Technical Forum

Measurements and the SI system in Australia

When reading a recipe, you will be dealing with weights and volumes. If the source of the recipe is from overseas you may be dealing with unfamiliar units such as pounds, pints and gallons. If you are lucky the metric equivalent of these weights and volumes will be given.

Australia has adopted the metric system based on the International System of Units or simply the SI system. The SI system is based on seven base units from which all other units can be derived. Of interest to the winemaker is the metre (m) and the kilogram (kg). These units may be used alone or combined with a prefix to form measurements of a more convenient magnitude.

Prefixes indicate powers of 1000 and the prefix to choose is that which give a value of the measurement between 0.1 and 1000, e.g 1234 metres should be written as 1.234 kilometres. The prefixes centi (one hundredth) and deci (one tenth) are still allowed to be used with the metre such as centimetres.

micro (μ) one millionth                     0.000 001 10-6
milli (m) one thousandth                     0.001 10-3
centi (c) one hundredth                     0.01 10-2
deci (d) one tenth                     0.1 10-1
kilo (k) a thousand               1000 103
mega (M) a million        1 000 000 106
giga (G) a thousand million 1 000 000 000 109

Multiplying two lengths gives an area, the SI unit being a square metre (m2). Multiplying three lengths give a volume with the SI units of a cubic metre (m3). This volume contains the equivalent of 1000 litres of water. Litres are used as a more convenient unit for liquids and gases. In Australia a capital L is used to signify a litre.

1000 micrograms (μm) =      1 millimetre (mm)
10 millimetres (mm) = 1 centimetre (cm)
10 centimetres (cm) = 1 decimetre (dm)
1000 millimetres (mm) = 1 metre (m)
1000 metres (m) = 1 kilometre (km)

 

1000 cubic centimetres (cm3) =      1 litre (L) or 1 cubic decimetre (dm3)
1000 cubic decimetres (dm3) = 1 kilolitre (kL) or 1 cubic metre (m3)
1 cubic centimetre (cm3) = 1 millilitre (mL)
1000 millilitres (mL) = 1 litre (L)
1000 litres (L) = 1 kilolitre (kL) or 1 cubic metre (m3)
1000 kilolitres (kL) = 1 megalitre (ML)

Rules for using the metric system in Australia

  • Names are written in full or by their correct symbols, e.g. metre per second or m/s but not mixed as in metre/second or m per s.
  • Plural unit names are used for numbers greater than 1 and numbers less than -1. The singular is used for numbers in between -1 and 1, e.g. 30 grams, 0.2 kilometre.
  • All unit names are written in lower case except Celsius e.g. gram, metre
  • All unit symbols are lower case except the symbols derived from people's names and the symbol for litre (L) e.g. metre (m), gram (g), newton (N), hertz (Hz), pascal (Pa), watt (W).
  • Prefix symbols are lowercase except for quantities exceeding a million such as mega, giga tera (M, G, T)
  • The prefix symbol is joined directly to the unit symbol e.g. milligram (mg), millimetre (mm)
  • Multiplication of one unit by another is represented by a dot or a space. The dot is preferred, e.g. newton metres N.m or N m.
  • Division of one unit by another can be represented by a horizontal line, an oblique line or by the use of a negative power. The latter being preferred. e.g. grams per litre g.L-1 , g L-1 , g/L and kilograms per cubic metre kg.m-3, kg m-3 , kg/m3
  • The number is followed by a space then the symbol, e.g. 60 km/h
  • The decimal symbol may be a dot on the line, a dot at mid letter height or a comma. A dot on the line is preferred, e.g. 4.76
  • The thousand marker is indicated by a space, e.g. 12 000 000. In the case of four digit numbers the space may be omitted, e.g. 1250 or 1 250. When expressing sums of money, the numbers are printed without any spaces, e.g. $23500.00.
  • Numbers less than 1 are always preceded by a zero before the decimal marker, e.g. 0.135 60.

SI units are now the sole legal units of measurement in Australia. Now you know how it is done delight yourself in seeing the many errors on food labels, in newspaper articles and the like. You'll find contents in ml instead of mL; numbers joined to their units; a mix of abbreviations and symbols and all from food technologists and other professionals who should know better.

Different unit systems.

American and English recipes use a different system of units based on a weight in pounds (lb) and a volume in pints (pt), quarts (qt) or gallons (gal). To complicate matters a little, the volume of an English imperial gallon is different from an US gallon. Fortunately, it is quite easy to convert between these non-SI units and their metric equivalent. And what does a teaspoon really mean?

The following table allows you to convert the unit on the left to the unit on the right simply by using the multiplying factor in between. If you want to go the other way, then divide by the factor.

x
÷

Ounces (oz) 28.4 Grams (g)
Pounds (lb) 454 Grams (g)
Pounds (lb) water 0.454 Litres (L)
US gallons (gal) 3.63 Litres (L)
Imperial gallons (gal) 4.54 Litres (L)

For example, a typical demijohn holds an imperial gallon and so to find the number of litres this is equivalent to we need to multiply the number of gallons by 4.54 i.e. 1 x 4.54. So 4.54 is the number of litres this is equivalent to. By definition a US gallon is the volume of 8 pounds of water and 8 pounds is 3.63 L while the British gallon is the volume of 10 pounds of water or 4.54 L.

How many pounds is 1.5 kg? In this case, we know 1.5 kg is 1500 g and when going left across the table we need to divide so 1500 / 454 is 3.3 lb. The decimal part, 0.3 lb is easily converted into ounces knowing there are 16 oz to a lb and three tenths of a pound is 0.3 x 16 or about 5 ounces.

A tea spoon is a nebulous quantity usually implied to be 5 g or 5 mL. The actual amount depends upon the size of the spoon, how big the heap in the spoon is and the nature of the material you are measuring. While the situations in which it is used is usually not critical it's a measurement best avoided when writing down a recipe.

In the US temperatures are still measured using the Fahrenheit system. Since the action of yeasts and enzymes are temperature dependant it's important to get the temperature right. Developing the skill to convert between Fahrenheit and Celsius is important for using the correct process temperatures.

C = (5 F - 160) / 9
60°F can be converted to Celsius by firstly multiplying 60 by 5 then subtracting 160 and dividing this answer by 9.

C = (5 x 60 -160) / 9
C = (300 -160) / 9
C = 140 / 9
Celsius temperature =15.6°

Going from Celsius to Fahrenheit the formula becomes:

F = (9C + 160) / 5

Of course, you can always use a temperature conversion table and so avoid making a calculation.

°C °F °C °F °C °F °C °F °C °F
0 32 20 68 40 104 60 140 80 176
1 34 21 70 41 106 61 142 81 178
2 36 22 72 42 108 62 144 82 180
3 37 23 73 43 109 63 145 83 181
4 39 24 75 44 111 64 147 84 183
5 41 25 77 45 113 63 149 85 185
6 43 26 79 46 115 66 151 86 187
7 45 27 81 47 117 67 153 87 189
8 46 28 82 48 118 68 154 88 190
9 48 29 84 49 120 69 156 89 192
10 50 30 86 50 122 70 158 90 194
11 52 31 88 51 124 71 160 91 196
12 54 32 90 52 126 72 162 92 198
13 55 33 91 53 127 73 163 93 199
14 57 34 93 54 129 74 165 94 201
15 59 35 95 55 131 75 167 95 203
16 61 36 97 56 133 76 169 96 205
17 63 37 99 57 135 77 171 97 207
18 64 38 100 58 136 78 172 98 208
19 66 39 102 59 138 79 174 99 210

In Summary

The International System (SI) is a comprehensive and practical system of units of measurement of all physical quantities for technical, scientific and general use. The unit of measurement of every physical quantity is derived from, and described in terms of, one or more base units. The seven base units are the metre (m), kilogram (kg), second (s), ampere (A), kelvin (K) candela (cd) and mol (mol). A complete list of the physical units and prefixes used in Australia follows.

 
Length metre (m) atto (a) 10-18
Area square metre (m2) femto (f) 10-15
Volume cubic metre (m3) pico (p) 10-12
Capacity litre (L)(a non-SI unit) nano (n) 10-9
Mass gram (g) micro (μ) 10-6
Density kilogram per cubic metre (kg/m3) milli (m) 10-3
Time second (s)  
Velocity metre per second (m/s) kilo (k) 103
Acceleration metre per second squared (m/s2) mega (M) 106
Temperature kelvin (K) giga (G) 109
Force newton (N) tera (T) 1012
Pressure pascal (Pa) peta (P) 1015
Energy joule (J) exa (E) 1018
Power watt (W)    
Resistance ohm(Ω)    
Current ampere (A)    
Frequency hertz (Hz)    
Molecules mole (mol)    

The tonne, the special name for megagram, is used with the prefixes 'kilo' and 'mega' only.

Non-SI units

There are still some non-SI units used in Australia such as the hectare for land measurement (10 000 m2) and degree Celsius (°C ) for temperature measurement (the SI unit for temperature is kelvin (K) (°C = K -273.15). Notice we don't say °K just K. These Non-SI Units have been retained because of their practical importance or their use in specialised fields.

minute (min) 1 min = 60 s 60 s
hour (h)   3600 s
tonne (t) 1 ) t = 1 megagram (Mg 1000 kg
litre (L) ) 1 L = 1 cubic decimetre (dm3) 0.001 m3
hectare (ha)   10 000 m2
degree Celsius (°C) °C = K -273.15
nautical mile (n mile) 1852 1 n mile = 1.852 km 1852 m
knot (kn) 1.852 km/h 1 kn = 1 n.mile/h 1.852 km/h
kilowatt hour (kW.h)   3.60 MJ
angles (°) 1° = π/180 rad 0.017 45 rad

Non-SI units are not to be used for scientific and technical calculations. The hectare is a common unit for land area in Australia. The degree Celsius is the common unit for the measurement of temperature. Technical and scientific calculations should employ the kelvin(K). Sea and air navigation still use the nautical mile for distance and the knot for speed in navigation. The kilowatt hour is internationally recognised as the commercial unit for selling electricity.