Metric System

Because the meter (1.0936 yards) is much too big for measuring an atom and much too small for measuring the distance between two cities, we need a variety of smaller and larger units of length. But instead of inventing different-sized units with completely different names, as the English-American system does, we can create a metric unit of almost any desired size by attaching a prefix to the name of the unit. For example, since kilo- is a Greek form meaning a thousand, a kilometer (kil-OM-et-er) is a thousand meters. Similarly, a kilogram is a thousand grams; a gigagram is a billion grams or 10⁹ grams; and a nanosecond is one billionth of a second or 10^-9 second.

Minutes are permitted to remain in the metric system for convenience or for historical reasons, even though they don't conform strictly to the rules. The minute, hour, and day, for example, are so customary that they're still defined in the metric system as 60 seconds, 60 minutes, and 24 hours—not as multiples of ten. For volume, the most common metric unit is not the cubic meter, which is generally too big to be useful in commerce, but the liter, which is one thousandth of a cubic meter. For even smaller volumes, the milliliter, one thousandth of a liter, is commonly used. And for large masses, the metric ton is often used instead of the kilogram. A metric ton (often spelled tonne in other countries) is 1,000 kilograms. Because a kilogram is about 2.2 pounds, a metric ton is about 2,200 pounds: 10% heavier than an American ton of 2,000 pounds. Another often-used, non-standard metric unit is the hectare for land area. A hectare is 10,000 square meters and is equivalent to 0.4047 acre.