Look up the specific gravity of your gasoline and convert it to a density in kilograms per cubic meter. Specific gravities are already in metric and to convert them to kilograms per cubic meter, multiply by 1,000. According to the Material Safety Data Sheet for standard unleaded gas, it has a specific gravity of 0.71 to 0.77. Other types of gas, such as aviation fuel, have different densities.
Convert the number of liters of gas to cubic meters. There are 1,000 liters per cubic meter, therefore divide your liters by 1,000 to get the volume in cubic meters.
Multiply the volume from Step 2 by the density, in kilograms per cubic meter, from Step 1. The result is the total weight of your gas in kilograms.
Divide the weight by 1,000 to get metric tons, because there are 1,000 kilograms per ton.