Units of Measure

Standards Reference

The authoritative standards that define how units and prefixes are named, written, and used across science, engineering, and software.

SI Units and Prefixes

The International System of Units (SI) is the modern form of the metric system and the world's most widely used system of measurement. It defines seven base units — metre, kilogram, second, ampere, kelvin, mole, and candela — along with 24 decimal prefixes for expressing multiples and submultiples.

Key authority: BIPM — Bureau International des Poids et Mesures (bipm.org) publishes the SI Brochure, the definitive reference for units, symbols, and prefix definitions.

→ View all SI decimal prefixes    → View all base units

Binary Prefixes and Data Storage

To remove ambiguity between the decimal use of "kilo" (1,000) and the binary use in computing (1,024), the IEC introduced binary prefixes: kibi, mebi, gibi, tebi, pebi, exbi, zebi, yobi. Each represents a power of 2¹⁰.

Key authority: IEC — International Electrotechnical Commission defines binary prefixes in IEC 80000-13 (previously IEC 60027-2, Amendment 2, 1999).

→ View decimal vs. binary data size comparison    → View binary prefix table

Naming Conventions

Correct use of unit names and symbols reduces errors and improves interoperability. Key rules from SI and BIPM:

  • Unit names are written in lowercase in running text, even if derived from a proper noun (e.g., kelvin, hertz, pascal).
  • Unit symbols are case-sensitive (e.g., m = metre, M = mega-)
  • Symbols are never pluralized and do not take a period unless at the end of a sentence.
  • A space is placed between the numerical value and the unit symbol (e.g., 100 kg, 25 °C).
  • Prefix symbols are attached directly to the unit symbol with no space (e.g., km, MHz, GiB).

Reference: BIPM SI Brochure (9th edition, 2019), NIST SP 811 (Guide for the Use of the International System of Units)

Data Contracts and APIs

When representing physical quantities in APIs and data payloads, consistency and clarity are critical:

  • Always include both the numeric value and its unit in the data contract.
  • Use the canonical unit symbol (e.g., "unit": "kg") rather than the full name where space is limited.
  • Distinguish between decimal and binary prefixes explicitly — GB vs GiB.
  • Document the unit system (SI, IEC, Imperial) in the API specification.
  • Prefer base SI units in storage and computation; apply prefixes only in presentation layers.

Reference: IEEE standards for software documentation; BIPM SI Brochure Chapter 5 (Writing unit symbols and names)

Key Reference Sources

  • BIPM — Bureau International des Poids et Mesures · bipm.org · SI Brochure (9th ed.)
  • NIST — National Institute of Standards and Technology · nist.gov · NIST SP 811
  • IEC — International Electrotechnical Commission · iec.ch · IEC 80000-13
  • IEEE — Institute of Electrical and Electronics Engineers · ieee.org · IEEE Std 260.1