# 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 and prefixes

These units should really be known as SI units. The SI stands for Systeme Internationale because the system was invented by the French. The older system was MKS system (for metre/kilogram/second). The British spell metre and litre, as the French do, and the Americans spell these meter and liter.

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

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

Effective from 20 May 2019, the International System of Units, the SI, is based on this set of universal constants:

Name of constantValue of constantbase unitother unit
unperturbed ground state hyperfine transition frequency of the caesium 133 atom9 192 631 770 Hzsecond (time)
speed of light in vacuum299 792 458 m/smetre (length)
Planck constant6.626 070 15 x 10-34J skilogram (mass)joule (energy)
elementary charge1.602 176 634 x 10-19Campere (electricity)coulomb (electric charge)
Boltzmann constant1.380 649 x 10-23J/Kkelvin (temperature)
Avogadro constant6.022 140 76 x 1023mol-1mole (numbers of atoms/molecules)
luminous efficacy of monochromatic radiation of frequency 540 x 1012 Hz683 lm/Wcandela (light)lumen (light)

All other S.I. units are based on these 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 for length was 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.

Now 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. I found this table in 2021. Note that temperature is specified as only centigrade.

### 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. 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.

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.

Spare the 5.03 metres and spoil the child.

### 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.