Communications of the ACM
SI and Binary Prefixes: Clearing the Confusion
Credit: Alicia Kubista / Andrij Borys Associates
The International System of Units (SI), organized by the International Bureau of Weights and Measures (BIPM), defines several prefixes to denote the quantities between 10−24 and 10+24, given in Table 1. It allows expression of very large or small quantities of all units of measurement in common scientific notation accepted by the international community. For example, the prefixes kilo (k), mega (M), giga (G), and tera (T) indicate 103n, for n= 1, 2, 3, and 4, respectively. However, when expressing data quantities and memory addresses, SI prefixes (SIPs) are misused to indicate binary multiples. In this non-standard adoption, for a number n, where n=1, 2, and 8, each prefix specifies 210n instead of their original values (103n). Almost everyone, including scientists and engineers, thinks that this non-standard acceptance is correct. On the other hand, the prefixes for binary multiples were already defined by the relevant standards approximately 23 years ago. This non-standard notation confuses at the very least or can cause rather serious problems. Hence, all parties should immediately abandon this misconception and disseminate the correct information to everyone.
Table 1. SIPs and their values.
This Viewpoint provides first a historical view of the origins of this non-standard use of SIPs for binary multiples, encountered problems and suggested solutions, and the relevant standards. Then, this article introduces new prefixes for binary multiples and proposes several actions to be undertaken to disseminate the correct information.
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