Garage heater sizing

I have a garage that I want to heat.  I have looked

at the heat loss in a couple ways and get very different answers.  When I

look at volume and rise though various heater manufacturers', the

calculations say that i need about 50 mbtu/hr.  When I put it in web

calculators that are more complicated heat loss calculators that use

wall, roof, and floor construction and window/door area, I get a

something around 100 mbtu/hr.





The garage is 16'w x 24' deep x 7.5'h.  The peak of the roof

is 5.5' above the walls and it is a gable roof with wood walls at the

gables.  The garage walls themselves are cinderblock.  It is a single

car garage door, it has an uninsulated wood side door and 2 3'x4' steel

single pane windows.





It is in ann arbor michigan so the max rise is ~70 degrees f.



The volume is 3936 cubic feet.



Which

calculation is correct?  Is the calculation from the heater sites only

calculating what is needed to heat the air and not accounting for the

heat that contact and radiation to the outer shell materials is removing

from the air?



Thanks



Matt