How Many?
A Dictionary of Units of Measurement
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For information on a specific unit, click on the first
letter of its name:
A
B C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
Answers to the three most frequently asked
questions:
How many micrograms (µg, ug or mcg) in a
milligram (mg)?
1000 micrograms = 1 milligram, and 1000 milligrams = 1
gram.
How can I convert from international units (IU)
to milligrams or micrograms?
Generally speaking, you can't. IU's measure the potency of a
drug, not its mass or weight.
What countries besides the U.S. have not adopted the
metric system?
Many U.S. teachers think the answer is "Liberia and
Burma" (make that Myanmar). Let's give Liberia and Myanmar a
break! All countries have adopted the metric system,
including the U.S., and most countries (but not the U.S.)
have taken steps to eliminate most uses of traditional
measurements. However, in nearly all countries people still
use traditional units sometimes, at least in colloquial
expressions. Becoming metric is not a one-time event that
has either happened or not. It is a process that happens
over time. Every country is somewhere in this process of
going metric, some much further along than others.
Commentary and Explanation
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What's New
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Index of Tables and Scales
Introduction
Americans probably use a greater variety of units of measurement
than anyone else in the world. Caught in a slow-moving transition
from customary to metric units, we employ a fascinating and sometimes
frustrating mixture of units in talking about the same things. We
measure the length of a race in meters, but the length of the long
jump event in feet and inches. We speak of an engine's power in
horsepower and its displacement in liters. In the same dispatch, we
describe a hurricane's wind speed in knots and its central pressure
in millibars.
Furthermore, our English customary units
do not form a consistent system. Reflecting their diverse roots in
Celtic, Roman, Saxon, and Norse cultures, they are often confusing
and contradictory. There are two systems for land measurement (one
based on the yard and the other on the rod) and a third system for
distances at sea. There are two systems (avoirdupois and troy) for
small weights and two more (based on the long and short tons) for
large weights. Americans use two systems for volumes (one for dry
commodities and one for liquids) and the British use a third (British
Imperial Measure).
Meanwhile, only a few Americans know that the legal definitions of
the English customary units are actually based on metric units. The
U. S. and British governments have agreed that a yard equals exactly
0.9144 meter and an avoirdupois pound equals exactly 0.453 592 37
kilograms. In this way, all the units of measurement Americans use
every day are based on the standards of the metric
system. Since 1875, in fact, the United States has subscribed to
the International System of Weights and
Measures, the official version of the metric system.
This dictionary began as a collection of notes describing the
relationship between various English and metric units. It gradually
grew until it finally became too large a word-processing document; I
couldn't find my way around in it any more. So I turned it into a
folder of html documents and added it to my Internet site. For many
months, no one looked at the site except me and my students. Then,
gradually, the dictionary began to attract users from around the
world. Many users were kind enough to point out errors; others
suggested additions and improvements. Questions about units began to
appear in my email inbox. Sometimes I could answer the questions,
sometimes not.
Today the dictionary has become a kind of interactive resource. It
grows slowly and steadily, mostly through suggestions from readers
and my efforts to answer questions posed by readers. You can
participate in this process! Please
let me know if you find any errors on the site, or if you can't
find what you wanted to know, or if you know of units used in your
field of study or in your part of the world that aren't included.
I hope you find the dictionary useful and informative.
Formalities
Written by:
Russ Rowlett,
Director,
Center for Mathematics
and Science Education
University of North Carolina at
Chapel Hill.
You are welcome to email
the author (rowlett@email.unc.edu) with comments and
suggestions.
All material in this folder is copyright © 2005 by Russ Rowlett and the
University of North Carolina at Chapel Hill. Permission is granted for personal
use and for use by individual teachers in conducting their own classes. All
other rights reserved. You are welcome to make links to this or any page of
the dictionary. Please do not copy the contents of any page of the dictionary
to another site. The material at this site is updated frequently as new
information is added, so linking to the site rather than copying it is in the
best interests of everyone.
The information contained in the dictionary is as accurate as I can make it;
please notify me if you find any
errors. Neither the author nor the University of North Carolina assumes
any liability for uses made of the information presented by this web site. The
dictionary is not designed to promote any system of measurement. Its only
purpose is to present information useful to anyone interested in the subject.
July 11, 2005