How much does 100LL avgas weigh?

It weighs approximately 6.02 lbs per US gallon ( approx 0.72 kg/liter) at "standard temperature" - defined as 15 degrees celsius (59 degrees Fahrenheit). Its density increases to 6.4 lbs per US gallon (0.77 kg/l) at -40 C (also -40 F) and 5.9 lbs per gallon ( 0.7kg/l) at 38 degrees C (100 degrees F). 100LL " avgas " is used in most small aircraft with reciprocating (piston) engines, though some use mogas or ethanol and some new engines are using diesel and/or Jet A. Turbine engines generally use "Jet A" which is more similar to kerosene or diesel. (which is about 7 lbs per gallon)