A gallon of milk weighs 8.61 lbs.
The specific gravity of milk is 1.032.
Take the weight of water per gallon, 8.34 and multiply that by the specific gravity of milk and that equals 8.61
Alternatively, you could take the longer way below:
The weight of water is 62.4 lb/ft3.
Take 62.4 x 1.032 = 64.4 lb/ft3
Take the weight in cubic feet and divide that by 7.48 = 8.61
There are 7.48 gallons per cubic foot