Measure your quantity in gallons. Most water jugs or large containers have measurements marked on the side. If so, simply read the number of gallons from the measurements. If your container does not have built-in measurements, you can measure your quantity in gallons by pouring your liquid into an empty one-gallon milk carton and determining how many cartons it takes to clear the liquid. This, however, can be cumbersome and time consuming.
Multiply your measurement from Step 1 by 128. For example, if you measured 6.2 gallons in Step 1, you will calculate 6.2 x 128 = 793.6 fluid ounces. If you measured 3.3 gallons in Step 1, you will calculate 3.3 x 128 = 422.4 fluid ounces.
If you do not have a calculator, use a conversion chart for volume measurements. The conversion chart will list the number of fluid ounces in any whole number of gallons, so you have to round to the nearest gallon. For example, if your quantity is 6.3 gallons, use the conversion measurement for 6 gallons.