Prefixes

​Orders of magnitude and a scientific notation are handy ways to write very small and very large numbers. Here comes another method - prefixes. A prefix is an addition that changes a word's meaning. Milli, for example, means thousandth, so one millimeter is one thousandth of a meter, or ten to the power of minus three meters. And one kilometer, a thousand meters - ten to the power of three. ​ ​Prefixes can be used with more than just meters.

A kilogram is a thousand grams, and a kilovolt is a thousand volts. Here are a few prefixes that make a unit larger. Decca means ten, and hecto means 100, but we don't use those very often, except when we buy candy or shrimp by weight. Then the price is given in hectograms instead of kilograms. The reason is probably that this way they seem cheaper. ​ ​We've mentioned a kilo.

That means 1,000. After kilo, the prefixes are not ten times larger, but 1,000 times larger for every step. Mega means a million, and a giga means a billion. Then come tera, peda, exa, zetta and yotta. Each one is worth 1,000 times more than the last.

You might have heard some of these prefixes in descriptions of large amounts of stored data. ​ ​Look at the fourth column. There you see what the number is called in Swedish. From billion and up, this might seem a bit confusing to an English speaker. One miljard in Swedish is called a billion in English. And what is called a biljon in Swedish is a trillion in English.

Quite simply, there are two different standards for naming large numbers. They are called the long and short scales. If you speak several languages, you might know which scale they use in each language. The prefixes, however, are always the same, and so is the short abbreviation you see in the second column. ​ ​Prefixes can not only make things larger, as with kilo, but also smaller, like milli. You may already know that deci is one tenth, a decimeter is one tenth of a meter.

It's the same with centi - one hundredth. Centimeters and centiliters are hundredths of a meter and a liter. Milli is a thousandth, and micro is a millionth. One micrometer is therefore one millionth of a meter, or one thousandth of a millimeter. The symbol for micro is the Greek letter Mu.

Then comes nano, which is a billionth, and pico, femto, atto, zepto, and yocto. From milli and down, every unit is one-thousandth of the unit before. ​ ​Since you jump one-thousandth for every prefix, you can no longer follow the same rule as for scientific notation. In other words, you cannot always have a single digit coefficient. An Ebola virus is approximately eight times ten to the negative eight meters in diameter. But there is no named prefix for 100 millionths, so we say it is 80 nanometers, 80 billionths of a meter. ​ ​You don't need to know all this by heart, but it is good to know the most common prefixes, and to know how they correlate with scientific notation and words like billionth and million, and to be especially alert when you are talking about big numbers in a different language.