propane bottles are rated in pounds. The little one you are talking about is a 1 pound bottle, or 16 0z of fuel inside the tank (so they say)
the big tanks for a grill is 20lbs. You tank should say how many pounds it holds. A 20 lb tank holds about 4.1 galons of liquid propane.
you said it took 1.5 gallons,
128 oz in a gallon
192 oz in 1.5 gallons
192 oz % 16 oz (1 pound) = 12 1 pound bottles