A unitless ratio measuring the amount of water to cement, expressed as a decimal fraction.

Every gallon of water weighs 8-1/3 pounds
Every sack of cement weighs 94 pounds

Multiply the number of gallons of water by 8.333
Multiply the number of sacks of cement by 94
Divide the former by the latter. The result is the water to cement ratio.

((Num. Gal. Water)*8.333)/((Num. Sacks of Cement)*94)