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Overview of important units You may come across metric units of measure with which you are unfamiliar. This page is intended to introduce you to the units that a literate person might be expected to know (such as the bar), or which you may come across (such as the Newton). Some of this material may be rather technical. If you are looking for a basic introduction to the metric system, only some of the following information may be of use. In that case, just skip over the more arcane parts. I recommend that you also look at the previous page, "How to learn the metric system," if you have not already done so. Descriptions of these units are below the table
Descriptions Prefixes
*These prefixes have limited usage. You will usually only see them in the centimeter, hectopascal, and hectare. The symbol for "micro" is the Greek letter mu: µ. To type this character:
In Windows, with NumLock on, hold down Alt and type 230 on the keypad (not
the numbers at the top of the keyboard). The length units are probably what most people think of when they think of the metric system, so most people are probably fairly familiar with them. A meter is about the distance from the floor to a doorknob. Sometimes, you may see a kilometer informally referred to as a "klick". If you read scientific or technical publications, you may come across the terms micron or angstrom. A micron is another word for micrometer, and an angstrom is a tenth of a nanometer (in other words, one ten billionth of a meter). The word micrometer, when it means the unit of measure, is pronounced "MY-cro-mee-ter". When the word refers to a measuring instrument, it is pronounced "my-CROM-it-er". Would you like a ruler? Print one! Word format PDF format (not currently available) The metric system does not include the acre, which is 43,560 square feet. To specify area, use square meters (m2), hectares (ha), or square kilometers (km2). A hectare is much more fun to use than an acre. A hectare is the area of a square 100 m on a side, which means it is 10,000 m2. There are 100 ha in one km2. A hectare covers about two and a quarter football fields. Incidentally, with the metric system it is easy for farmers and others to figure out how much water covers how much area. For example, one liter of water covers one square meter to a depth of one millimeter. So, 10 cm of water over 1 m2 is 100 L. Likewise, 1 mm of water over 1 ha equals 10,000 L, and 1 mm of water over 1 km2 equals 1 million liters. (Try doing that with acre-feet, gallons, square miles, and fractions of inches of rain!) You may already be familiar with liters and milliliters. Just as there are 1000 milliliters in one liter, there are 1000 liters in one cubic meter. In various metric areas of the world, prefixes besides "milli" are often used with liters. So in France you may find a bottle labeled in centiliters, and in Australia you may get a water bill showing your consumption in kiloliters, or see a news report mentioning megaliters. Mass (or weight in normal speech) Mass measures come in multiples of 1000: a thousand milligrams per gram, a thousand grams per kilogram, etc. The metric ton is no exception to this—it is 1000 kg. The word tonne refers to the metric ton. The word "ton" can refer to either 2000 pounds or the metric ton, depending on the context. There is a correlation between volume measures and mass measures. Water weighs one gram per milliliter, one kilogram per liter, and one metric ton per cubic meter. You have seen medicine and vitamins labeled in milligrams, and perhaps in micrograms well. Sometimes, labels will indicate micrograms by "mcg" rather than the more correct way of µg. The metric unit for force is the Newton (N). This is one area that may seem less familiar to newcomers to the metric system. Normally, you won't have to worry about Newtons, but they do come up in a few situations. For example, the lift on the back of a truck may have its capacity measured in Newtons. A Newton is about the weight of 100 grams. So, an apple weighs about 1 N. A good rule of thumb is that an object's weight in Newtons is ten times its mass (weight) in kilograms. So, since I am about 59 kg, I weigh approximately 590 N. For a more technical description of this unit, see its Wikipedia entry The measurement of pressure can seem bewildering at first glance, because there are so many units that have historically been used. However, there are now just two metric units that are generally used, and they are closely related: the Pascal and the bar. Measuring in millimeters of mercury (torr) is obsolete except for a few applications where it is in common use (such as in chemistry). The modern unit for pressure is the Pascal (Pa). The Pascal is a very small unit, so people often use another unit called the bar, which is defined to be 100 000 Pa. The bar is about the pressure of the atmosphere, hence its name (the barometric pressure). In weather reports, pressure is usually given in millibar (mb or mbar) or hectopascal (hPa). They both the same: 100 Pa. Sometimes, pressure is given in kilopascals (kPa). Air pressure is normally in the range of 980 mb to 1030 mb. In everyday life, we measure temperature in degrees Celsius when we use metric, of course. For scientific measurements, or for very hot or cold things, we may use Kelvins (such as for the temperature of the sun or of liquid helium). Kelvin is like Celsius, but shifted so that zero is at absolute zero instead of the freezing point of water. Water freezes at 273.15 Kelvins. Incidentally, we do not speak of degrees Kelvin—it's just Kelvins. To understand Celsius temperatures just remember: That is really all you need to know most of the time, except perhaps the boiling point of water (100 °C) and body temperature (37 °C). |