Metric System, Conversion Factors, and Water Properties
The following information is summarized from the Federal Highway Administration, National
Highway Institute (NHI) Course No. 12301, "Metric (SI) Training for Highway Agencies." For
additional information, refer to the Participant Notebook for NHI Course No. 12301.
In SI there are seven base units, many derived units and two supplemental units (Table A.1).
Base units uniquely describe a property requiring measurement. One of the most common
units in civil engineering is length, with a base unit of meters in SI. Decimal multiples of
meter include the kilometer (1000m), the centimeter (1m/100) and the millimeter (1 m/1000).
The second base unit relevant to highway applications is the kilogram, a measure of mass
which is the inertial of an object. There is a subtle difference between mass and weight. In
SI, mass is a base unit, while weight is a derived quantity related to mass and the
acceleration of gravity, sometimes referred to as the force of gravity. In SI the unit of mass is
the kilogram and the unit of weight/force is the newton. Table A.2 illustrates the relationship
of mass and weight. The unit of time is the same in SI as in the English system (seconds).
The measurement of temperature is Centigrade. The following equation converts Fahrenheit
temperatures to Centigrade, -C = 5/9 (-F - 32).
Derived units are formed by combining base units to express other characteristics. Common
derived units in highway drainage engineering include area, volume, velocity, and density.
Some derived units have special names (Table A.3).
Table A.4 provides useful conversion factors from English to SI units. The symbols used in
this table for metric units, including the use of upper and lower case (e.g., kilometer is "km"
and a newton is "N") are the standards that should be followed. Table A.5 provides the
standard SI prefixes and their definitions.
Table A.6 provides physical properties of water at atmospheric pressure in SI system of
units. Table A.7 gives the sediment grade scale and Table A.8 gives some common
equivalent hydraulic units.