The Metric System

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

gram (g) 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 base-ten 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 = 10³ 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., two-dimensional) units of length:

1 m² = 1.20 yd² = 1,550 in.² = 1.550 x 103 in.²

1 cm² = 100 mm² = 0.16 in.²

1 hectare = 10,000 square meters (m²) = 2.47 acres

1 km² = 0.39 mi²

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 mm² or 1.6 x 10^-11 in.²

Skin area of average-sized woman 1.58 m² or 17 ft²

U.S. paper currency 104 cm² or 16.1 in.²

Measurements of area and volume can use the same units:

1 m³ = 35.3 ft³ = 1.31 yd³

1 cm³ = 0.000001 m³ = 0.061 in.³

The following comparisons will help you appreciate these conversions of volume:

Coke can 355 ml or 12.0 oz

Legal-sized filing cabinet 0.4 m³ or 14 ft³

Sugar cube 1 cm³ or 0.061 in.³

Units of Mass

The gram is the basic unit of mass.

1 g = mass of 1 cm³ 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 x 10^-18 lb

Giant amoeba 10^-8 kg or 2 x 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., three-dimensional) units of length.

1 L = 1,000 cm³ = 1,000 mL = 0.001 m³

1 L = 2.1 pt = 1.06 qt = 0.26 gal = 1 dm³

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

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 cm³ = 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).

The following site contains interesting information and an on-line tutorial about the metric system:

deci (d) 0.1 10^-1 (tenth part), deka (da) 10 10¹ (tenfold)

centi (c) 0.01 10^-2 (hundredth part), hecto (h) 100 10² (hundredfold)

milli (m) 0.001 10^-3 (thousandth part), kilo (k) 1,000 10³ (thousandfold)

micro (µ) 0.000001 10^-6 (millionth part), mega (M) 1,000,000 10⁶ (millionfold)

nano (n) 0.000000001 10⁹ (billionth part), giga (G) 1,000,000,000 10⁹ (billionfold)

pico (p) 0.000000000001 10^-12 (trillionth part), tera (T) 1,000,000,000,000 10¹² (trillionfold)