SI Units & Conversion Factors

1. Definition

The SI ( Système Internationale d'Unités) is a practical, coherent unit system suitable for adoption by all those using the metric system. Its clarity and uniformity ensures consistency in all scientific reporting through easy conversion and universal understanding. It is composed of seven base units, two supplementary units and derived units. The seven base units are combined to form the derived units. Standard prefixes are used with the base units to make larger and smaller units.

2. Base Units

Table 1. The seven base units

QUANTITY	UNIT	SYMBOL
Length ( l )	meter	m
Mass ( m )	kilogram	kg
Time ( t )	second	s
Electric current ( I )	ampere	A
Thermodynamic temperature ( T )	kelvin	K
Luminous intensity ( I )	candela	cd
Amount of substance (n Q )	mole	mol

Standard prefixes are used with the base units to make larger and smaller units (Table 2).

Table 2. SI Prefixes

PREFIX	SYMBOL	FACTOR	PREFIX	SYMBOL	FACTOR
deka	da	10	deci	d	10-1
hecto	h	102	centi	c	10-2
kilo	k	103	milli	m	10-3
mega	M	106	micro	µ	10-6
giga	G	109	nano	p	10-9
tera	T	1012	pico	n	10-12
peta	P	1015	femto	p	10-15
exa	E	1018	atto	f	10-18
zetta	Z	1021	zepto	a	10-21
yotta	Y	1024	yocto	z	10-24

There are a large number of derived units, which are made by
manipulating the base units (Table 3). There should be a space or a multiplication dot between unit symbols, e.g. mol m^-3 ormol.m^-3.

Table 3. Derived Units of interest to Agroecologists

QUANTITY (symbol)	UNIT NAME	SYMBOL	DEFINITION
Area (A)	square meter	m2	m.m
Celsius temperature (t)	degree Celsius	oC	t K -273.15 K
Concentration (c)	moles per cubic meter	mol.m-3	mol.m-3
Energy (E)	joule	J	N.m (m2.kg.s-2)
Force (F)	newton	N	kg.m.s-2
Irradiance (energy, E )	watts per square meter	W.m-2	J.s-1.m-2
Irradiance (moles of photons)	moles per square meter per second	mol.m-2.s-1	mol.m-2.s-1
Power (P)	watt	W	J. (m2.kg.s-3)
Pressure (p)	pascal	Pa	N.m-2 (kg.s-2.m-1)
Speed or Velocity (v)	meters per second	m.s-1	m.s-1
Volume (V)	cubic meter	m3	m.m.m

• The candela and its derivatives were based on the sensitivity of the human eye. As plant sensitivities may differ, the candela and its derivatives should be avoided
• Although the liter is not an official SI unit, it may be used by plant scientists due to its traditional and wide use by non-scientists. As a precaution, the symbol "L" should be used instead of "l", in order to avoid any confusion with "1"
• Concentrations, whenever possible, should not be expressed in terms of molarity or molality, as has been the common practice. These units are not commonly understood outside of the field, whereas mol/L or mol/kg are much more widely comprehended
• The dalton is considered a dated unit, and should be replaced with the atomic mass unit, u
• Discarded units should be avoided, particularly the micron (replace with micrometer or nanometer), the bar(replace with megapascal or kilopascal), the calorie(use joule), and parts per million or billion(use the equivalent in units of mass, volume or amount of substance). As well, degrees should be reported in Celsius and not Fahrenheit.

• minute (min)
• hour (h)
• day (d)
• degree (^o)
• liter (L)
• metric ton or tonne (t)
• unified atomic mass unit (u)
• standard acceleration due to gravity (g_n)

• Salisbury, F.B. (ed.) 1996. Units, symbols, and terminology for plant physiology: a reference for presentation of research results in plant sciences . Oxford University Press, New York, Oxford. Dal. Lib. QK 710.5 U55 1996