Units and Values

SI units should be used at all times unless expressly requested by the instructor If values are needed in a different unit system, add them in parentheses. This is useful when stating values that were standardized using a different unit. For example: ``Nails should be placed every 40.6cm (16in.) to line up with the center of the studs." The SI has prefixes which should be used and are reproduced in Table 9.1. 1.2GPa is the correct way of expressing 1200 $\times 10^6$ Pa. This helps avoid the use of scientific notation in graphs.

Table 9.1: SI Prefixes from Quantities and Units [2]

	Prefix
Factor	Name	Symbol
$10^{24}$	yotta	Y
$10^{21}$	zetta	Z
$10^{18}$	exa	E
$10^{15}$	peta	P
$10^{12}$	tera	T
$10^9$	giga	G
$10^6$	mega	M
$10^3$	kilo	k
$10^2$	hecto	h
$10$	deca	da
$10^{-1}$	deci	d
$10^{-2}$	centi	c
$10^{-3}$	milli	m
$10^{-6}$	micro
$10^{-9}$	nano	n
$10^{-12}$	pico	p
$10^{-15}$	femto	f
$10^{-18}$	atto	a
$10^{-21}$	zepto	z
$10^{-24}$	yocto	y

The SI recommends the comma as a radix point but a dot is acceptable if it is placed on the line ($.$ ) not half high ($\cdot$ ). A cipher should always be used in the unit place of all values less than one; i.e. use $0.123$ not $.123$ . To facilitate the reading of numbers with many significant digits, digits should be grouped into threes. Starting from the decimal point, separate each group of three decimals and/or integers with a thin space never a comma. Hence $1234.56789$ should be set as $1 234.567 89$ . You will however rarely have to write numbers with more than 5 significant digits.