The metric system is a standardized system of measurement used by scientists throughout the world. It is also the measurement system used in everyday life in most countries. Although the metric system is the only measurement system ever acknowledged by Congress, the United States remains "out of step" with the rest of the world by clinging to the antiquated English system of measurements involving pounds, inches, and so on.
Metric units commonly used in biology include
meter (m) The basic unit of length
liter (L) The basic unit of volume
kilogram (kg) The basic unit of mass
degree Celsius (°C) The basic unit of temperature
Unlike the English system with which you are already familiar, the metric system is based on units of ten, thus simplifying interconversions (table B.1). This baseten system is similar to our monetary system, in which 10 cents equals a dime, 10 dimes equals a dollar, and so on. Units of ten in the metric system are indicated by Latin and Greek prefixes placed before the base units:
Thus, we multiply by
0.01 to convert cg to g
0.1 to convert dm to m
1,000 to convert kg to g
0.000001 to convert µL to L
0.1 to convert mm to cm
For example, 620 g = 0.620 kg = 620,000 mg = 6,200 dg = 62,000 cg.
Units of Length
The meter (m) is the basic unit of length.
1 m = 39.4 inches (in.) = 1.1 yards (yd)
1 km = 1,000 m = 10^{3} m = 0.62 miles
1 cm = 0.01 m = 10^{2} m = 0.39 in. = 10 mm
1 nm = 10^{9} m = 10^{6} mm = 10 angstroms (Å)
470 m = 0.470 km
1 in. = 2.54 cm
1 ft = 30.5 cm
1 yd = 0.91 m
1 mi = 1.61 km
Units of area are squared (i.e., twodimensional) units of length:
1 m^{2} = 1.20 yd^{2} = 1,550 in.^{2} = 1.550 ´ 103 in.^{2}
1 cm^{2} = 100 mm^{2} = 0.16 in.^{2}
1 hectare = 10,000 square meters (m^{2}) = 2.47 acres
1 km2 = 0.39 mi^{2}
The following comparisons will help you appreciate these conversions of length and area:
Height of the Washington Monument 169.1 m or 555 ft
Length of a housefly 0.5 cm or 0.2 in.
Distance kangaroos can hop in one leap ~7.6 m or ~25 ft
Diameter of a pore in a hen’s egg 17 µm or 0.0007 in.
Surface area of a flu virus 10^{8} mm^{2} or 1.6 ´ 10^{11} in.^{2}
Skin area of averagesized woman 1.58 m^{2} or 17 ft^{2}
U.S. paper currency 104 cm^{2} or 16.1 in.2
Measurements of area and volume can use the same units:
1 m^{3} = 35.3 ft^{3} = 1.31 yd^{3}
1 cm^{3} = 0.000001 m^{3} = 0.061 in.^{3}
The following comparisons will help you appreciate these conversions of volume:
Coke can 355 ml or 12.0 oz
Legalsized filing cabinet 0.4 m^{3} or 14 ft3
Sugar cube 1 cm^{3} or 0.061 in.^{3}
Units of Mass
The kilogram is the basic unit of mass.
1 g = mass of 1 cm^{3} of water at 4°C = 0.035 oz
1 kg = 1,000 g = 103 g = 2.2 lb
1 mg = 0.001 g = 10^{3} g
The following comparisons will help you appreciate these conversions of mass:
Penicillin molecule 10^{18} kg or 2 ´ 10^{18} lb
Giant amoeba 10^{8} kg or 2 ´ 10^{8} lb
Human 102 kg or 220 lb
Nickel 5 g or 0.176 oz
Remember that mass is not necessarily synonymous with weight. Mass measures an object’s potential to interact with gravity, whereas weight is the force exerted by gravity on an object. Thus, a weightless object in outer space has the same mass as it has on earth.
Units of Volume
The liter (L) is the basic unit of volume. A typical thermos bottle holds about 1 liter; a standard flush toilet flushes about 20 liters of water. Units of volume are cubed (i.e., threedimensional) units of length.
1 L = 1,000 cm^{3} = 1,000 mL = 0.001 m^{3}
1 L = 2.1 pt = 1.06 qt = 0.26 gal = 1 dm^{3}
1 mL = 0.035 fl oz
1 tsp = 5 mL
1 cup = 0.24 L
Units of Temperature
You are probably most familiar with temperature measured with the Fahrenheit scale, which is based on water freezing at 32°F and boiling at 212°F. Celsius temperatures are synonymous with Centigrade temperatures, and these scales measure temperature in the metric system. Celsius (°C) temperatures are easier to work with than Fahrenheit temperatures since the Celsius scale is based on water freezing at 0°C and boiling at 100°C. You can interconvert °F and °C with the following formula:
5(°F) = 9(°C) + 160
For example,
45°C (113°F) temperature at which any area of skin of an average person will feel pain
40°C (104°F) typical for a hot summer day
75°C (167°F) hot coffee
37°C (98.6°F) human body temperature
16°C (60.8°F) coldest body temperature a person has been known to have and survive
Celsius versus Centigrade
The Celsius temperature scale was developed in 1742 by Anders Celsius, a Swedish astronomer. Interestingly, Celsius originally set 0°C as the boiling point of water and 100°C as the freezing point of water. Soon thereafter, J. P. Christine revised the scale to its present form—with 0°C as the freezing point and 100°C as the boiling point of water. The Celsius scale is identical to the Centigrade scale.
The following comparisons will help you appreciate the conversions of Fahrenheit and Centigrade temperatures:
Water freezes 0°C 32°F
Butter melts 31°C 87°F
Water boils 100°C 212°F
Fireplace fire 827°C 1,520°F
Hints for Using the Metric System
1. Express measurements in units requiring only a few decimal places. For example, 0.3 m is more easily manipulated and understood than 300,000,000 nm.
2. When measuring water, the metric system offers an easy and common conversion from volume measured in liters to volume measured in cubic meters to mass measured in grams: 1 mL = 1 cm^{3} = 1 g.
3. Familiarize yourself with manipulations within the metric system. Work within one system, and do not convert back and forth between the metric and English systems.
4. The metric system uses symbols rather than abbreviations. Therefore, do not place a period after metric symbols (e.g., 1 g, not 1 g.). Use a period after a symbol only at the end of a sentence.
5. Do not mix units or symbols (e.g., 9.2 m, not 9 m, 200 mm).
6. Metric symbols are always singular (e.g., 10 km, not 10 kms).
7. Except for degree Celsius, always leave a space between a number and a metric symbol (e.g., 20 mm, not 20mm; 10°C, not 10°C).
8. Use a zero before a decimal point when the number is less than one (e.g., 0.42 m, not .42 m).
Web Sites
The following sites contain interesting information and an online tutorial about the metric system:
http://edie.cprost.sfu.ca/~rhlogan/metric.html
http://www.essex1.com/people/speer/metric.html
Prefix Division of Metric Unit Equivalent Prefix Multiple of Metric Unit Equivalent (Latin) (meter) (Greek) (meter)
deci (d) 0.1 10^{1} (tenth part), deka (da) 10 10^{1} (tenfold)
centi (c) 0.01 10^{2} (hundredth part), hecto (h) 100 10^{2} (hundredfold)
milli (m) 0.001 10^{3} (thousandth part), kilo (k) 1,000 10^{3} (thousandfold)
micro (µ) 0.000001 10^{6} (millionth part), mega (M) 1,000,000 10^{6} (millionfold)
nano (n) 0.000000001 10^{9} (billionth part), giga (G) 1,000,000,000 10^{9} (billionfold)
pico (p) 0.000000000001 10^{12} (trillionth part), tera (T) 1,000,000,000,000 10^{12} (trillionfold)
