How Many?

A Dictionary of Units of Measurement

For information on a specific unit, click on the first letter of its name:


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

 What's New

Index of Tables and Scales


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.


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 ( 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