Fractions in the SI System?

Date: 11/07/2001 at 16:21:02
From: steve gissendanner
Subject: Proper metric terminology

When using metric, can 4 cm and 5 mm be written as 4 1/2 mm?
I think 4.5 cm would be the only correct terminology.

The question arises from a standardized test question.

Thanks,
Giss

Date: 11/08/2001 at 12:03:09
From: Doctor Peterson
Subject: Re: proper metric terminology

Hi, Steve.

It's my understanding, too, that fractions are disparaged in the 
metric system; but most of the metric style guides I know of don't 
seem to mention them - though they don't give any examples with 
fractions, either. For your reference, here are some of them:

   NIST Reference on Constants, Units, and Uncertainty
   http://physics.nist.gov/cuu/Units/index.html   

   NIST Guide for the Use of the International System of Units (SI)
   http://physics.nist.gov/Pubs/SP811/sp811.html   

   Correct SI* metric system usage - U.S. Metric Association), Inc. 
   http://lamar.colostate.edu/~hillger/correct.htm   

   The Metric System (SI)- A Concise Reference Guide - David Bartlett
   http://www.hlalapansi.demon.co.uk/Metric/index.html   

But I ran into two that do explicitly say not to use fractions:

   Metrics the Right Way
   http://www.pnl.gov/ag/usage/metrics.html   

   Mixed units (or mixed multiples), which are commonly used in the
   inch-pound system, are not used in metric practice. For example,
   you may write an inch-pound length as 5 ft, 8 in., but write the
   metric length as 1.73 m, not 1 m, 73 cm. Binary fractions (such
   as 1/2 or 3/8) are not used with metric units. For example, you
   would give a person's weight as 70.5 kg, not 70-1/2 kg. When
   spelling out numerical values, decimal fractions are acceptable.
   For example, 0.4 A would be spelled out as four-tenths ampere. 

By "binary fractions" they apparently mean "common fractions," which 
is what we are talking about.

Again,

   The International System of Units (SI) by Dennis Brownridge
   http://lamar.colostate.edu/~hillger/brownridge.html#correct-use   

   Use decimals, not fractions. Do not use more than one prefix
   with a unit, or a mixture of units.

   Correct   Incorrect 
   8.5 kg    8 1/2 kg
   3.15 m    3 m 15 cm 

So "officially," fractions are not used in the SI system; but in more 
informal circumstances, particularly with children, I wouldn't 
personally make strong objections. You might be interested in this 
page on household use of metric, which starts off with a fractional 
metric measurement:

   METRIC LAND - Joan Pontius
   http://www.rci.rutgers.edu/~jup/metric/metric_land.html   

- Doctor Peterson, The Math Forum
  http://mathforum.org/dr.math/