A Guide to SI Usage for the Reluctant
by Stan Jakuba
A great number of Americans are reluctant to deal with the metric
system. Unfortunately for them, its a language spoken by nearly
everyone else on the planet. Stan Jakuba offers some plain and
simple information on the ins and outs of metric.
(This manuscript is published with the permission of the author
and ASME Mechanical Engineering magazine where most of the text
was published in the April 2001 issue (www.asme.org).)
Engineers in the United States rarely need one of the requirements
for their counterparts in most countries. They typically dont
have to learn a second language. In most of the world, however,
engineers are at least bilingual. Most of them speak English in
addition to their native tongue.
The arrangement may make it convenient for Americans to venture
abroad, but it also contributes to cultural isolation. And the
isolation extends to the system of measurement.
There has been an undue amount of criticism over the years condemning
the metric system for its inconsistency (as compared to the Englishor
inch-poundsystem, presumably). We little realize how inconsistent,
illogical, and unsystematic the English system is because we
are not readily confronted with alternatives.
The inconsistencies the critics of the metric system focus on
are miniscule and most are due to the systems evolution. The
evolution is inevitable. Any system that wants to keep up with
the evolving society must change.
Among metric users, as in any population, there are people who
do not want to accept change, or are not aware of revisions or
the need for them, who prefer to cling to established ways. Inconsistencies
arise, then, in the way that people use the system, not in the
The Metric System Is a Standard
There indeed is one, and only one, metric systemalways the latest
revision of the international standard that describes it. For
engineers worldwide, the standard is ISO 1000. This standard is
subject to change, as any standard must be, and a user is expected
to follow the latest version, which is the practice with all standards.
The standard describes a universal, international language of
Essentially, all units created in a coordinated manner in modern
times are metric in every country of the world, including the
United States. The evolution is guarded by an international committee
in which the United States has participated since 1875.
It may be comforting to many to learn that the standard, in the
section most people use, is not expected to change for a long
time. And the last major revision took place in 1960two generations
Note that there is no officially recognized forum for the development
of any other system of units, of which there used to be dozens,
including the English system and any version of it.
The modern system of measurement is properly called SI, not metric.
SI, for the French Systeme International, is based on seven units,
called base units, and a set of names, called prefixes, that stand
for certain multiples.
Base units measure such basic physical quantities as length, mass,
or time. Alone or in combination, they let mankind measure everything.
There are, of course, hundreds of units needed for measuring everything,
but they are all derived from those seven. The derivation is done
in a way that provides the marvelous and unique feature of SI:
there are no conversion factors.
Some of the derivations were given a special name. This was applied
in cases where the combination would be too long and cumbersome
for frequent use, or where confusion could result. Most have been
used in the English system also because no official non-metric
equivalent ever existed.
All derived units can be expressed in terms of the base units.
The derivations are as straightforward as the relationship of
length to breadth to compute area, m2. The unit newton, for example, is derived from mass times acceleration,
the kilogram accelerating a metre per second per second or, in
graphic symbols, kg·m/s2.
As convenient, units may be expressed in a combination of both
base and derived units. Knowing that torque, for example, means
force times distance, leads to the newton metre, N·m. Or, for
energy density (energy per mass or volume), the same logic leads
to the units J/kg or J/m3. Pressure is force per area, hence the unit pascal is N/m2.
Many derived units can be expressed in more than one form, but
professional use usually settles on a single convention. For example,
the unit of dynamic viscosity could be expressed as kg/(m·s) or
N·s/m2 or Pa·s. Only the last form is prevalent.
Holdovers From the Past
As pointed out by SIs critics, there are inconsistencies in the
sense that non-SI units and terms remain in local (and, in some
cases, general and approved) use. They mostly reflect a tradition
that is slow to die.
Here are several examples of terms carried over from the past
that are still in common and approved use.
The degree Celsius (symbol °C) designates a temperature on the
Celsius scale. Note that as an increment, the degree Celsius is
identical to the kelvin.
The degree in plane angle (symbol °) is an alternative to the
The litre and millilitre are the everyday usage alternates for
dm3 and cm3, respectively.
Handling Long Numbers
Prefixes (kilo-, centi-, milli-, etc.) often precede the name
of an SI unit. They were devised to shorten long numbers; for
example, 20 000 kg shortens to 20 Mg, and 0.0008 A to 0.8 mA.
Prefixes are the names of power multiples (hundred, thousand,
million, etc.). Their use provides an alternative to the scientific
notation (50 000 expressed as 5x104), eliminates the need for the creation of unnecessary new units
(5,280 feet grouped into one mile), and helps retain only significant
The 10n notation is impractical for the non-scientific person, and the
creation of the new units is impractical for everybody, because
in the modern world it would necessitate coining thousands of
names and subjecting each of us to memorizing hundreds of them.
Instead, the prefixes shorten numbers to make them convenient
for everybody to use.
While the SI committee has so far established 20 prefixes, far
fewerperhaps eightare needed in daily life and in common technical
Ten are more than most people need.
*The use of these four prefixes is declining all over the world
with hekto (hecto) and deka (deca) not in engineering use anymore,
and deci and centi surviving in engineering only with m3 and m4 (and, to be picky, deci- alone with the non-SI unit bel as
in decibel (dB)).
With rare exception, Americans consider km and mm or cm separate
units because schools teach converting among them. That misguided
practice leads to the persistent argument against metric for having
too many and long-named units.
But km or mm is just a way of saying in shorthand a thousand
metres or a thousandth of a metre. One cannot convert among
them as one does not convert between a thousand inches and a
thousandth of an inch. Prefixes are a languagethe words can
be translated, not converted.
There is hope. We nowadays seem to treat kilobyte, megabyte,
and gigabyte quite comfortably as one unit. This author has
not heard as yet anyone claiming them to be three different units
nor any teacher suggesting converting among them.
Getting To Like SI
Most people, once they understand it, like SI for its logic, for
featuring only one unit per physical quantity, and for its lack
of conversion factors. On the other hand, some older engineers
dont like to use it. This is understandable. One tends to dislike
anything that one does not understand and has little feel for.
The author hopes that this article provides an understanding of
the system. To get the feel for the units takes longer, and personal
initiative is requiredsuch as reading about SI and trying to
remember the commonly used values in units of either system.
Table 1 presents a sample of units and reference values. With them, each
unit is shown accompanied by the most suitable prefix. All engineers
new to performing their jobs in SI should take the time to write
down a similar table for the ball-park figures in their fields,
and pin it onto the office wall at eye-level.
Writing the reference numbers and returning to them repeatedly
helps in getting the feel for values in SIa feel of utmost importance
for overcoming the resistance to using SI in ones work.
Seek Brevity and Universality
SI units, prefixes, and rules were established to facilitate data
communication worldwide. They represent a compromise intended
to suit all languages, to ease arithmetic manipulations, to prevent
ambiguity, and to retain some of the traditions of the metric
For technical documentation, the preferred way of writing SI prefixes
and units is by their symbols; for example, 5 kg, not 5 kilogram,
or five kilograms.
If each symbol is written according to the SI rulesdistinguishing
between uppercase and lowercase letters, and between the Latin
and Greek lettersit will be intelligible everywhere, regardless
of the script and language a nation uses.
People facing the need to acquire a new skill often delight in
debating the pros and cons of the new venture. Nowhere is this
more prevalent than with a new system of units. But it should
be easy to realize that there cannot be one that satisfies all,
just as there cannot be one language that would combine only the
best features of other languages. What is the best unit in one
profession may be the worst in another. SI is the best compromise;
its unceasing advancement into the standards writing communities
and daily usage in all countries of the world attests to its being
the best. //
Copyright 2001, ASTM