An index generally refers to a quantity derived from a ratio of 2 (or more) measurable quantities and often is used to compare individuals with each other or with normal values. Except for products or quotients representing specific derived SI units of measure (see also 18.2.2, Expressing Unit Names and Symbols, Products and Quotients of Unit Symbols), the ratio of SI units used to create indexes does not represent an SI convention.
At first mention in the text, the formula used to calculate the index should be described; thereafter, the numerical value for the index may be given without units attached to it. For figures or tables, the formula should be included in legends or in footnotes, respectively. However, the formula used to calculate an index need not be included in the abstract of an article.
body mass index (BMI), calculated as the weight in kilograms divided by height in meters squared
cardiac index, calculated as cardiac output in liters per minute divided by body surface area in square meters (L/min/m2)