当量（符号为：eq 或Eq），有时又称为莫耳当量、克当量，是一个在化学或生物科学中使用的单位，用于表示物质的量。 当量正式定义为一个物质的量，它可以是：

一物质含一当量所需要的质量，称为此物质之当量重。
分别来说，首先，以酸碱中和反应为第一例：一莫耳的H₂SO₄为两当量，因为它在反应 中提供两莫耳的氢离子。它的当量重(49.04)为它的分子量(98.07)的一半。
其次，以KMnO₄的氧化还原为第二例：当KMnO₄被用以氧化足量As₄O₆时， 一莫耳的KMnO₄为五当量。这是因为一莫耳的KMnO₄与五莫耳电子反应，使得Mn的氧化数从+7转换为+2。
当量在历史上也被定义为：一个物质能与一克氢、或八克氧、或35.5克氯反应的质量。在实际使用中，物质的当量也可以使用更小的单位，例如，毫当量（mEq或meq），微当量（uEq或ueq）。1 Eq = 1000 mEq，1 mEq = 1000 μEq.！！！

The Equivalent (symbol: Eq), was an archaic unit used in chemistry and the biological sciences in the era before chemical formulas and equations were known. It represented the quantity of a substance which reacted exactly with (or was equivalent to) an arbitrarily fixed quantity of another substance in a particular reaction. As knowledge increased, the theoretical basis for these 'chemical equivalents' became understood and the need for a common standard using the concept of Equivalent weights developed. The equivalent weight of an element is the gram atomic weight divided by its valence. The equivalent can also be formally defined through the amount of substance which will either: