How Many?

A Dictionary of Units of Measurement

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


Commentary and Explanation

The International System of Units (SI)
The International System of Units
Base and derived units; other units included (and some that are not included).
Defining the Base Units
Current definitions for the second, meter, kilogram, ampere, mole, kelvin, and candela, including those revised in November 2018.
SI Derived and SI Compatible Units
22 derived units that are in the SI, and 10 more that aren't.
Metric and SI Unit Prefixes
From yotta- (septillion) to yocto- (septillionth).
Using Numbers and Unit Symbols in Measurements
SI rules for numbers (decimal markers, digit grouping, etc.) and the SI rules for handling unit symbols.

The Long Road to the SI
English Customary Weights and Measures
A brief account of a long (but fascinating) history.
The Metric System
A new idea -- now more than 200 years old.
CGS and MKS Units
Two contending coherent unit systems. Spoiler alert: MKS wins.
The Metric System in the United States
It's not true that the U.S. has failed to adopt the metric system. What is true?

Other Matters of Interest
Roman and "Arabic" Numerals
How Roman numerals work, and how we got our current number system.
Names of Large Numbers
The never-ending controversy: how many is a billion?
Spelling of Metric Units
Meter? metre? metr? metro? Depends on your language, but the SI doesn't care.

Links to Tables and Scales

Introduction to the Dictionary

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.

It is said sometimes that the United States is one of only two or three countries that have failed to adopt the metric system. This a gross misrepresentation of history. Since 1875 the United States has subscribed to the International System of Weights and Measures, the official version of the metric system. Congress has repeated this commitment on several occasions, but it has not forced the general public to use metric units in all phases of ordinary life. So Americans have adopted the metric system, they simply don't use it in many settings.

The metric units form what is called a coherent system; this means they fit together precisely and it is easy to convert from one metric unit to another or to combine the units to make new units for different phenomena. The English customary units do not form a coherent 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).

This dictionary includes:

Each unit's definition includes conversion factors you can use to convert that unit into other units measuring the same concept. However, a caution is in order. In the case of the traditional units, remember that in many cases the "precise" definition for an older unit (such as the league or the hogshead) was not established until the nineteenth century. It's not wise to rely too much on these relatively modern definitions when reading older works. Also, many units which have precise meanings now, such as the barrel and the gallon, formerly had a variety of special meanings when applied to particular commodities; there isn't space in the dictionary for all these meanings.

I hope you find the dictionary useful and informative. Suggestions (and corrections if you find errors) are always welcome and can be sent to rowlett at


Written by:

Russ Rowlett
Retired Professor of Education and Adjunct Professor of Mathematics
University of North Carolina at Chapel Hill.

All material in this folder is copyright 2018 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.