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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.


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

   NIST Guide for the Use of the International System of Units (SI)

   Correct SI* metric system usage - U.S. Metric Association), Inc. 

   The Metric System (SI)- A Concise Reference Guide - David Bartlett

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

   Metrics the Right Way

   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.


   The International System of Units (SI) by Dennis Brownridge

   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

- Doctor Peterson, The Math Forum
Associated Topics:
Elementary Fractions
Elementary Terms & Units of Measurement
Middle School Fractions
Middle School Terms/Units of Measurement

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