A tonne, or metric ton, is a single unit of mass equal to 1,000 kilograms or 2,204.6 pounds. This unit is the approximate mass of a cubic meter of water at 4 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 1 gallon of your substance to get its weight in pounds. For instance, if you have a gallon of water, that typically weighs about 8.34 pounds.

Divide 2,204.6 pounds by the weight of the object from Step 1. Carry out the answer to at least four decimal places. In the example, 2,204.6 pounds per tonne / 8.34 pounds per gallon = 264.3405 gallons per tonne.

Divide the number of gallons you wish to convert into metric tons by your answer from Step 2. For example, 50 gallons / 264.3405 gallons per tonne = 0.189 tonnes. This example means that 50 gallons of water weighing 8.34 pounds per gallon is approximately 0.189 tonne -- or 18.9 percent of 1 metric ton.

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You can check your calculation by multiplication. For instance, 50 gallons times 8.34 pounds per gallon equals 417 pounds. When you divide 417 pounds by 2,204.6 pounds per ton, you get the same answer.