The percent sign (sometimes per cent sign in British English) is the symbol used to indicate a percentage, a number or ratio as a fraction of 100. Related signs include the permille (per thousand) sign and the permyriad (per ten thousand) sign (also known as a basis point), which indicate that a number is divided by one thousand or ten thousand, respectively. Higher proportions use parts-per notation.
Correct style
Form and spacing
English style guides prescribe writing the percent sign following the number without any space between (e.g. 50%). However, the International System of Units and ISO 31-0 standard prescribe a space between the number and percent sign, in line with the general practice of using a non-breaking space between a numerical value and its corresponding unit of measurement.
Other languages have other rules for spacing in front of the percent sign: In Czech and in Slovak, the percent sign is spaced with a non-breaking space if the number is used as a noun. In Czech, no space is inserted if the number is used as an adjective (e.g. “a 50% increase”), whereas Slovak uses a non-breaking space in this case as well. In Finnish, the percent sign is always spaced, and a case suffix can be attached to it using the colon (e.g. 50 %:n kasvu 'an increase of 50%'). In French, the percent sign must be spaced with a non-breaking space. According to the Real Academia Española, in Spanish, the percent sign should be spaced now, despite the fact that it is not the linguistic norm. Despite that, in North American Spanish (Mexico and the US), several style guides and institutions either recommend the percent sign be written following the number without any space between or do so in their own publications in accordance with common usage in that region. In Russian, the percent sign is rarely spaced, contrary to the guidelines of the GOST 8.417-2002 state standard. In Chinese, the percent sign is almost never spaced, probably because Chinese does not use spaces to separate characters or words at all. According to the Swedish Language Council, the percent sign should be preceded by a space in Swedish, as all other units. In German, the space is prescribed by the regulatory body in the national standard DIN 5008. In Turkish and some other Turkic languages, the percent sign precedes rather than follows the number, without an intervening space. In Persian texts, the percent sign may either precede or follow the number, in either case without a space. In Arabic, the percent sign follows the number; as Arabic is written from right to left, this means that the percent sign is to the left of the number, usually without a space. In Hebrew, the percent sign is written to the right of the number, just as in English, without an intervening space. This is because numbers in Hebrew (which otherwise is written from right to left) are written from left to right, as in English. In Dutch, the official rule (NBN Z 01-002) is to place a space between the number and the sign (e.g. "een stijging van 50 %"), but most of the time, the space is missing (e.g. "een stijging van 50%").
Usage in text
It is often recommended that the percent sign only be used in tables and other places with space restrictions. In running text, it should be spelled out as percent or per cent (often in newspapers). For example, not "Sales increased by 24% over 2006" but "Sales increased by 24 percent over 2006".
Evolution
Prior to 1425, there is no known evidence of a special symbol being used for percentage. The Italian term per cento, "for a hundred", was used as well as several different abbreviations (e.g. "per 100", "p 100", "p cento", etc.). Examples of this can be seen in the 1339 arithmetic text (author unknown) depicted below. The letter p with its descender crossed by a horizontal or diagonal strike (ꝑ in Unicode) conventionally stood for per, por, par, or pur in Medieval and Renaissance palaeography.
At some point, a scribe used the abbreviation "pc" with a tiny loop or circle (depicting the ending -o used in Italian ordinals, as in , , etc.; it is analogous to the English "-th" as in "25th"). This appears in some additional pages of a 1425 text which were probably added around 1435. This is shown below (source, Rara Arithmetica p. 440).
The "pc" with a loop eventually evolved into a horizontal fraction sign by 1650 (see below for an example in a 1684 text) and thereafter lost the "per".
In 1925, D. E. Smith wrote, "The solidus form () is modern."