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 Quantity that was used in chemistry and the biological sciences (see Equivalent weight § In history). The mass of an equivalent is called its equivalent weight.

The formula from milligrams (mg) to milli-equivalent (mEq) and back is as follows:

$${\displaystyle {\begin{aligned}{\text{mg}}\to {\text{mEq}}&:\quad {\text{mg }}\times {\frac {V}{MW}}\\[4pt]{\text{mEq}}\to {\text{mg}}&:\quad {\text{mEq }}\times {\frac {MW}{V}}\end{aligned}}}$$

where V is the valence and MW is the molecular weight.

For elemental compounds:

$${\displaystyle {\text{mg}}\to {\text{mEq}}:\quad {\frac {\text{element mass [mg]}}{\text{mass fraction}}}\times {\frac {V}{MV}}}$$

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

• react with or supply one mole of hydrogen ions (H⁺) in an acid–base reaction
• react with or supply one mole of electrons in a redox reaction.^[6][7]

The "hydrogen ion" and the "electron" in these examples are respectively called the "reaction units."

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.^[8]

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.