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How many millimeters in a meter

Browsing Q & A sites like answers.yahoo.com, wiki.answers.com is not so hard to notice that many people post questions similar to: “How many millimeters are in a meter”, “How many milliliters are in liter”,¬† “How many meters are in a kilometer”, … . Authors of these questions probably don’t know that the metric system of measurement is established on the set of prefixes.

Let’s see some examples, how the names of units looks like:

Examples Of Length Units: Micrometer, Millimeter, Kilometer.
Examples Of Mass Units: Microgram, Milligram, Kilogram.
Examples Of Capacity Units: Microliter, Milliliter, Kiloliter.

You can easily notice that the names can be divided into two parts: prefix (micro, milli, kilo) and the name of the unit (meter, gram, liter).

Every prefix has its own meaning and always means the same part or multiplication of something. E.g.

micro means 1/1.000.000 (microgram, microliter, micrometer)
milli means 1/1000 (milligram, milliwatt, millisecond)
centi means 1/100 (centimeter, centiliter)
deci means 1/10 (decimeter, deciliter)
kilo means 1000 (kilometer, kilogram, kilowatt)
mega means 1.000.000 (megaton, megawatt )

All 24 metric prefixes (called si-prefixes), introduced and governed by Bureau International des Poids et Mesures
( International Bureau of Weights and Measures ), you can find here : metric prefixes list

Prefixes used in computing:
If you are a computer user, you should be familiar with prefixes: kilo, mega, giga, tera or (maybe even with) peta. The same names are used in SI-prefix system but there are small difference between metric and computing systems. Since informational systems are based on power of 2, kilo means 1024 units (bits or bytes) (1024=2^10), mega means 1024 kilos, giga means 1024 mega and so on. In the SI (metric) system next prefix is 1000 times bigger then the previous (kilo means 1000 units, mega means 1000 kilos, giga means 1000 megs, and so on) 

Exceptions:
Generally, you can add every prefix to every unit name. It should be understandable and acceptable, however there are some ‘everyday language’ exceptions of this rule.
Exception 1: There is a widely used mass unit called ‘metric ton’. It is equal to 1000 kilogram. In everyday language we will say 1 ton rather then 1 megagram but 1 megaton, 1 kiloton, 1 gigaton is ok.
Exception 2: prefixes mega and greater(giga, tera, …) are often added to power (megawatt, gigawatt), energy (megajoule,gigajoule), pressure (megabar, gigapascal) units but ‘the metric man’ will say ‘thousand liters’ rather then ‘megaliter’ or 1000 kilometers or 1.000.000 meters instead of 1 megameter.¬†

Answering the question in the title of this post:
There are 1000 milliliters in one liter. Remember prefix milli means 1/1000 (one thousandth) of something.
Respectively, there are 1000 millimeters in a meter and 1000 meters in one kilometer.

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