The English system is used in the United States for general use, the scientific community frequently uses the metric system, so it is sometimes necessary to convert measurements from English to metric. Gallons are an English measure of volume while kilograms are a Metric unit of mass. Therefore, you need to know the density of the liquid you are measuring to convert gallons to kilograms. Weigh the substance for which you have the volume measurement, then convert this weight to kilograms.
Measures of Volume

Volume is a measure of how much liquid fits into a container. The amount of water that fits into a container, for example, is a measure of volume. Liquids, solids and gases all have volume. English units that measure volume include cups, pints, quarts and gallons. Metric units that measure volume include liters and milliliters.
Measures of Mass

Mass is a measure of the density of a substance. The weight of the water that fits into a glass, for example, is a measure of mass. Liquids, solids and gases all have mass. English units that measure volume include ounces, pounds and tons. Metric units of mass include milligrams, kilograms and grams.
Obtain the Mass

Measure the density of the substance you wish convert by weighing it. The substance can be weighed in either English or Metric units. If the substance is weighed in a unit other than kilograms, the amount must be converted to kilograms.
Convert to Kilograms

Conversion to kilograms can be done manually or by using a conversion table. If converting from pounds to kilograms, multiply the pounds by 0.453 because one pound is equivalent to .453 kilograms. Similarly, 1 ounce = 0.028 kilograms 1 ton = 907.18 kilograms. If converting within metric units, 1 milligram = 0.000 001 kilogram and 1 gram = 0.001 kilogram.
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