Thousands separator

The thousands separator is a blank space, point or comma located between the digits of a number whose function is to facilitate its reading, grouping the figures in threes.

International standards for writing numbers

In accordance with the guidelines of the International System of Units, mandatory in countries where it is the only legal one, the writing of numbers will be done subject to the following rules:

• The decimal separator will be in line with the figures and the comma will be used (,) or the point (.), depending on what is customary in the context it contends. The Academy of Language in the Orthography of 2010 establish (p. 666): "In order to promote a process towards unification, the use of the point is recommended as a separating sign of decimals".

Examples: 123.45 123.45

The CGPM at its 22nd meeting in 2003 indicates that "some international organizations, including some international standardization organizations, specify that the decimal separator should be the comma on the line, in all languages," but the CGPM does not set a preference for either of the two options by admitting both valid. ISO/IEC 80000-1:2009, and contrary to what was established in the previous ISO 31, also supports both coma and point (section 7.3.2).

• If the number is between +1 and -1, then the decimal separator will be preceded by a zero.

Examples: 0.345 -0.123

• No other sign should be placed between the figures.

Example: 12345,67

• To facilitate the reading of large numbers, the figures of three in three can be grouped from the comma or decimal point, separating them with a blank space, both in the entire part and in the fraction. No point or comma should be used as a mile separator.

Example: 123 456 789,987 546

• However, when there are only four digits, both in front and behind the decimal sign, it is customary not to isolate a digit.

Example: 1234,5678

From opting for space, ISO/IEC 80000-1:2009 states that it must be done both in the entire part and in the decimal (section 7.3.1).

Example: 234,567 8 (and not 1 234,5678 or 1234,567 8)

For very large or very small numbers it is recommended:

• the scientific notation, in which the number is expressed as a product of a number between zero and ten (with a whole figure) by a power of 10.

Examples: 1234,5678 = 1,234 567 8 × 10³0,000 012 345 = 1.2345 × 10^-5

• use the corresponding prefixes of the units.

Examples: 12 345 678 m = 12,345 678 Mm 0,000 012 345 m = 12,345 μm

Financial use

In the event that the number refers to a monetary amount, the custom prior to the current regulations, mainly for security reasons against counterfeiting and fraud, followed the following scheme:

The symbol used for the thousands separator depends on the one used to separate the entire part from the decimal part, so that the symbols are not confused. For example, countries using decimal coma use a point as a mile separator, while countries using the decimal point use a comma as a mile separator.

Example: "Twelve million three hundred thousand euros" is written 12,300,000.00 EUR or 12,300.000,00 EUR.

Other cultures

The separation of figures in groups of 3 is due to the fact that in Western culture the use of the powers of a thousand (10³) predominates: million (10⁶), billion (10⁹), trillion (10¹²), etc, although the ways of calling them have small variations called short scale and long scale. However, this is not so in other countries and cultures.

In India, two numbers are used from a traditional numeral system: the lakh (one hundred thousand) and the crore (10 million). Because of this, the numbers are separated in a peculiar way, for example, the number above would be written 1.23.00,000, that is, "one crore, twenty-three lakhs and zero thousands" instead of "twelve million three hundred thousand".

In China and Japan, large numbers are separated into groups of four digits, because they have special words for powers of ten thousand, and not for powers of one thousand. In the expression, Arabic numerals can be mixed with traditional Chinese characters for powers of 10,000, and so the previous number could be written 1230万0000, or 1230.0000.