A tonne, or metric ton, is a single unit of mass equal to 1,000kg or 2,204lbs. This unit is the approximate mass of a cubic meter of water at four degrees Celsius. Converting standard units to metric units often involves only a single step, but because a gallon is a standard unit of volume, you must first convert it into a standard unit of weight before you can complete any calculation.
Weigh one gallon of your substance to get its weight in pounds. For instance, if you have a gallon of water, that typically weighs about 378kg.
Divide 1000kg. by the weight of the object from Step 1. Carry out the answer to at least four decimal places.
Example: 1000kg. per ton / 378kg. per gallon = 264.2686 gallons per ton
Divide the number of gallons you wish to convert into metric tons by your answer from Step 2.
Example: 50 gallons / 264.2686 gallons per ton = .189 tons
This example means that 50 gallons of water weighing 378kg. per gallon is approximately .189 -- or 18.9 per cent -- of one metric ton.
You can check your calculation by multiplication. For instance, 50 gallons times 378kg. per gallon equals 189kg. When you divide 189kg. by 1000kg. per ton, you get the same answer.