普朗克常数记为，是一个物理常数，用以描述量子大小。在量子力学中占有重要的角色，马克斯·普朗克在1900年研究物体热辐射的规律时发现，只有假定电磁波的发射和吸收不是连续的，而是一份一份地进行的，计算的结果才能和试验结果是相符。这样的一份能量叫做能量子，每一份能量子等于普朗克常数乘以辐射电磁波的频率。这关系称为普朗克关系，用方程序表示普朗克关系式：

其中， 是能量， 是普朗克常数， 是频率。

普朗克常数的值约为：

其中电子伏特（eV）为能量单位：

普朗克常数的量纲为能量乘上时间，也可视为动量乘上位移量：

（牛顿（N）·公尺（m）·秒（s））为角动量单位

The Planck constant (denoted h, also called Planck's constant) is a physical constant that is the quantum of action, central in quantum mechanics.

First recognized in 1900 by Max Planck, it was originally the proportionality constant between the minimal increment of energy, E, of a hypothetical electrically charged oscillator in a cavity that contained black body radiation, and the frequency, f, of its associated electromagnetic wave. In 1905 the value E, the minimal energy increment of a hypothetical oscillator, was theoretically associated by Einstein with a "quantum" or minimal element of the energy of the electromagnetic wave itself. The light quantum behaved in some respects as an electrically neutral particle, as opposed to an electromagnetic wave. It was eventually called the photon.