Numbers, units and operators

Last revised by Frank Gaillard on 16 Jun 2023

Numbers, units and operators are used frequently in articles. We often quote the incidence of a condition as a range or say that x happens in more than y% of cases.

To try and keep the style similar across the site, and maximize the readability of these figures, we have some simple suggestions.

In general:

  • under 10: if a number is 10 or less, use full text, e.g. four

  • over 10: if a number is over 10, use digits, e.g. 5000

  • thousands separators can be helpful for very big numbers and as a rule of thumb we advise using a comma for numbers larger than 10,000, e.g. 10,700,000

  • decimal places

    • more than two is usually a special case, e.g. 1.76

    • leading number is required, e.g. 0.05 NOT .05 including numbers where their value cannot exceed 1.0 e.g. p-values

  • fractions: no spaces either side of the slash, e.g. 1/4, 1/2 and avoid ¼, ½, etc.

When ranges are included there should be no space on either side of the dash, e.g. "22-27". If the range is described within prose, there should be no dash, e.g. "the mean length is between 10 and 15 centimeters", not "the mean length is between 10-15 cm". Notice too, that the unit should be included in full if used in prose.

When giving a range, especially percentages, it is useful to pick a mean or median value that is representative of the range and then include the range in brackets after. Please see note on figures and percentages for further details and examples. 

When using an ordinal number put the last two letters of the written word in superscript after the numeral, e.g. 1st, 2nd, 3rd, 4th

Some biomedical classification systems and nomenclatures employ Roman numerals (e.g. I, II, III...) instead of Arabic numerals (e.g. 1, 2, 3...). The most well-known usage is probably the numbering of the cranial nerves, which are designated I through XII 2. The Bosniak classification of cystic renal masses also uses Roman numerals 3.

Historically, hepatic segmental numbering used Roman numerals. In 2000 an updated terminology 1 introduced and promoted the use of Arabic numerals instead, and Radiopaedia has adopted this convention.

When using units, there should always be a space between the number and unit. This stems from the fact that the number is a numeric representation of a word and that there is always a space between two words. If the number is written in text, the unit should be written in full:

  • two milliliters

  • 15 mL (remember that it is mL, not ml or mls)

Also, see units of measurement for further specific details on various units, but in general we prefer the use of SI units on, as per best scientific practice. 

An operator is a term from computer programming and refers to symbols used to indicate relative sizes of numeric quantities such as

  • '<', e.g. a<b; a is less than b

  • '>', e.g. a>b; a is greater than b

  • '≤', e.g. a≤b; a is less than or equal to b

  • '≥', e.g. a≥b; a is greater than or equal to b

  • where a percent sign is used, it should not have a preceding space, e.g. "22%"

  • where an operator is used, it should not have a space after it, e.g. "<45"

  • when the tilde '~' is used, it should not have a space after it, e.g. "~15"

When writing about ages, use the following styles: 

  • 30 years

  • 25-45 years

  • 20-year-old

Read more about rounding off patient ages in patient data article

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