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### Fractions in the SI System?

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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
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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/
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Associated Topics:
Elementary Fractions
Elementary Terms & Units of Measurement
Middle School Fractions
Middle School Terms/Units of Measurement

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