Units of Measure index

Metric System


index --- alpha --- intro --- length --- area --- volume --- weight --- money --- angles --- weather --- other --- foreign --- trades --- documents --- metric --- tables

This website is about Imperial units and other older British units, but here is one page is about the metric system. Metric units are logical, easy to understand, easily convertible, easy to learn, unambiguous, and boring!

Names
SI base units
History
Does Britain uses metric or Imperial units?
Relationship to older British units
Metric facts
Metric versions of sayings
Problems

Names and prefixes

These units should really be known as SI units. The SI stands for Système Internationale because the system was invented by the French. It can be called the MKS system (for metre/kilogram/second). This is why the British spell metre and litre in the French way, either as a compliment, or to point out that these foreign units are nothing to do with us. I'll leave you to make up your mind!

These are the units covered by this website (see below for conversion to British units):

Main unitOther common units
lengthmetrekilometre, centimetre, millimetre
areaarehectare
volumelitremillilitre
weightgramkilogram
anglegrad
temperatureCelsius

I'm not sure if the grad is an official SI unit. They seem to prefer the radian. But grads were the metric degrees.

Once you know the basic name of a system, then you can generate all the other names. Take, for example, a metre:

1 metre =  10 decimetres
100 centimetres
1,000 millimetres
1,000,000 micrometres
1,000,000,000 nanometres
1,000,000,000,000 picometres
1,000,000,000,000,000 femtometres
1,000,000,000,000,000,000 attometres
1,000,000,000,000,000,000,000 zeptometres
1,000,000,000,000,000,000,000,000 yoctometres
10 metres = 1 decametre
100 metres = 1 hectometre
1,000 metres = 1 kilometre
1,000,000 metres = 1 megametre
1,000,000,000 metres = 1 gigametre
1,000,000,000,000 metres = 1 terametre
1,000,000,000,000,000 metres = 1 petametre
1,000,000,000,000,000,000 metres = 1 exametre
1,000,000,000,000,000,000,000 metres = 1 zettametre
1,000,000,000,000,000,000,000,000 metres = 1 yottametre

You will probably only have heard of millimetres, centimetres, metres and kilometres, but the rest are there if you want them.

Once you have grasped the idea, then you can work out other units, for example, 1000 millilitres = 1 litre, and 1000 grams = 1 kilogram. You can see where Kilobytes, Megabytes and Gigabytes came from. A kilo (which we use to mean kilogram) merely means a thousand of something. A micron is really a micrometer. The mil is a milliradian. The system is very logical, but people in the real world tend to adjust it a little!

There has been some suggestion that only prefixes in powers of multiples of three should be allowed. This would make the centimetre (10-2 metres) and the hectare (102 ares) unofficial units. This doesn't seem to have happened yet. The hectare would be particular embarrassing, as the hectare is recognised, but the 'are' isn't!




SI base units

MeasuresNameabbrev.Definition
lengthmetremlength of the path travelled by light in vacuum during a time interval of 1/299792458 of a second
masskilogramkgequal to the mass of the international prototype of the kilogram
timeseconds duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom
electricityampereAthat constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2 x 107 newton per metre of length
temperaturekelvinK1/273.16 of the thermodynamic temperature of the triple point of water
numbers of atoms/moleculesmolemol1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12.
2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
lightcandelacdluminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

Seconds, amperes and moles are used universally.

Note that while the other base units are based on fundamental physical constants, a kilogram is the same mass (not weight!) as the standard kilogram. This worries people a bit...

All other S.I. units are based on these base units. You can obviously get square and cubic measurements from length ones, and volumes are just a type of cubic measurement. One millilitre is equal to 1 cubic centimetre.



History

The metric system was first proposed by the French astronomer and mathematician Gabriel Mouton (1618-94) in 1670, and was standardised in France under the Republican government in the 1790s, its use being made compulsory there in 1801. They defined a metre as one ten-millionth of the length of a line from the North Pole to the Equator, going through Paris. It was intended that a gradian of Latitude should be 100 Kilometres. The centigrad (Kilometre) would replace the nautical mile. They got it slightly wrong - the poles are 10,002 Km from the equator. Now SI units are strictly defined using scientific constants, or in the case of the kilogram, a specific lump of metal (see above).

Now most (all?) of mainland Europe uses metric measures. Australia changed to metric from 1970-1980, although informally older people still use Imperial quite a lot. Britain is partly metric, partly Imperial (see index page). America seems fairly aggressively non-metric (they describe their system of measures as U.S. Customary systems) apart from their scientists, of course, although I have been informed that the American Military also use the metric system.

Websites about SI units
Modern definitions of SI units
More information about SI units including their history
United Kingdom Metric Association


British use

Some people may be interested to know whether Britain uses metric or Imperial units. This is not easy to answer. Legally, at the moment (2009), Britain is mostly a metric country. The Statutory Instrument 1995 No. 1804 says so. But it also specifies exceptions, such as

A correspondent has pointed out that "As from 1-Jan-2010, the acre could no longer be used for the registration of land. In practice the Land Registry Office had stopped using acres some years ago, so the removal of the acre was a bureaucratic tidying up exercise."

Many British people, especially older people, continue to use Imperial units informally.



Relationship to older British units

lengthkilometre1 km = 0.62 mile
1 km = 1093.61 yards
metre1 m = 1.09 yards
centimetre1 cm = 0.39 inches
millimetre1 mm = 0.039 inches
1 mm = 39.37 thou or mil
areahectare1 hectare = 2.47 acres
volumelitre1 litre = 1.76 pints
millilitre1 ml = 0.04 fluid ounce
1 ml = 16.89 minim
weightkilogram1 kg = 2.2 lbs
gram1 gram = 0.04 oz
1 gram = 15.43 grains
anglegrad1 grad = 0.9°
temperatureCelsius0°C = 32°F, then for each 1°C, add 1.8°F

An inch is legally defined in Britain as being 2.54 centimetres, so Imperial units are really part of the metric system!



Metric facts (some silly)

You may notice that after liquid measures on packages in Britain (and elsewhere within the European Union), you will see an 'e'. This is a legal requirement, and says that the number is accurate within a certain pre-defined limit (only downwards - they don't care if you give too low a figure!) It stands for 'estimation' rather than 'Europe'.

This appeared in New Scientist: "Drinking a litre of beer is like emptying a bottle the length of the universe with the cross-sectional area of a medium-sized nucleus." So there!

The universe is about 100 yottametres across (but it's getting larger all the time).



Metric versions of sayings (frivolous)

A miss is as good as 1.6 kilometers.

Put your best 0.3 of a metre forward.

Spare the 5.03 metres and spoil the child.

28 grams of prevention is worth 453 grams of cure.

Give a man 2.5 centimetres and he'll take 1.6 kilometres.

Peter Piper picked 8.8 litres of pickled peppers.



Problems

My generation spent a lot of time at school having to learn Imperial units. For example, we learnt that a yard was 36 inches and 3 feet make a yard. However, as Britain started to use metric units more and more, the school children were taught metres and centimetres, and knew nothing about the older units.
Some time ago, my husband and I wanted to buy 2 nets curtains. One was 5 foot by 6 foot, and the other 3 foot or a yard square. A local shop showed some in their window, marked with Imperial (rather than metric) units. We went in to buy them, and found the young shop assistant, who valiantly tried to serve us despite obviously not knowing a thing about Imperial units. We described the first curtain, and she asked what the drop was. We said 5 foot. She asked "What was in inches?" Patiently we said 60 inches (that was the sort of thing we had drilled into us at school). She got the right roll, then asked what the width was. We said 6 feet. "What was that in yards?" she asked. Trying to keep a straight face, we said 2 yards. She cut the right amount, then asked about the second curtain. My husband, trying to hurry things up, said it was 36 inches by 1 yard. "Which was the drop and which was the width?" she asked. We were tempted to get them the wrong way round and then convert them to show that the curtain was square, but instead we solemnly said that 36 inches was the drop, and the width a yard.
This story is not intended to crow over her ignorance. It demonstrates how complicated Imperial units are, and how much of our school time was wasted drilling us.

Another, more serious problem arose with the Mars Climate Orbiter in 1998, which was to study the Martian weather. It was intended to enter orbit at an altitude of 140-150 km above Mars. However, a navigation error caused the spacecraft to reach as low as 57 km. The spacecraft was destroyed by atmospheric stresses and friction at this low altitude. The navigation error arose because a NASA subcontractor (Lockheed Martin) used Imperial units (pound-seconds) instead of the metric system.

However, we mustn't gloat over the metric system too much. There seems to be wide-spread disagreement whether a pound is a unit of mass or weight, rather an important distinction! The Imperial system has a mess of units, for example, 'quart', 'quarter' or 'quarto' were widely used to mean a quarter of something else. School children had to learn long tables of conversions and be drilled in their use. The metric system is easier. But still boring!