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METRIC LANGUAGE AND RULES

This document gives a brief and general overview of the basic units of metric measure and the department conventions in writing and speaking them.

The kilo (k) and milli(m) are most commonly used prefixes for metric units in design and construction.

The prefixes mega (M) for on million, giga (G) for one billion, micro (u) for one millionth, and nano (n) for one billionth are used in some engineering calculations.

The prefixes deci (d) for one tenth, centi (c) for one hundredth, and deca (da) for ten should not be used.

CONVERSION AND ROUNDING

When converting number from English to metric, the metric value must be carried to a sufficient number of digits to maintain the accuracy implied or required in the English number (11 miles at 1.609 km/mi equals 17.699 km, which should be rounded to 18 km.

When converting mixed English units (feet and inches, pounds and ounces), the primary concern is to maintain the same order of magnitude. Convert the mixed English units to either the smaller or the larger English unit before converting to metric and rounding (10 feet 3 inches = 123 inches; 123 inches x 25.4 mm/inch = 3124.2 mm which should be rounded to 3120 mm; or 10 feet 3 inches = 10.25 ft.; 10.25 ft. x 0.3048 m/ft. = 3.1242 m which should be rounded to 3.12 m).

In a "soft" conversion, an English measurement is converted to its mathematical metric equivalent. With "hard" conversion, a new, rounded, standard rationalized metric number is created that is convenient to work with and remember.

RULES FOR LINEAR MEASUREMENT

The meter and millimeter are preferred. Do not use the centimeter, decimeter and decameter.

Use the kilometer for long distances and the millimeter for precision measurement.

RULES FOR AREA

The square meter is the preferred unit of measurement.

Very large area may be expressed in square kilometers and very small areas in square millimeters.

Use the hectare for land and water measurement only.

Do not use the square centimeter, decimeter and decameter.

Linear dimensions such as 40 mm x 90 mm may be used; if so, indicated width first and height second.

RULES FOR VOLUME AND FLUID CAPACITY

The cubic meter is preferred for volumes in construction and for large storage tanks.

Use liter (L) and milliliter (mL) for fluid capacity.

RULES FOR CIVIL AND STRUCTURAL ENGINEERING

The metric units used in civil and structural engineering are:
meter (m) length
kilogram (kg) mass
second (s) time
newton (N) force or weight
pascal (Pa) pressure

Use these rules in civil and structural engineering:
there are separate units for mass and force.

The kilogram (kg) is the base unit for mass, which is the unit quantity of matter independent of gravity.

The newton (N) is the derived unit for force (mass times acceleration). It replaces the unit "kilogram-force" (kgf), which should not be used.

Do not use the joule to designate torque; always use newton meters.

The pascal (Pa) is the unit for pressure and stress. The term "bar" is not a metric unit and should not be used. Structural calculations should be shown in MPa or kPa.

Plane angles in surveying (cartography) will continue to be measured in degrees (either decimal degrees, or degrees, minutes and seconds) rather than metric radian.

Slope is expressed in nondimensional ratios with the vertical component shown first and then the horizontal (V:H). The vertical component is unitary for slopes less than 45 degrees and the horizontal is unitary for slopes greater than 45 degrees. The units that are compared are the same (metric to meters, millimeters to millimeters). Base tapers normally shown as 8:1 will now be shown as 1:8.

RULES FOR USING DUAL UNITS

Dual units should not be used except when documents are used specifically for public viewing. The metric unit should be shown first followed by the English equivalent in parenthesis. A note should be appended to documents with dual units advising that the English units are provided for information only.

For test procedures where dual units are used, the following statement will be included under "Scope" of the test procedure.

The value stated in either SI units or English units are to be regarded as standard. Within the text, the English units are shown in parenthesis. The values stated in each system are not exact equivalents; therefore, each system must be used independently of the other.

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Updated October 08, 2003