You cannot.
A tonne is a measure of mass. A gallon is a measure of volume. The two measure different things and, according to basic principles of dimensional analysis, conversion from one to the other is not valid.
If you are not convinced, consider a gallon of air. How many tonnes (or grams)? Next consider a gallon of Mercury. How many kilograms?
The volume occupied by a gallon of air and of water will clearly be very different. So there is no direct conversion between mass and volume: you need to know the density of the substance to enable you to carry out the conversion.
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Metric tons can't be converted to gallons. Metric tons measure mass, while gallons measure volume.
The conversion rate of metric tons to gallons depends on the density of the substance being measured. Bunker fuel, a type of fuel oil used in ships, has a density of around 0.96 metric tons per cubic meter. Therefore, to convert metric tons of bunker fuel to gallons, you would need to know the volume in cubic meters and then use the conversion factor of 1 cubic meter being approximately equal to 264.172 gallons.
7,000 kg = about 6.889 metric tons.
Divide by 1000
To convert cubic meters to metric tons of fuel oil, you need to know the density of the fuel oil in metric tons per cubic meter. Once you have the density value, you can multiply the number of cubic meters by the density to get the mass in metric tons. The formula for this conversion is: Mass (in metric tons) = Volume (in cubic meters) x Density (in metric tons per cubic meter).
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