Insulin Units and Conversion Factors: A Story of Truth, Boots, and Faster Half-Truths

J Diabetes Sci Technol. 2019 May;13(3):597-600. doi: 10.1177/1932296818805074. Epub 2018 Oct 13.


Conventional insulin concentration units (IU/mL or just U/mL) are bioefficacy based, whereas the Système International (SI) units (pmol/L) are mass based. In converting between these two different approaches, there are at least 2 well-accepted conversion factors, where there should be only 1. The correct value is not the most-used or well-accepted using online calculators, some journal styles, laboratory reports, and published articles. In short, an incorrect insulin conversion factor is widely used which underreports insulin concentrations by ~15%, with potentially significant research and clinical implications. This short commentary describes the history of insulin IU definitions and conversion factors, and highlights the widespread nature of conversion factor misuse, to provoke deeper interest and thought regarding numbers we so often use without thinking.

Keywords: insulin; insulin history; insulin potency; insulin standards; insulin unit conversion; international units.

Publication types

  • Historical Article
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Cross-Cultural Comparison
  • Dosage Forms
  • Drug Dosage Calculations*
  • History, 20th Century
  • Humans
  • Insulin / administration & dosage*
  • Insulin / analysis
  • International System of Units* / history
  • Internationality
  • Osmolar Concentration
  • Reference Standards
  • World Health Organization / history


  • Dosage Forms
  • Insulin