. 20-30 rpms. i was thinking of an electric motor/gear reduction which spins a small pulley wheel which in turn spins a larger pulley wheel via a "v" belt. i can buy motors with various gear reductions (100:1, 20:1, 10:1, etc)
i guess my question is: how do i determine the reduction ratio for the pulley wheels? if the small pulley is 4" dia. and the big one is 16" dia. does that mean it's a 4:1 reduction? or is it determined by circumference of the pulley wheels?
thanks for any tips...

 

[diamondjim]

Greg,
since the circumference ratio and the diameter ratios
are the same, either works.

 

thanks jim. i've worked it out with both circumference and diameter and you're right, it doesn't matter which i use.
thanks again