When dealing with military a/c, several types of fuels have been used throughout the years, and each has it's own density. Depending on how in-depth you want to be with your a/c or drop tanks, here are the service years, branches, and densities for jet fuels:
JP-4 Density = 6.5 lb/gal (1951 - 1995, Primarily used by the USAF)
JP-5 Density = 6.8 lb/gal (1952 - Present; projected to 2025, Primarily used by the USN, and other navies)
JP-7 Density = 6.6 lb/gal (used by the SR-71)
JP-8 Density = 6.7 lb/gal (1996 - Present, Primarily used by the USAF)
Jet-A Density = 6.70 lb/gal (used by civil turbines)
Example:
1x F-111 External 600gal Tank holds 3900lbs [600 x 6.5 = 3900] of JP-5 (I'm using JP5 since that's what was in service at that time).
Now, to insert the fuel weight into the game, it must be converted from pounds to kilograms:
3900 (lbs.) x.45 = 1755 (kgs).
Once you've converted the weight, insert it into the 'Capacity' box in the 'Fuel Tank Data' section in the weapons editor.