How Many? A Dictionary of Units of Measurement© Russ Rowlett
and the University of North Carolina at Chapel Hill |

In science, there are definite rules of syntax for using numbers and units. This dictionary tries to follow those rules.

**Large numbers:** How many is a billion? In America, the word
"billion" means the number 1 000 000 000, or 10^{9}. In Britain,
this number is (or was) traditionally called "one milliard" or "one
thousand million," and "billion" means the number 1 000 000 000
000 or 10^{12}, which is what Americans call a "trillion."
Most other European languages have similar expressions, such as *mil millones*
for 10^{9} and *billón* for 10^{12} in Spanish.

The Dictionary uses the American definitions for large number names. Although the General Conference on Weights and Measures endorsed the European names in 1948, the American definitions seem to be gaining ground in Britain and perhaps elsewhere. See Names for Large Numbers for more on this question.

There's always interest, especially among school children, in knowing the names of still larger numbers: quadrillions, quintillions, etc. These names are listed in on the Names for Large Numbers page.

**Decimal Points and Numbers:** In English-speaking
countries, the decimal marker (decimal point) is the period. In
continental Europe and most other places, the decimal marker is
the comma. The International System (SI) allows either
one to be used; the Dictionary
uses the period. Since the comma is often a decimal marker, the SI recommends
that large numbers, like the billions above, be represented as groups of three
digits separated by narrow spaces. The Dictionary observes this
convention. The SI also allows large numbers to be written without any grouping,
as in 158346, but it does not allow the use of commas or any other grouping marks
other than narrow spaces.

**Scientific Notation:** Many authorities, including the U.S.
NIST (*Guide for the Use of the International System of Units*, sec.
7.9), also recommend that in scientific notation (when numbers are represented
using powers of ten), the exponent of the 10 should be a multiple of 3. For
example, the distance from the Earth to the Sun should be stated as approximately
149.6 x 10^{6} km, not 1.496 x 10^{8} km. This rule fits the
structure of the prefixes used for metric units.
The Dictionary follows this rule for most numbers, but not for stating the ratios
between CGS and MKS units.

**Symbols (Abbreviations):** Following the rules
of the International System, the Dictionary uses no periods after
abbreviations or symbols for units,
and never uses an "s" on an abbreviation or symbol to
indicate a plural.

Return to the Dictionary Contents page.

You are welcome to email the author (rowlett@email.unc.edu) with comments and suggestions.

All material in this folder is copyright 2012 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 page, but please do not copy the contents of any page in this folder to another site. The material at this site will be updated from time to time.

February 14, 2000; last updated August 8, 2012.