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.

### Step 1

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.

### Step 2

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.

### Step 3

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.