What Unit Is Larger Than 'Tera' but Smaller Than 'Nano'? [Reading Science]

To express these large numbers in units, the scale of the units must also increase accordingly. What are the largest and smallest units?
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[Asia Economy Reporter Kim Jonghwa] When using units in daily life, the 'metric system' is followed. The units decrease in size in the order of 'meter (m)', 'centimeter (cm)', 'millimeter (mm)', 'micrometer (μm)', and 'nanometer (nm)'. Conversely, the units increase in size as 'kilometer (km)', 'mega (M)', 'giga (G)', 'tera (T)', and so on. In these cases, prefixes such as 'centi-', 'milli-', 'micro-', 'nano-', 'kilo-', 'mega-', 'giga-', and 'tera-' are attached before the unit.

What rules govern these prefixes, and what meanings do they carry? These prefixes first appeared in 1793, initially only 'kilo-' and 'milli-' were used, but nowadays various prefixes are employed. This is because the limits of expressing units with just kilo and milli have been surpassed.

The officially used prefixes in the International System of Units (SI) total 20. Since these units are so numerous that one cannot count them all in a lifetime, they are rarely used. Generally, prefixes up to 'tera' are well known.

The largest unit currently expressible is 'yotta (Y)'. One yottameter (Ym) equals 10²⁴ (1,000,000,000,000,000,000,000,000) meters. This means 10 to the 24th power, which corresponds to 10,000 hae (a Korean large number unit). The next largest unit is 'zetta (Z)', where one zettameter (Zm) equals 10 to the 21st power, or 10 hae.

Next is 'exa (E)', where one exameter (Em) equals 10 to the 18th power, followed by 'peta (P, 10 to the 15th power)', 'tera (T, 10 to the 12th power)', 'giga (G, 10 to the 9th power)', 'mega (M, 10 to the 6th power)', and 'kilo (k, 10 to the 3rd power)'. Below these are 'hecto (h, 10 squared)' and 'deca (da, 10¹)'. 'Giga' means giant in Latin, and 'tera' means monster.

When writing, prefixes up to mega are capitalized as Pm, Tm, Gm, Mm, etc., but kilometer uses a lowercase letter and is written as km. We usually write 100 m, 10 m, etc., but 100 m can also be expressed as 1 hectometer (hm), and 10 m as 1 decameter (dam or Dm).

Prefixes used in the International System of Units (SI). [Table=Korea Research Institute of Standards and Science (KRISS)]

Units smaller than 1 meter start with 'decimeter (dm)', where 1 dm equals 0.1 m or 10 to the -1 power, commonly representing 10 cm. Next is centimeter, 10 to the -2 power or 0.01 m, followed by millimeter, 10 to the -3 power or 0.001 m.

From 'micrometer (μm)' onward, the units decrease by a factor of 1000. One micrometer equals 10 to the -6 power or 0.000001 m, followed by 'nanometer (nm)' at 10 to the -9 power, 'picometer (pm)' at 10 to the -12 power, 'femtometer (fm)' at 10 to the -15 power, 'attometer (am)' at 10 to the -18 power, 'zeptometer (zm)' at 10 to the -21 power, and 'yoctometer (ym)' at 10 to the -24 power.

Recently, the nanometer is a frequently used unit, representing one billionth of a meter. 'Nano' originates from the Greek word 'nanos', meaning dwarf. For units not commonly used, 1000 km equals 1 Mm (megameter), and 1 million km equals 1 Gm (gigameter).

Also, there is no need to be alarmed if you encounter units like pm, fm, am, zm, or ym in documents or books. These are not radio frequencies FM or AM, but metric units picometer, femtometer, attometer, zeptometer, and yoctometer, which you now know.