# Metric System

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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 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
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.

Modern definitions of SI units
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

• the use of the mile, yard, foot or inch for road traffic signs, distance and speed measurement
• the use of the pint for dispensing draught beer and cider
• the use of the pint for milk in returnable containers
• the use of the acre for land registration (see below)
• the use of the troy ounce for transactions in precious metals

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

 length kilometre 1 km = 0.62 mile1 km = 1093.61 yards metre 1 m = 1.09 yards centimetre 1 cm = 0.39 inches millimetre 1 mm = 0.039 inches1 mm = 39.37 thou or mil area hectare 1 hectare = 2.47 acres volume litre 1 litre = 1.76 pints millilitre 1 ml = 0.04 fluid ounce1 ml = 16.89 minim weight kilogram 1 kg = 2.2 lbs gram 1 gram = 0.04 oz1 gram = 15.43 grains angle grad 1 grad = 0.9° temperature Celsius 0°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.