When the fuel comes out of our fuel trucks the indicators on the truck are in gallons. Typically an F-15E will take about 3,300 gallons or so with conformal fuel tanks installed if the airframe is down to about 1500lbs. Once they are done fueling the airframe they log the amount of gallons input into the forms. Then later on that night they will do a power on check to verify the fuel load and lighting systems.
I'm curious to know if maintenance on smaller civilian airframes actually have to calibrate their fuel systems. I'm surprised they aren't using a hydrometer to calculate the fuel load. Reason I say that is because 6.7lbs per gallon isn't always correct. I've calibrated systems when the hydrometer read as low as 6.4 and 6.5 in the past. I've also calibrated systems with the reading as high as 6.8 and at 6.9. Granted it's not much but I know it's critical enough that we calibrate our systems because of it.
