Don't estimate how much fuel you have to load...let your mobile phone compute it. With this app, this is very easy.
Fuel Converter will help you to determine the required amount of kerosene or avgas (100LL in US), to order. You enter the amount of fuel remaining in the tanks and the required block fuel and get your required upload.
--liters or gallons
--kerosene or avgas (100LL)
--specific weight of kerosene: 0.8 kg/l
--specific weight of avgas: 0.721 kg/l
--Unit of mass: pounds (lbs)