Why numbers need their own rules
Numbers are quick to say and awkward to show. A speaker can say “twenty-five thousand three hundred and fifty” in a couple of seconds, while the digits take up very little room but must be read and understood just as quickly. Dates, times, units, currencies and symbols add further choices. Subtitle guides therefore include number formatting rules, and these differ more between guides than almost any other topic.
Below are the common approaches and the reasoning behind them. Follow your own project’s guide where one exists, and keep to one approach throughout a project.
Digits or words?
A frequent convention is to spell out smaller numbers and use digits for larger ones. A common threshold is to write numbers from one to ten in words and larger ones in digits, though some guides spell out up to twenty or a hundred. Others use digits everywhere to save space.
The reasoning:
- Words for small numbers read naturally, like ordinary text, and they stand out less.
- Digits for larger numbers are shorter and easier to scan than long strings of words.
- Digits take less room, which helps when a line is nearly full.
Common additions to this rule:
- Do not start a sentence with a numeral. Spell the number out, or rephrase the sentence. Some guides allow exceptions for years.
- Keep numbers in the same list consistent. If one number in a series needs digits, use digits for the others too.
- Spell out approximations and casual use, as in “a hundred times” or “one in a million”, where the number is part of an expression rather than a measurement.
- Use digits for measurements, scores and technical values, where precision matters.
Large numbers
Long numbers need separators or abbreviations so the reader can parse them. Separator conventions vary by language: some use commas for thousands, some use spaces or periods, and decimals also vary, with some languages using a comma where English uses a point. Subtitles should follow the conventions of the language of the viewer, not the source, which is a task for translators.
Words like “million” and “billion” are often kept in words after a digit, for example “3 million”. Be careful with “billion”, since it has not meant the same thing everywhere historically, although modern usage is generally one thousand million. If the audience is likely to be confused, spell out or rephrase. Do not guess; if you are unsure what a number means, check it.
Dates and times
Dates are a classic trap. The same string of digits can mean different dates in different regions: a day-first order and a month-first order both exist. Writing the month as a word avoids confusion. Speech often gives dates in a particular form (“the fourth of July”), and the subtitle may use the viewer’s own convention, though it should still be clear.
Times can be written in a twelve-hour or twenty-four-hour style, with or without a colon or a point. Use the style the target audience expects, and use it the same way throughout. Time-of-day abbreviations, such as a.m. and p.m., take up room, and some guides prefer to spell out “in the morning” in ordinary dialogue.
Years are normally written in digits. Decades can be written with an apostrophe or without, and conventions differ by language and guide.
Units, currency and percentages
- Units can be abbreviated or written out. Abbreviations save space, but may be unfamiliar to some viewers. Writing “kilometers” in full may be clearer than “km” in dialogue, whereas a technical video may prefer the symbol.
- Conversion. When a unit is unfamiliar to the audience, a translator may convert it or keep it with a short explanation. Any conversion must be correct, so calculate and verify it. This is a localization choice, discussed in localization basics.
- Currency may use a symbol, a code or a word. Symbols should be placed according to the target language: before or after the number varies.
- Percentages are normally written with digits and the percent sign, or with the word “percent” in some guides.
Symbols
Symbols save space, but they can also confuse.
- Ampersand and plus sign. Usually avoided in dialogue, where the word is spelled out, but appropriate when they appear as part of a name or on-screen text.
- Degree sign, percent sign, currency signs. Common and compact; check they display properly, because some fonts or files fail to show them. This is related to character encoding.
- Hash, at sign and similar. Fine when the speaker is naming an address or tag, though some need explanation.
- Musical note. Often used to mark song lyrics, though its use depends on the guide and on font support. See also song lyrics.
- Mathematical symbols. Reserve them for technical content where the audience expects them.
Where the speech is “plus” or “percent”, ask whether the written symbol is clearer or not. In a casual conversation, the word probably is. In a chart or a recipe, the symbol may be.
Telephone numbers, codes and names
Strings of digits, such as phone numbers or codes, are hard to read in a short time. Group digits as they are said, and keep the whole number in one cue and on one line. Do not split a number across lines or cues, and keep numbers with their units on the same line, a point covered in line breaks.
Consistency and checking
Inconsistent numbers are an easy way for a subtitle to look careless. Pick rules, write them down and check them. Numbers are also a classic source of real mistakes: a transposed digit, a wrong unit, or a misheard figure can change the meaning completely. In important material, such as medical dosages, financial figures or safety instructions, have a qualified reviewer verify every number against a reliable source. Automatic transcripts in particular can mishear numbers, so they should always be checked, as noted in the guide to subtitle errors.
A final test: read the number aloud as the viewer would. If you stumble, simplify it.