An equivalent (symbol: officially equiv;[1] unofficially but often Eq[2]) is the amount of a substance that reacts with (or is equivalent to) an arbitrary amount (typically one mole) of another substance in a given chemical reaction. It is an archaic unit of measurement that was used in chemistry and the biological sciences (see Equivalent weight#In history). The mass of an equivalent is called its equivalent weight.

In a more formal definition, the equivalent is the amount of a substance needed to do one of the following:

By this definition, the number of equivalents of a given ion in a solution is equal to the number of moles of that ion multiplied by its valence. For example, consider a solution of 1 mole of NaCl and 1 mole of CaCl2. The solution has 1 mole or 1 equiv Na+, 1 mole or 2 equiv Ca2+, and 3 mole or 3 equiv Cl.

An earlier definition, used especially for chemical elements, holds that an equivalent is the amount of a substance that will react with 1 g (0.035 oz) of hydrogen, 8 g (0.28 oz) of oxygen, or 35.5 g (1.25 oz) of chlorine—or that will displace any of the three.[5]

In medicine and biochemistry

In biological systems, reactions often happen on small scales, involving small amounts of substances, so those substances are routinely described in terms of milliequivalents (symbol: officially mequiv; unofficially but often mEq[2] or meq), the prefix milli- denoting a factor of one thousandth (10−3). Very often, the measure is used in terms of milliequivalents of solute per litre of solution (or milliNormal, where meq/L = mN). This is especially common for measurement of compounds in biological fluids; for instance, the healthy level of potassium in the blood of a human is defined between 3.5 and 5.0 mEq/L.

A certain amount of univalent ions provides the same amount of equivalents while the same amount of divalent ions provides twice the amount of equivalents. For example, 1 mmol (0.001 mol) of Na+ is equal to 1 meq, while 1 mmol of Ca2+ is equal to 2 meq.


  1. ^ "CAS Standard Abbreviations & Acronyms". Retrieved 2017-07-26.
  2. ^ a b American Medical Association, "14.12: Units of Measure", AMA Manual of Style, retrieved 2019-10-23.
  3. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "equivalent entity". doi:10.1351/goldbook.E02192
  4. ^ International Union of Pure and Applied Chemistry (1998). Compendium of Analytical Nomenclature (definitive rules 1997, 3rd. ed.). Oxford: Blackwell Science. ISBN 0-86542-6155. section 6.3. "Archived copy" (PDF). Archived from the original (PDF) on July 26, 2011. Retrieved 2009-05-10.((cite web)): CS1 maint: archived copy as title (link)
  5. ^ "Atome", Grand dictionnaire universel du XIXe siècle (in French), vol. 1, Paris: Pierre Larousse, 1866, pp. 868–73