Systems of Units
Provided by Dr. Kenneth D. Walsh
In the calculations required for most engineering- and construction-related tasks, the numbers in the calculation represent some
property of a physical thing.
As such, these numbers are more than just numbers, they are in fact a combination of a number and a
It is very important to understand the unit systems which could be presented to you on a problem, and to take care with the units
in your calculations so that you arrive at a result with a physical meaning.
Remember that a number is meaningless without units.
U.S. Customary and SI Systems
The most basic measurements that might carry units include length, mass, and time.
Most other measurements (velocity, power,
etc.) are merely combinations of these units, although often with other names.
For instance, a speed is a length per time, and might be
called ft/sec or mph.
Even though England itself and most of the rest of the world converted to the SI system long ago, the most
commonly used system in the United States continues to be the old English system.
In this system, the basic units are feet (length),
slugs or pounds-mass (mass), and seconds (time).
However, the US Customary system nearly ignores the mass unit, and instead uses
a force unit (pounds).
This fuzziness about the mass unit is often cited as one of the drawbacks of the US Customary system.
overpowering familiarity of the system to Americans is its primary advantage.
There is at present a move to change the units system in the US to the SI (often called metric) system.
The basic units of the
metric system are the meter (length), kilogram (mass), and second (time).
Prefixes are added to describe the scale of the number, with
a new prefix for each thousand-fold change in the value.
Most Americans are less familiar with this system, and so often have no
“feel” for metric values, and the system has a large number of special names for unit combinations.
For example, the pressure unit in
the English system is pounds per square foot (or inch), while in the SI system it is the Pascal, which is a special name for the Newton
per square meter (1 Pa = 1 N/m
The Newton is in turn a special name for the kg-m/sec
Still, the metric system is widely used
outside the US, and government pressure for a change is slowly increasing.
US contractors will probably have to have some fluency
with both systems for some time.
In fact, several years ago the federal government has mandated that all construction projects with
federal funding use SI units.
The base units for each system are shown in Table 1.
Derived units are shown in Table 2.
Table 3 gives scale prefixes for the SI
system, and important scale units for the English system are shown in Table 4.
Finally, a number of useful conversion factors are
given in Table 5.
Table 1: Base Units in the English and SI System